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Biological  Services  Program 


FWS/OBS-81/15 


May  1981  1 

THE  ECOLOGY  OF  INTERTIDAL 
OYSTER  REEFS  OF  THE  SOUTH 
ATLANTIC  COAST:  A  COMMUNITY  PROFILE 


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Fish  and  Wildlife  Service 


U.S.  Department  of  the  Interior 


The  Biological  Services  Program  was  established  within  the  U.S.  Fish 
and  Wildlife  Service  to  supply  scientific  information  and  methodologies  on 
key  environmental  issues  that  impact  fish  and  wildlife  resources  and  their 
supporting  ecosystems.  The  mission  of  the  program  is  as  follows: 

t  To  strengthen  the  Fish  and  Wildlife  Service  in  its  role  as 
a  primary  source  of  information  on  national  fish  and  wild- 
life resources,  particularly  in  respect  to  environmental 
impact  assessment. 

•  To  gather,  analyze,  and  present  information  that  will  aid 
decisionmakers  in  the  identification  and  resolution  of 
problems  associated  with  major  changes  in  land  and  water 
use. 

•  To  provide  better  ecological  information  and  evaluation 
for  Department  of  the  Interior  development  programs,  such 
as  those  relating  to  energy  development. 

Information  developed  by  the  Biological  Services  Program  is  intended 
for  use  in  the  planning  and  decisionmaking  process  to  prevent  or  minimize 
the  impact  of  development  on  fish  and  wildlife.  Research  activities  and 
technical  assistance  services  are  based  on  an  analysis  of  the  issues,  a 
determination  of  the  decisionmakers  involved  and  their  information  needs, 
and  an  evaluation  of  the  state  of  the  art  to  identify  information  gaps 
and  to  determine  priorities.  This  is  a  strategy  that  will  ensure  that 
the  products  produced  and  disseminated  are  timely  and  useful. 

Projects  have  been  initiated  in  the  following  areas:  coal  extraction 
and  conversion,  power  plants;  geothermal ,  mineral  and  oil  shale  develop- 
ment; water  resource  analysis,  including  stream  alterations  and  western 
water  allocation,  coastal  ecosystems  and  Outer  Continental  Shelf  develop- 
ment, and  systems  inventory,  including  National  Wetland  Inventory, 
habitat  classification  and  analysis,  and  information  transfer. 

The  Biological  Services  Program  consists  of  the  Office  of  Biological 
Services  in  Washington,  D.C.,  which  is  responsible  for  overall  planning  and 
management.  National  Teams,  which  provide  the  Program's  central  scientific 
and  technical  expertise  and  arrange  for  contracting  bioloaical  services 
studies  with  states,  universities,  consulting  firms,  and  others,  Regional 
Staffs,  who  provide  a  link  to  problems  at  the  operating  level; and  staffs  at 
certain  Fish  and  Wildlife  Service  research  facilities,  who  conduct  in-house 
research  studies. 


Cover  design  by  Graham  (ioUcn 


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FWS/CCS-81/15 
May  1981 


THE  ECOLOGY  OF  INTERTIDAL  OYSTER  REEFS  OF  THE 
SOUTH  ATLANTIC  COAST:  A  COWUNITY  PROFILE 


by 


Leonard  f".  Bahr 
William  P.  Lanier 
Louisiana  State  University 
3aton  Rouge,  Louisiana  70803 


Project  Officer 

Wiley  V,.    Kitchens 

National  Coastal  Ecosystens  Team 

U.S.  Fish  and  Wildlife  Service 

1010  Gause  Boulevard 

Slidell,  Louisiana  70458 


Performed   for 

National  Coastal  Ecosystems  Team 

Office  of  Biological  Services 

Fish  and  Wildlife  Service 

U.S.  Department  of  the  Interior 

Washington,  D.C.  20240 


DISCLAIMER 

The  findings  in  this  report  are  not  to  be  construed  as  an  official 
U.S.  Fish  and  Wildlife  Service  position  unless  so  designated  by  other 
authorized  documents. 


This  report  should  be  cited: 

Bahr,  L.  M.,  and  W.  P.  Lanier.  1981.  The  ecology  of  intertidal  oyster 
reefs  of  the  South  Atlantic  coast:  a  community  profile.  U.S.  Fish 
and  Wildlife  Service,  Office  of  Biological  Services,  Washington, 
D.C.   FWS/OBS-81/15.  105  pp. 


PREFACE 


This  oyster  reef  comnunity  profile  is 
the  second  in  a  developing  series  of  pro- 
files of  coastal  habitats.  The  purpose  of 
this  profile  is  to  describe  the  structure 
and  ecological  function  of  intertidal  oys- 
ter reefs  in  the  salt  niarsh  estuarine  eco- 
systeni  of  the  Southeastern  United  States. 
The  intertidal  oyster  reef  habitat,  as 
described  here,  is  classified  by  Cowardin 
et  al.  (1979)  as  occurring  in  the  Carolin- 
ian province,  in  the  euhaline  estuarine 
system,  in  the  intertidal  subsysterri,  in 
the  reef  class,  and  in  the  riollusk  sub- 
class, v/ith  the  eastern  oyster  Crassostrea 
vi rqinica  as  the  dominance  type. 

This  profile  provides  a  handy  refer- 
ence which  synthesizes  the  voluminous  sci- 
entific literature  on  oysters  and  focuses 
on  aspects  of  the  less-studied  oyster  reef 
community.  The  profile  also  points  out 
some  of  the  many  deficiencies  in  the  cur- 
rent level  of  understanding  of  the  oyster 
reef  subsystem  and  of  the  entire  estuarine 
ecosystem.  If  additional  research  efforts 
are  thereby  initiated,  this  profile  vji  1 1 
have  been  a  success.  (The  observant  read- 
er will  notice  that  in  many  instances 
where  quantitative  data  were  not  avail- 
able, extrapolations  from  other  communi- 
ties or  educated  judgments,  or  both,  were 
necessary. ) 

The  information  in  the  profile  will 
be  useful  to  environmental  managers,  re- 


source planners,  coastal  ecologists,  ma- 
rine science  students,  and  interested  lay- 
men who  wish  to  learn  about  the  oyster 
reef  community  and  its  role  in  the  coastal 
ecosystem.  The  format,  style,  and  level 
of  presentation  should  make  this  report 
adaptable  to  a  diversity  of  needs,  from 
the  preparation  of  environmental  assess- 
ment reports  to  supplementary  reading 
material  in  college  marine  science 
courses. 

This  profile  proceeds  from  a  descrip- 
tion of  the  estuarine  setting  (Chapter  1), 
to  a  discussion  of  oyster  biology  (Chapter 
2),  to  a  characterization  of  the  oyster 
reef  per  se  (Chapter  3),  to  a  discussion 
of  the  development  and  role  of  the  reef 
system  in  the  coastal  ecosystem  (Chapter 
4).  Chapter  5  is  a  summary  of  the  role  of 
the  oyster  reef  as  expressed  in  three  con- 
ceptual models,  and  Chapter  6  includes  a 
brief  synopsis  of  the  first  five  chapters, 
along  with  implications  for  management. 

Any  questions  or  comn:ents  about  or 
requests  for  this  publication  should  be 
directed  to: 

Information  Transfer  Specialist 
iJational  Coastal  Ecosystems  Team 
U.S.  Fish  and  l.'ildlife  Service 
NASA-SI idell  Computer  Complex 
ICIC  Cause  Boulevard 
SI idell,  LA  70458 


CONTENTS 


Page 

PREFACE iii 

FIGURES vii 

TABLES viii 

ACKNOWLEDGMENTS  ix 

CHAPTER  1.  COMMUNITY  PROFILE  BACKGROUND  AND  OBJECTIVES;  DESCRIPTION 

OF  THE  COASTAL  SETTING 1 

1.1  Introduction  and  Objectives  1 

1.2  General  Characteristics  of  the  South  Atlantic  Bight  ...  3 

1.3  Estuarine  Producers  8 

1.4  Estuarine  Consumers  10 

CHAPTER  2.  FUNCTIONAL  OYSTER  BIOLOGY  AND  AUTECOLOGY  17 

2.1  Taxonomy  and  Evolution 17 

2.2  Oyster  Reproduction  and  Development 20 

2.3  Oyster  Feeding,  Digestion,  and  Assimilation  24 

2.4  Stresses  on  Oyster  Populations:  Natural  and  Cultural.  .  .  28 

2.5  Energy  Summary 32 

CHAPTER  3.  OYSTER  REEF  DESCRIPTION  AND  SYNEC0L06Y  37 

3.1  General  Reef  Description 37 

3.2  Reef -Associated  Macrofauna  42 

3.3  Reef  Community  Energetics 48 

3.4  Reef  Predation 53 

3.5  Colonial  Aspects  of  the  Reef  Community 55 

CHAPTER  4.  OYSTER  REEF  DEVELOPMENT,  DISTRIBUTION,  PHYSICAL  EFFECTS, 

AND  AREAL  EXTENT 57 

4.1  Reef  Development 57 

4.2  Distribution  of  Oyster  Reefs  in  the  Marsh  Estuarine 

Ecosystem 58 

4.3  Physical  Effects  of  Oyster  Reefs  on  the  Marsh- 

Estuarine  Ecosystem  61 

4.4  Areal  Extent  of  Oyster  Reefs  in  the  Coastal  Ecosystem  .  .  62 

CHAPTER  5.  CONCEPTUAL  MODELS  OF  THE  INTERTIDAL  OYSTER 

REEF  COMMUNITY 65 

5.1  Objectives  and  Level  of  Resolution 65 

5.2  Regional  Level  Conceptual  Model 71 

5.3  Drainage  Unit  Level  Conceptual  Model  71 

5.4  Reef  Level  Conceptual  Model 74 


CONTENTS  (continued) 

Page 

CHAPTER  6.  SUMMARY  AND  MANAGEMENT  IMPLICATIONS  AND  GUIDELINES  ....  79 

6.1  Summary  and  Oyster  Reef  Significance 79 

6.2  Management  Implications  and  Guidelines  82 

REFERENCES 84 

APPENDIX:  OYSTER  BIOENERGETICS  93 


VI 


FIGURES 


Number  Page 

la   The  study  area  (South  Atlantic  Bight)  extends  from  Cape 

Fear,  North  Carolina,  to  Cape  Canaveral,  Florida  2 

lb   Tidal  characteristics  of  the  Atlantic  and  Gulf  of  Mexico 

coasts  2 

2  Sedimentary  regions  of  estuary  6 

3  A  comparison  of  primary  productivity  for  different  kinds 

of  ecosystems 9 

4  Model  of  energy  flow  in  the  Georgia  salt  marshes 12 

5  Trophic  spectrum  of  an  estuarine  community  (Lake 
Pontchartrain  Estuary,  Louisiana) 15 

6  The  distribution  of  Crassostrea  virqinica  19 

7  Anatomy  of  the  oyster  (Crassostrea  virqinica)  and  diagram 
showing  the  correct  method  of  measuring  the  height,  length, 

and  width  of  oyster  shells 21 

8  Transverse  section  of  the  dorsal  part  of  an  adult 
Crassostrea  virginica  22 

9  A  schematic  representation  of  the  rhythmic  nature  of  the 
feeding  process  and  extracellular  and  intracellular 
digestive  mechanisms  in  oysters  25 

10  Effects  of  turbidity  on  pumping  rate 27 

11  Effects  of  crude  oil  extract  on  Mytilus  edulis  carbon 
budgets  calculated  for  100-mg  mussels  held  at  31  /oo 
salinity  under  summer  conditions  (15°  C,215  ug  C/liter).  .  .  31 

12  Summary  of  energy  flow  through  intertidal  reef  oysters  ...  35 

13  Diagrammatic  section  through  oyster  reef  illustrating 
relative  elevation  with  respect  to  mean  tidal  levels  and 
corresponding  fouling  pattern  on  piling  38 

14  Several  generations  of  oysters  (£.  virqinica)  growing 
vertically  on  muddy  bottom  of  Altamaha  Sound,  Georgia  ...  39 

15  Seasonal  oxygen  consumption  of  reef  community  49 

16  Seasonal  energy  partitioning  estimates  for  the  entire 

reef  community 52 

17  Typical  distribution  of  oyster  reefs  in  small  tidal 

creeks 60 

18  Comparison  of  two  systems  of  concentrated  consumers  whose 
survival  depends  on  strong  flows  that  bring  in  fuels  and 
oxygen  and  outflow  wastes:   (a)  reef  of  oysters  and 
other  marine  animals  characteristic  of  many  estuaries; 

(b)  industrialized  city 66 

19  Three  hierarchical  levels  of  oyster  reef  organization.  ...  67 

20  Reef  distribution  in  a  single  drainage  basin,  the  Half 

Moon  River  Estuary,  Wilmington  Island,  Georgia  .  68 

21  Recent  and  historical  reef  distribution  in  the  Duplin  River 
Estuary,  Sapelo  Island,  Georgia  69 

22  Regional  level  conceptual  model  and  explanation  of 

symbols 72 

23  Drainage  unit  level  conceptual  model  73 

24  Reef  organization  conceptual  model  75 

vii 


FIGURES  (continued) 


Number  Page 

A-1    Age-dependent  annual  production  of  soft  tissue,  shell 

organics,  gonad  output,  and  respiration  in  an  oyster  ....  95 
A-2    Seasonal  variation  in  water  temperature  affecting  oyster 

reefs  in  South  Carolina 95 

A-3    Seasonal  changes  in  size -frequency  distribution  of  reef 

oysters  in  Georgia 99 

A-4    Seasonal  changes  in  intertidal  oyster  size -frequency 

distribution  in  South  Carolina  100 

A-5    Reef  oyster  height  frequency  relationship  and  cumulative 

biomass  curves  102 

A-6    Schematic  representation  of  percentage  distribution  of 

potential  food  expressed  in  kilocalories  for  1-year  period 

in  1  m^  of  subtidal  Crassostrea  gigas  population  104 


TABLES 


Number  Page 

1  General  effects  of  man-induced  (cultural)  stress  on 

oysters 29 

2  Pounds  of  meat  and  ex-vessel  value  (dollars)  of  oysters 
harvested  in  four  South  Atlantic  States  from  1973-1975  .  .   33 

3  Macrofauna  found  in  Georgia  oyster  reefs  43 

4  Mean  annual  frequency  distribution  of  reef  macrofauna  .  .   45 

5  Ranked  biomass  of  16  major  oyster  species  or  groups  of 
species  and  proportion  of  total  macrofaunal  biomass  ...   45 

6  Ranking  of  macrofaunal  metabolic  dominance  based  on 

biomass 51 

7  Community  respiration  in  aquatic  systems  54 

8  Time  scales  relating  ecosystem  processes  and  components  at 
the  three  conceptual  levels  of  oyster  reef  organization 

and  function 70 

A-1    Conversion  factors  for  oyster  biomass  units  (intertidal 

oysters) 98 

A-2    Comparison  of  two  sets  of  oyster  reef  energy  parameters 

collected  within  the  study  area 103 


vn  1 


ACKNOWLEDGMENTS 


During  the  construction  of  this  pro- 
file it  was  necessary  to  request  inforina- 
tion  from  a  variety  of  individuals,  all  of 
whom  were  generous  with  their  time  and 
suggestions.  An  incomplete  list  of  those 
who  helped  includes:  Clay  Adams,  Jim 
Bishop,  Norman  Buroker,  Elgin  Dunnington, 
Skipper  Keith,  Kel  Lehman,  Bob  Reimold, 
and  Tony  Reisinger.  Special  thanks  are 
also  due  to  Joy  Bagur,  Pauline  Jolly,  and 
Francisco  Ley,  who  assisted  in  the  compi- 
lation of  the  reference  material;  and  to 
Carolyn  Lusk  and  Josie  Williams,  who 
typed  the  manuscript. 

The  draft  manuscript  was  reviewed  for 


its  scientific  content  by  Ed  Cake,  John 
Hall,  James  Kirkwood,  Stuart  Stevens,  Jim 
Stone,  Martha  Young,  and  Tim  Sipe.  All 
reviews  were  extremely  helpful.  Gaye 
Farris  and  Elaine  Bunce  edited  the  manu- 
script and  were  assisted  by  the  efforts  of 
typists,  Elizabeth  Krebs  and  Daisy  Single- 
ton,  and   illustrator,   Graham  Golden. 

Finally  we  would  like  to  acknowledge 
the  unselfish  assistance  of  Wiley  Kitch- 
ens, who  ably  administered  this  project. 
Factual  errors  or  faulty  conclusions  are, 
of  course,  the  sole  responsibility  of  the 
authors. 


IX 


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« ■ 


This  view  of  the  intertidal  zone  at  Sapelo  Island,  Georgia,  shows  the  position  of 
oyster  reefs  relative  to  the  surrounding  marshes  and  mudflats.  Photo  by  Leonard 
Bahr,  Louisiana  State  University. 


CHAPTER  1 

COMMUNITY  PROFILE  BACKGROUND  AND  OBJECTIVES; 

DESCRIPTION  OF  THE  COASTAL  SETTING 


1.1  INTRODUCTION  AND  OBJECTIVES 

Oysters  occupy  a  unique  status  among 
marine  and  estuarine  invertebrates.  As  a 
group,  they  are  the  most  widely  studied 
and  thus  best  known  of  all  these  animals, 
primarily  because  of  their  universal 
socioeconomic  value.  Some  oyster  species 
are  prized  for  their  flavor  and  high  qual- 
ity protein;  some  are  valued  for  their 
pearls.  Oyster  shell  calcium  carbonate 
has  long  been  used  as  a  building  material; 
e.g.,  "Tabby"  houses  of  oyster  shell  were 
once  common  in  coastal  Georgia  and  South 
Carolina.  Oyster  shells  are  fed  to  chick- 
ens and  are  even  the  source  of  supplemen- 
tary calcium  in  tablets  for  humans.  Oys- 
ters of  various  kinds  have  been  cultured 
for  centuries,  so,  in  a  sense,  some  spe- 
cies qualify  as  domestic  animals. 

Much  of  the  information  gathered 
about  oysters  has  been  collected  by  sci- 
entists, but  a  significant  amount  has  been 
collected  by  observant  laymen,  natural- 
ists, and  aquaculturists.  The  objectives 
of  most  of  those  observations,  however, 
were  to  learn  to  grow  more  oysters  in 
given  areas  faster  and  with  fewer  losses. 
In  the  vast  oyster  literature,  there  are 
relatively  few  "pure"  ecological  studies 
that  treat  the  oyster  objectively  as  an 
ecosystem  component.  For  example,  animals 
associated  with  oysters  are  usually 
referred  to  as  "pest"  species,  as  coyotes 
are  to  sheepherders.  But  just  as  no  orga- 
nism is  autonomous,  and  all  organisms 
operate  within  the  framework  of  ecosys- 
tems, so  the  oyster's  importance  extends 
beyond  its  socioeconomic  value. 

The  primary  objective  of  this  commu- 
nity profile  is  to  describe  the  function 
of  one  species  of  oyster  in  a  portion  of 
its  habitat.  More  specifically,  we  present 
a  profile  of  a  community  associated  with, 
dependent  on,  and  dominated  by  the  Ameri- 
can or  eastern  oyster,  Crassostrea  virgin- 
ica  (Gmelin).   The  study  area  comprises 


the  coast  of  the  South  Atlantic  Bight  be- 
tween Cape  Fear,  North  Carolina,  and  Cape 
Canaveral,  Florida,  (Figure  la),  as  dis- 
cussed in  Section  1.2. 

Because  the  range  of  the  American 
oyster  extends  over  a  wide  latitude  (from 
20°  N  to  54°  N),  the  ecological  conditions 
encountered  are  diverse  and  the  "oyster 
community"  is  not  uniform  throughout  the 
range  (see  Section  2.2).  The  present  de- 
scription applies  primarily  and  specifi- 
cally to  those  populations  of  oysters  and 
associated  organisms  occurring  in  the  in- 
tertidal  zone  in  the  Southeastern  United 
States.  In  some  portions  of  its  range, 
particularly  in  the  area  being  described 
(which  has  a  large  tidal  range),  the  oys- 
ter builds  massive,  discrete  reefs  in  the 
intertidal  zone. 

The  vertical  elevation  of  intertidal 
oyster  reefs  above  mean  low  water  is  max- 
imal within  the  central  Georgia  coastal 
zone,  where  mean  tidal  amplitude  exceeds 
2  m  (2.2  m  [7.2  ft]  at  Sapelo  Island,  ap- 
proximately at  the  center  of  the  South 
Atlantic  Bight).  Approximate  isopleths  or 
contours  of  the  tidal  range  along  the 
Atlantic  and  gulf  coasts  are  indicated  in 
Figure  lb.  Local  areas  experience  tidal 
variation  because  of  local  hydrologic 
effects.  The  most  extensive  contiguous 
oyster  reefs  occur  in  the  South  Carolina 
coastal  zone.  Oyster  reefs  diminish  in 
size  and  significance  south  of  Georgia  and 
north  of  South  Carolina,  but  there  are  in- 
tertidal patch  reefs  in  northeastern 
Florida  and  southern  North  Carolina.  In- 
formation reported  in  this  document  is 
applicable  to  reefs  from  Cape  Fear, 
North  Carolina,  to  Cape  Canaveral,  Flor- 
ida, except  as  noted  in  the  text. 

The  term  oyster  reef  often  is  inter- 
changed loosely  with  other  terms  for  local 
estuarine  areas  inhabited  by  oysters, 
including  oyster  bar,  oyster  bed,  oyster 
rock,  oyster  ground,  and  oyster  planting. 


DIURNAL  TIDE 
One  high  water  and  one  low  water  each 
tidal  day  (about  24  hours,  50  minutes) 

SEMIDIURNAL  TIDE 
Two  nearly  equal  high  waters  and  two 
nearly  equal  low  waters  each  tidal  day 

MIXEDTIDE 

Two  unequal  high  waters  and/or  two 
unequal  low  waters  each  tidal  day 


Corange  lines  connect 
points  of  equal  tide 
range.   Numerals  Indicate 
the  mean  maximum 
semimonthly  spring  tide' 
ranges  in  feet 

Direction  of  tide  pro- 
gression 


Figure  1.    (a)  The  study  area   (South  Atlantic  Bight)   extends  from  Cape  Fear, 
North  Carolina,  to  Cape  Canaveral,   Florida,      (b)  Tidal   characteristics  of 
the  Atlantic  and  Gulf  of  Mexico  coasts    (adapted  from  U.S.   Geological   Survey 
1970). 


Throughout  this  document,  oyster  "reefs" 
are  strictly  defined  as  "the  natural 
structures  found  between  the  tide  lines 
that  are  composed  of  oyster  shell,  live 
oysters,  and  other  organisms  and  that  are 
discrete,  contiguous,  and  clearly  distin- 
guishable (during  ebb  tide)  from  scattered 
oysters  in  marshes  and  mud  flats,  and  from 
wave-formed  shell  windrows."  Intertidal 
reefs,  as  defined  here,  are  also  distinct 
from  natural  and  planted  subtidal  oyster 
populations. 

Ecologists'  opinions  differ  as  to 
whether  benthic  communities  (and  some 
other  communities)  exist  as  tightly  inter- 
active and  interdependent  systems  of  orga- 
nislns,  or  whether  such  communities  are 
merely  loose,  chance  associations  to  which 
member  species  belong  solely  by  geographic 
accident.  A  proponent  of  the  former  argu- 
ment long  ago  chose  the  oyster  community 
as  an  example  of  a  biocoenosis,  or  inter- 
active community  (Mobius  1883).  Whether  or 
not  some  species  that  occur  in  the  oyster 
reef  community  are  dispensable,  the  Ameri- 
can oyster  is  the  "keystone"  species  (or 
indispensable)  in  the  sense  intended  by 
Paine  (1959)  when  he  coined  the  term. 

Specific  objectives  of  this  report 
are  as  follow:  (1)  to  synthesize  a  state- 
of-the-art  systems  view  of  the  oyster  reef 
community  in  the  study  area  from  existing 
literature;  (2)  to  address  the  effects  of 
various  potential  cultural  and  natural 
perturbations  on  the  oyster  reef  subsys- 
tem, including  pollution  effects,  physical 
alterations  to  the  estuary,  and  natural 
changes;  (3)  to  condense  the  above  infor- 
mation into  conceptual  ecosystem  models 
constructed  at  a  level  understandable  by  a 
variety  of  readers,  including  those  inex- 
perienced  in  using  ecological  models. 

The  American  oyster  is  the  quintes- 
sential or  most  typical  estuarine  animal. 
It  can  tolerate  a  wide  range  of  salinity, 
temperature,  turbidity,  and  oxygen  ten- 
sion, and  therefore  is  adapted  to  the  pe- 
riodic and  aperiodic  changes  in  water 
quality  that  characterize  estuaries.  Some 
physiological  and  anatomical  reasons  for 
its  adaptive  plasticity  are  described  in 
Chapter  2,  which  treats  the  autecology  of 
the  oyster.  Other  aspects  of  the  success 
of  the  intertidal  oyster  are  related  to 
its  colonial  lifestyle  and  mutual  inter- 
dependence and  cannot  be  comprehended  from 


information  gathered  for  individual  oys- 
ters. Chapter  3  is  devoted  to  a  discus- 
sion of  the  entire  reef  community.  Chap- 
ter 4  discusses  the  reef's  role  in  the 
coastal  ecosystem  and  Chapter  5  presents 
three  models  expressing  the  reef's  role. 
Chapter  6  summarizes  the  other  chapters 
and  gives  implications  for  management. 

This  chapter's  remaining  sections  de- 
scribe the  specific  estuarine  environment 
of  the  oyster  reef  community.  They  in- 
clude the  physical,  chemical,  and  biolog- 
ical settings. 


1.2  GENERAL  CHARACTERISTICS 
OF  THE  SOUTH  ATLANTIC  BIGHT 

The  geographic  area  on  which  this 
profile  primarily  focuses  is  the  portion 
of  the  South  Atlantic  Bight,  extending 
along  the  southeastern  coast  of  the  United 
States  between  Cape  Fear  ,  North  Caro- 
lina, and  Cape  Canaveral,  Florida.  This 
section  of  the  southern  coastal  plain  ex- 
hibits a  continuum  of  change  in  coastal 
morphology,  but  is  characterized  by  exten- 
sive lagoon-marsh  systems  and  estuaries 
bound  at  their  eastern  extent  by  barrier 
island  complexes.  The  morphology  of  coast- 
al barrier  island  systems  and  extent  of 
the  lagoon-marsh  are  the  results  of  a  com- 
plex interplay  of  physical  and  biological 
processes. 

In  general,  this  area  can  be  consid- 
ered a  mixed-energy  coast  (Hayes  1975) 
since  coastal  processes  and  morphologies 
are  determined  by  the  varying  influence  of 
both  waves  and  tides.  Wave  and  tidal  con- 
ditions in  this  area  are  largely  a  func- 
tion of  the  changing  profile  of  the  inner 
continental  shelf  (Hayden  and  Dolan  1979; 
Hubbard  et  al.  1979).  Average  wave  heights 
decrease  from  a  maximum  of  1.2  m  (4  ft) 
along  the  North  Carolina  coast  to  a  mini- 
mum of  0.1  m  (0.5  ft)  along  the  central 
Georgia  coast  (Hubbard  et  al.  1979).  Where 
the  shelf  is  broad,  nearshore  wave  heights 
are  reduced  through  frictional  loss  caused 
by  shoaling  on  the  ocean  floor  shelf. 

Shelf  width,  combined  with  the  arcu- 
ate shape  of  the  coastline,  also  influenc- 
es tidal  range.  The  southern  coast  of 
North  Carolina  is  classified  as  a  micro- 
tidal  coastline  (Davies  1964),  with  semi- 
diurnal tides  that  range  between  0  and  2  m 


(0-6.5  ft).  Tides  at  ^lasonboro  Inlet  on 
the  southern  coast  of  North  Carolina  range 
from  1.2  m  (4  ft)  to  1.4  m  (4.5  ft)  for 
mean  and  spring  conditions,  respectively 
(Vallianos  1975).  Wind  and  wave  processes 
are  the  principal  forces  dictating  coastal 
morphology  in  microtidal  coastal  systems 
(Hayes  1975).  Barrier  islands  in  North 
Carolina  tend  to  be  long  and  narrow,  and 
they  contain  relatively  few  tidal  inlets. 
Lagoon-marsh  systems  are  usually  narrow 
(1.5  km  or  0.6  mi),  shallow,  and  densely 
vegetated  (Cleary  et  al.  1979). 

Farther  south  in  South  Carolina  and 
Georgia,  the  coastal  system  has  been  clas- 
sified as  mesotidal  (Davies  1964),  having 
a  tidal  range  between  2  and  4  m  (6.5  and 
13  ft).  This  coastline  is  characterized 
by  short  (20  to  30  km  or  12  to  19  mi)  bar- 
rier islands,  with  a  wider  central  portion 
and  narrow  ends  broken  by  numerous  tidal 
inlets.  In  response  to  the  higher  tidal 
range,  larger  areas  of  lagoon-marsh  are 
broken  by  extensive  and  complex  networks 
of  tidal  drainage  channels.  Tidal  inlets 
between  barrier  islands  tend  to  be  rela- 
tively deep  (>10  m  or  34  ft)  and  are 
flanked  by  extensive  bars  and  spits. 

Estuaries  and  lagoons  with  associated 
marsh,  mudflat,  and  tidal  drainage  net- 
works compose  the  dominant  habitat  of  the 
American  oyster,  £.  virginica,  in  the 
Southeastern  United  States.  The  term  estu- 
ary from  the  Latin  aestus,  meaning  tide 
(Schubel  1971),  has  been  defined  in  vari- 
ous ways.  Geologists  tend  to  accept  the 
strictly  physical  interpretation  of  Prit- 
chard  (1967),  who  defined  an  estuary  as  "a 
semi-enclosed  coastal  body  of  water  which 
has  a  free  connection  with  the  open  sea 
and  within  which  sea  water  is  measurably 
diluted  with  fresh  water  from  land  drain- 
age." A  broader,  more  ecological  defini- 
tion proposed  by  Cowardin  et  al.  (1979)  is 
"deep-water  tidal  habitats  and  adjacent 
tidal  wetlands  which  are  usually  semi- 
enclosed  by  land,  but  have  open,  partially 
obstructed,  or  sporadic  access  to  the  open 
ocean  and  in  which  ocean  water  is  at  least 
occasionally  diluted  by  fresh  water  runoff 
from  the  land."  In  this  paper  we  define 
estuaries  even  more  broadly  to  include  all 
the  ecological  subsystems  that  interact  to 
form  the  coastal  marsh-estuarine  ecosys- 
tem. In  other  words,  to  quote  Odum  et  al. 
(1974),  "It  is  the  ecosystems  rather  than 


the  estuarine  waterbodies  that  are  dis- 
cussed ...here. 


Pr. 
waterbod 
types 


'itchard  (1967)  subdivided  estuarine 
)dies   into   four  geomorphological 

„^ , (1)  drowned  river  valleys;  (2) 

fjord-type  estuaries;  (3)  bar-built  estu- 
aries; and  (4)  estuaries  produced  by  tec- 
tonic processes.  All  southeastern  coastal 
plain  estuaries  fall  into  either  the  bar- 
built  or  the  drowned  river  valley  estua- 
rine types. 

General  Estuarine  Hydrography 

Water  circulation  patterns  are  of 
primary  significance  in  determining  the 
physical  and  chemical  conditions  of  the 
estuarine  ecosystem.  Water  circulation 
strongly  influences  salinity,  but  it  also 
directly  influences  sedimentation  pat- 
terns, turbidity,  temperature,  and  nutri- 
ent conditions.  Estuaries  with  signifi- 
cant riverine  sources  of  low  salinity 
water  are  distinctly  different  in  form  and 
hydrographic  character  from  those  without 
such  sources  (Oertel  1974). 

Classifications  of  estuarine  water 
circulation  patterns  are  based  largely  on 
the  relative  magnitude  of  either  riverine 
or  tidal  influence  (Ketchum  1951;  Stommel 
1951;  Pritchard  1955,  1967,  1971;  Bowden 
1967)  in  conjunction  with  the  geomorphol- 
ogy  of  the  estuarine  basin  (Schubel  1971). 
Estuaries  with  large  riverine  sources  of 
fresh  water  show  a  well-defined  vertical 
salinity  stratification.  Fresh  water  over- 
rides higher  density  salt  water  and  forms 
an  upper,  freshwater  layer.  The  entrain- 
ment  of  salt  water  from  the  lower  layer 
into  the  upper,  freshwater  layer  through 
eddy  diffusion  results  in  the  mass  move- 
ment of  the  saline  bottom  layer  into  the 
estuarine  basin  (Schubel  1971).  This 
mechanism  creates  the  salt-wedge  type  es- 
tuary as  described  by  Pritchard  (1971).  A 
partially  mixed  estuary  occurs  when  the 
tidal  flow  is  sufficiently  strong  to  pre- 
vent the  river  from  dominating  the  circu- 
lation pattern  (Schubel  1971).  Turbulence 
generated  by  the  movement  of  the  saline 
bottom  layer  results  in  increased  vertical 
mixing  and  moderate  salinity  stratifica- 
tion (Pritchard  1967).  Many  southeastern 
estuaries  with  relatively  large  freshwater 
sources  (e.g.,  Altamaha  and  Ossabaw  Sounds 
in  Georgia  and  Charleston  Harbor  in  South 


Carolina)  fall  into  this  second,  partial- 
ly mixed  classification  at  least  season- 
ally. 

Most  estuaries  in  the  study  area  are 
classified  as  vertically  homogeneous 
(Pritchard  1967,  1971;  Schubel  1971), 
where  tidal  mixing  is  the  dominant  physi- 
cal process.  These  systems  receive  fresh 
water  primarily  though  local  precipitation 
via  tidal  creek  drainage  systems  particu- 
larly during  spring  floods.  Sapelo  Sound, 
Georgia,  and  the  lagoon-marsh  complex  ad- 
jacent to  North  Inlet,  South  Carolina 
(Finley  1975),  are  two  examples  of  verti- 
cally homogeneous  systems.  Lagoon-marsh 
complexes  in  southern  North  Carolina  are 
not  fed  by  major  streams  (Cleary  et  al. 
1979);  therefore,  they  can  also  be  consid- 
ered vertically  homogeneous. 

In  estuaries  not  directly  influenced 
by  large  riverine  sources,  estuarine  cir- 
culation patterns  are  largely  determined 
by  tides,  wind,  and  by  the  water  storage 
capacities  of  lagoon-marsh  complexes 
(Oertel  1975).  The  lagoon-marsh  complexes 
in  Georgia,  for  example,  are  extensive  and 
average  6,5  to  7.5  km  (4.0  to  4.6  mi)  in 
width.  These  areas  store  large  volumes  of 
water  during  high  tide,  and  during  tidal 
drainage  they  contribute  significantly  to 
water  circulation  and  nutrient  exchange 
within  the  estuarine  ecosystem.  These 
large  lagoon  marshes  generally  occupy  a 
major  portion  of  the  watershed  of  the  es- 
tuarine basins,  and  therefore  direct  rain- 
fall is  the  major  source  of  freshwater  to 
these  sytems  (Tom  Williams,  Clemson  Uni- 
versity, Georgetown,  South  Carolina;  pers. 
comm. ) 

Estuarine  Sedimentation 

The  origin  of  sediments  in  estuaries 
and  the  processes  that  affect  their  dis- 
tribution and  deposition  have  been  the 
subject  of  extensive  research  and  scien- 
tific debate  for  over  25  years  (Guilcher 
1967).  Estuarine  sedimentation  patterns 
are  complex  and  influenced  by  tidal  cycle, 
wind  direction  and  duration,  waves,  sea- 
sonal riverine  flooding,  water  storage 
capacity  of  lagoon-marsh  complexes,  and 
sediment  availability.  The  biological 
animal-sediment  interactions  (bioturba- 
tion)  and  chemical  factors  are  also  impor- 
tant (Howard  1975).  These  factors  may  vary 


continuously  in  space,  time,  and  intensity 
(Oertel  1974). 

The  processes  of  sedimentation  can 
best  be  understood  if  the  estuarine  system 
is  divided  into  three  parts,  based  on  gen- 
eralized physical  and  hydrographic  charac- 
teristics: (1)  the  lower  sound  and  inlet 
entrance;  (2)  the  middle  region  of  the  es- 
tuary, including  the  main  rivers  feeding 
the  sound;  and  (3)  smaller  tidal  creeks 
draining  the  marsh  complex.  Naturally  oc- 
curring oyster  reefs  can  be  found  in  each 
of  these  main  zones  in  the  study  area.  The 
three  estuarine  sedimentation  zones  are 
illustrated  in  Figure  2. 

The  area  of  the  lower  sound  and  inlet 
entrance  is  influenced  primarily  by  marine 
processes.  Wind-wave  and  tidal ly  generated 
currents  exert  the  greatest  influence  in 
the  lower  sound,  creating  a  relatively 
high  energy  sedimentary  system.  Where  a 
sufficient  sediment  supply  is  present, 
this  area  is  characterized  by  medium-  to 
coarse-grained  and  commonly  cross-bedded 
sands.  Where  the  lower  sound  is  less  in- 
fluenced by  strong  tidal  currents,  bottom 
sediments  consist  of  a  mixed  medium-  to 
fine-grained  muddy  sand.  These  sands  be- 
come progressively  finer  grained  and  in- 
terbedded,  or  mixed  with  mud  farther  in- 
land. This  is  particularly  common  in  estu- 
aries without  fluvial  sources  of  coarser- 
grained  sediment.  Near  the  mouth  of  the 
sound,  influence  of  the  adjacent  shoreface 
is  indicated  by  the  increasing  grain  size 
and  higher  energy  bedforms,  sand  ripples, 
etc.  (Mayou  and  Howard  1975).  Sandflats 
and  mudflats  frequently  characterize  the 
intertidal  margins  of  the  lower  sound. 

In  estuarine  systems  characterized  by 
large  riverine  freshwater  input,  the  ver- 
tically stratified  lower  sections  of  the 
estuaries  become  natural  traps  for  fine- 
grained sediment  (Schubel  1971).  Fine- 
grained sediment  transported  in  the  upper 
freshwater  layer  frequently  will  settle 
into  the  lower  saline  layer  and  then  be 
carried  back  inland.  Suspended  sediment 
may,  therefore,  be  transported  back  and 
forth  many  times  within  the  lower  section 
of  an  estuary  before  it  is  finally  depos- 
ited (Postma  1967). 

The  middle  region  of  the  estuarine 
sedimentary   environment   includes   the 


sssm 


SCALE 


:/ 


2    km 


ZONE  1 


ZONE  2 


ZONE  3 


0 
0 


0 
0 


MARSH 

LOWER  SOUND  AND  INLET  ENTRANCE 

UPPER  SOUND  AND  MAIN  RIVER  MOUTHS 

MAIN  RIVERINE  FRESH  WATER  SOURCE 

TIDAL  CREEKS 

UPLAND 


Figure  2.  Sedimentary  regions  of  estuary. 


uppermost  portion  of  the  sound  and  the 
main  rivers  feeding  the  sound.  This  zone 
is  influenced  by  both  marine  and  riverine 
processes.  Bottom  sediments  in  the  upper 
reaches  of  the  estuary  are  characteristi- 
cally muddy  sands  or  interbedded  fine- 
grained sands  and  muds.  Farther  inland, 
if  the  river  transports  a  significant 
amount  of  coarse-grained  material,  bottom 
sediments  contain  a  decreasing  percentage 
of  mud  (Dorjes  and  Howard  1975).  Turbid- 
ity levels  are  generally  higher  in  this 
zone  (the  middle  region)  during  all  por- 
tions of  the  tidal  cycle  (Day  1951;  Howard 
et  al.  1975).  These  higher  turbidity  lev- 
els in  part  reflect  the  fact  that  tidal 
currents  (especially  ebb  currents)  attain 
the  highest  velocities  in  the  middle  re- 
gions of  the  estuary  before  they  are 
slowed  in  the  open  sound.  The  importance 
of  turbidity  to  oyster  populations  will  be 
explained  in  Section  2.3. 

The  complex  network  of  smaller  tidal 
creeks  that  drain  extensive  areas  of  salt 
marsh  forms  the  third  division  of  the  es- 
tuarine  sedimentary  environment.  Tidal 
creeks  exhibit  highly  sinuous  channel  pat- 
terns; laterally  migrating  point  bars  on 
the  convex  inner  sides  form  depositional 
banks.  The  concave  outer  banks  of  tidal 
creeks  are  areas  of  net  erosion,  where 
water  currents  attain  their  highest  veloc- 
ities. This  estuary  zone  can  be  classi- 
fied as  a  low-energy,  sedimentary  environ- 
ment. Current  velocities  in  tidal  creeks 
depend  on  the  extent  of  marsh  drainage 
area.  Fine-grained  mud-silts  and,  less 
frequently,  fine  sands  are  the  most  common 
bottom  sediments.  Despite  the  relatively 
fine  grain  size  of  bottom  sediments  in 
tidal  creeks,  the  bottom  includes  all  gra- 
dations, from  extremely  soft  and  organi- 
cally rich  to  hard  mud  and  clay  (Galtsoff 
and  Luce  1930).  The  degree  of  bottom  sed- 
iment consolidation  is  a  function  of  the 
interaction  between  depositional  and  ero- 
sion forces.  Hard  mud  bottoms  form  in 
areas  where  tidal  creeks  erode  into  con- 
solidated marsh  sediment. 

Physico-Chemical  Environment 

The  chemical  environment  of  the  estu- 
arine  ecosystem  is  strongly  influenced  by 
local  hydrography.  The  three  general  divi- 
sions (Figure  2)  of  the  estuarine  system 
used  in  the  discussion  of  sedimentation 


also  provide  a  convenient  framework  for  a 
discussion  of  the  chemical  environment. 

In  the  study  area,  estuaries  are 
characterized  by  highly  variable  lateral 
and  vertical  salinity  gradients.  Within 
any  particular  estuary,  however,  salinity 
trends  are  best  described  by  the  degree  of 
vertical  mixing  taking  place  between  fresh 
and  saline  water  masses.  Three  relatively 
well-defined  salinity  zones  exist  in  the 
majority  of  estuarine  systems:  (1)  a 
stable,  well-mixed,  and  marine-dominated 
lower  zone;  (2)  an  unstable  intermediate 
zone  where  large  changes  in  the  vertical 
salinity  gradient  occur  with  each  tidal 
cycle;  and  (3)  a  stable  upper  region  domi- 
nated by  riverine  fresh  water  influence 
(Howard  et  al.  1975).  The  juxtaposition  of 
these  three  zones  depends  upon  the  inter- 
action and  relative  magnitude  of  riverine 
and  tidal  influences.  In  the  lower  sound 
and  inlet  entrance,  corresponding  to  zone 
1,  mean  salinities  are  high,  ranging  from 
approximately  20  °/oo  (parts  per  thousand) 
to  32  °/oo,  and  the  water  column  tends  to 
remain  well  mixed  throughout  the  tidal 
cycle.  In  estuaries  receiving  large  river- 
ine inflows,  the  well-mixed,  high-salinity 
zone  may  be  displaced  seaward  several  kil- 
ometers (Oertel  1974).  The  upper  sound  in 
the  vicinity  of  the  river  mouths  is  influ- 
enced by  both  marine  and  riverine  process- 
es. Salinity  in  this  region  varies,  rang- 
ing from  5  °/oo  to  over  20  °/oo,  and 
strong  vertical  salinity  gradients  are 
common.  Upstream  of  the  river  mouths 
(zone  3),  salinities  reflect  riverine  in- 
fluence. The  water  column  remains  well 
mixed  at  all  times,  and  salinities  vary 
from  0  °/oo  to  10  °/oo.  Salinity  varia- 
tions in  marsh  tidal  creeks  correspond  to 
that  of  the  tidal  water  mass  flooding  the 
marsh.  As  might  be  expected,  these  values 
are  lowered  significantly  during  periods 
of  local  precipitation  in  the  marsh  and 
resultant  runoff  from  adjacent  uplands. 

In  general,  thermal  mixing  of  estua- 
rine water  masses  occurs  rapidly  (Oertel 
1974).  Hence,  over  most  of  the  lower 
sound,  vertical  temperature  gradients  in 
the  water  column  are  not  pronounced 
and  are  subject  to  daily  fluctuations 
(Oertel  1974).  In  summer,  lower  tempera- 
ture ocean  waters  have  a  cooling  influence 
on  the  estuary.  Water  temperatures  in 
marsh  creeks  are  slightly  higher  during 


ebb  tide,  the  result  of  solar  heating  in 
the  marsh  during  the  tidal  excursion  over 
dark  sediments  with  low  albedo  (reflec- 
tance). 

Dissolved  oxygen  concentrations  gen- 
erally increase  from  the  upper,  riverine- 
dominated  portion  of  the  estuary  to  the 
lower  sound  and  inlet.  This  pattern  close- 
ly parallels  that  of  salinity.  Howard 
et  al.  (1975)  found  that  during  the  summer 
dissolved  oxygen  values  ranged  from  4  to  6 
microliters/liter  for  a  portion  of  the 
Ossabaw  Sound,  Georgia.  These  relatively 
low  values  may  reflect  the  consumption  of 
oxygen  during  the  oxidation  of  organic  de- 
tritus in  suspension  in  the  upper  section 
of  the  estuary.  Frankenberg  and  Wester- 
field  (1968)  reported  that  the  dissolved 
oxygen  levels  in  estuarine  waters  in 
coastal  Georgia  were  extremely  sensitive 
to  sediment  disturbance;  during  the  summer 
the  oxygen  demand  of  a  single  milliliter 
of  disturbed  sediment  could  deplete  the 
dissolved  oxygen  contained  within  986  ml 
of  water. 

Oertel  (1976)  described  large  tempo- 
ral and  spatial  variations  in  turbidity  in 
estuarine  waters.  These  variations  relate 
to  riverine  input,  local  resuspension  of 
bottom  sediments  by  tidal  scour  and  waves, 
and  trapping  of  fine-grained  sediments  in 
the  lower  portions  of  estuaries  (Schubel 
1971).  Turbidity  is  greater  in  the  upper 
reaches  of  the  estuarine  system  than 
either  farther  upstream  in  the  source 
river  or  farther  seaward.  This  zone  has 
been  termed  the  "turbidity  maximum"  by 
Schubel  (1968).  Oertel  (1976)  found  sus- 
pended sediment  concentrations  in  the  up- 
per Wassaw  ranging  from  9.6  mg/liter  to 
585.6  mg/liter,  averaging  46.6  ng/liter. 
Higher  levels  of  turbidity  were  measured 
during  spring  tides.  In  tidal  creeks,  tur- 
bidity increases  significantly  during  per- 
iods of  local  rainfall  when  the  marshes 
are  exposed  at  low  tide  (Settlemyre  and 
Gardner  1975).  Oertel  (1976)  found  a  con- 
sistent inorganic-organic  ratio  in  sus- 
pended sediments  in  the  upper  estuary, 
averaging  70%  inorganic  material  and  30% 
combustible  organic  detritus. 


1.3  ESTUARINE  PRODUCERS 

The  estuary  is  perhaps  best  known 
ecologically  for  its  typically  high  net 


primary  productivity.  The  productivity  of 
estuarine  systems  relative  to  other  eco- 
systems is  illustrated  in  Figure  3.  A  de- 
tailed explanation  of  the  high  annual  net 
productivity  in  southeastern  estuaries  was 
presented  by  Schelske  and  Odum  (1962). 
They  listed  five  essential  factors:  (1) 
tidal  currents;  (2)  abundant  nutrients; 
(3)  rapid  turnover  and  conservation  of 
nutrients;  (4)  three  separate  groups  of 
producers;  and  (5)  year-round  productiv- 
ity. Factors  4  and  5  ensure  that  primary 
production  occurs  throughout  the  year; 
therefore,  energy  and  nutrient  sources  are 
optimally  exploited  and  net  production  is 
maximized.  The  three  primary  producers 
discussed  by  Schelske  and  Odum  (1962)  are 
emergent  macrophytes,  phytoplankton,  and 
benthic  algae.  Another  group  recently  has 
received  scientific  attention:  chemosyn- 
thetic  bacteria  (Howarth  and  Teal  1979). 
Each  group  is  briefly  discussed  below. 


Emergent  Macrophytes 

The  marsh-estuarine  complexes  within 
the  study  area  are  characterized  by  broad 
expanses  of  salt  marshes  dominated  by  two 
marsh  grass  species  which  compose  a  major 
portion  of  the  annual  primary  production 
of  these  systems.  These  are  the  saltmarsh 
cordgrass  (Spartina  alterniflora)  and  the 
black  needlerush  (Juncus  roemerianus). 
Spartina  is  dominant  overall,  and  large 
continuous  stands  of  this  plant  occur  be- 
hind the  barrier  islands  (Pomieroy  and 
Wiegert  1980).  The  annual  production  cycle 
of  these  marshes  peaks  in  late  summer, 
followed  by  a  long  period  of  decay  and 
gradual  export  of  dead  vegetation  (detri- 
tus) into  waterbodies  or  incorporation 
into  peat  deposits  within  the  marsh. 

In  terms  of  overall  primary  produc- 
tion, the  emergent  macrophytes  are  consid- 
ered to  contribute  a  major  portion  of  par- 
ticulate carbon  to  the  estuarine  ecosys- 
tem. Pomeroy  and  Wiegert  (1980)  reported 
that  Spartina  production  makes  up  79%  of 
the  particulate  organic  matter  annually 
produced  by  the  entire  marsh  estuarine 
ecosystem.  Spartina  also  produces  dis- 
solved organic  matter  that  leaches  into 
the  water  column  during  each  tidal  inunda- 
tion. This  leachate  is  thought  to  contri- 
bute significantly  to  the  total  carbon 
budget  of  the  estuarine  ecosystem  (Turner 
1978). 


5        4 
>. 

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O. 
> 

E 
a 


5  to  15            . 

/is 

Mm 

/ 

/ 

1.5  to  5 

z/ 

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X).3  to  1.5 

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1  to  1.5 

0.3 

-jT 

,- 

0.3 

1 

i^    ^ 

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-miWiihUiMa 

t^jg^^^j  -J-  —?-,-; 

^^^^^^ 

Desert  Dry  agriculture  Moist 


Estuarine  Coastal  Open  ocean 


Figure  3.  A  comparison  of  primary  productivity  for  different  kinds  of  ecosystems 
(adapted  from  Teal  and  Teal  1969). 


Estimated  annual  net  production  by 
emergent  nacrophytes  in  the  study  area  has 
been  reported  at  980  gC/mVyr,  when  pro- 
rated for  the  entire  marsh  estuarine  sys- 
tem (Pomeroy  and  Wiegert  1980).  This  num- 
ber does  not  include  leachate,  however, 
making  it  a  conservative  estimate. 

It  should  be  noted  that  some  contro- 
versy exists  regarding  the  paradigm  that 
emergent  macrophytes  are  the  primary 
source  of  particulate  carbon  in  coastal 
ecosystems  in  the  South  Atlantic  Bight. 
Arguments  have  been  advanced  (Haines  1976, 
1977)  that  perhaps  production  by  phyto- 
plankton  is  more  significant  than  that  of 
emergent  macrophytes.  Counterarguments 
and  hypotheses  by  Peterson  et  al.  (1980) 
provide  alternative  interpretations  of 
Haines'  (1976,  1977)  studies.  Until  more 
definitive  research  resolves  this  contro- 
versy, the  paradigm  is  still  viable. 

Phytoplankton 

The  major  phytoplanktonic  producers 
in  a  "typical"  estuary  in  Georgia  were 
listed  (Pomeroy  and  Wiegert  1980)  as  pela- 
gic diatoms  (and  occasional  benthic  pen- 
nate  diatoms  swept  up  from  the  bottom  into 
the  water  column),  dinoflagellates  and 
green  flagellates.  Their  combined  produc- 
tion rate  was  estimated  as  125  gC/m-^/yr 
(Pomeroy  and  Wiegert  1980)  when  prorated 
for  the  entire  marsh  water  surface  (water 
comprises  about  one-third  of  this  area). 

Benthic  Algae  and  Epiphytes 

The  principal  primary  producers  in 
the  marsh  sediments  are  benthic  pennate 
diatoms.  These  organisms  migrate  verti- 
cally in  the  sediment,  depending  on  the 
tidal  stage  and  light  conditions.  They 
are  often  clearly  visible  on  exposed  creek 
banks  as  a  golden  sheen  on  the  brown  mud. 
Pomeroy  and  Wiegert  (1980)  reported  that 
benthic  algae  account  for  about  11%  of 
total  net  primary  production  in  a  marsh 
estuarine  system  in  the  study  area,  or 
134  gC/m^/yr,  prorated  for  total  estuarine 
area. 

Another  group  of  primary  producers  in 
the  marsh-estuarine  ecosystem  is  the  com- 
munity of  epiphytic  algae  that  inhabits 
the  culms  or  stalks  of  marsh  grass.  This 
diverse  community  is  not  readily  apparent 


but  attracts  grazers,  especially  the  gas- 
tropod Littorina  irrorata.  In  terms  of 
production  rates,  the  epiphytic  community 
is  relatively  unimportant  compared  to 
Spartina  alterniflora  (Pomeroy  and  Wiegert 
1980). 


Mixotrophic 


Chemosynthetic  Bacteria: 
and  Photolithotrophic 


Two  groups  of  anaerobic  microbial 
organisms  inhabiting  sediments  within  the 
salt  marsh-estuarine  system  are  the  mixo- 
trophic sulfate-reducing  bacteria  and  the 
photolithotrophic  bacteria,  which  have  re- 
cently received  scientific  attention.  The 
abundance  of  sulfate  in  salt  marsh  sedi- 
ments makes  this  ion  the  obvious  substi- 
tute for  oxygen  as  the  electron  acceptor 
in  the  anaerobic  respiration  of  many  mi- 
crobes. These  organisms  use  dissolved 
organic  matter  as  an  energy  source.  Sul- 
fate reduction  is  now  recognized  as  an 
extremely  important  process  in  the  salt 
marsh  estuary  (Fenchel  and  Riedl  1970; 
Howarth  and  Teal  1979).  As  sulfate  is 
reduced  (primarily  by  a  bacterial  group 
known  as  Desulfovibrio),  the  resulting 
sulfide  diffuses  upward.  Its  reducing 
"power"  is  subsequently  used  along  with 
light  as  the  energy  source  to  fix  atmos- 
pheric carbon  dioxide  by  anaerobic  bacte- 
ria. The  release  of  the  resulting  organic 
matter  from  salt  marsh  sediments  is  prob- 
ably augmented  by  tidal  flushing  and  may 
be  quite  significant  along  creek  banks. 

A  major  implication  of  this  overall 
process  is  that  the  initial  carbon  source 
for  the  sulfate  reducers  is  leachate  from 
the  roots  of  macrophytes.  Thus,  the  wet- 
land macrophyte  production  is  ultimately 
the  source  of  this  (unknown)  amount  of 
extra  organic  carbon  that  goes  into  the 
ecosystem. 

1.4  ESTUARINE  CONSUMERS 

The  following  description  of  the  con- 
sumers of  the  salt  marsh  estuarine  ecosys- 
tem in  the  study  area  is  necessarily  in- 
complete because  it  describes  only  those 
groups  of  organisms  that  are  considered 
dominant,  functionally  significant,  or 
that  directly  affect  the  oyster  reef  com- 
munity. These  are  bacteria,  benthic  in- 
fauna,  zooplankton,  nekton,  and  terres- 
trial consumers. 


10 


Bacteria 

Two  basic  principles  relate  to  the 
consumption  of  the  net  energy  produced 
annually  in  the  marsh-estuarine  ecosystem. 
The  first  principle  is  that  most  energy 
produced  by  the  dominant  primary  producer 
in  the  system  (Spartina  alterniflora)  is 
not  consumed  directly  by  grazers.  Instead, 
at  least  90%  (perhaps  95%)  either  leaches 
into  the  water  column  from  living  and  dead 
plants  as  dissolved  organic  matter,  or 
through  various  processes  enters  the  sys- 
tem as  detritus.  Both  forms  of  this  or- 
ganic matter  are  then  attacked  by  micro- 
scopic decomposers  or  ingested  directly  by 
macroconsumers. 

The  second  major  principle  of  energy 
consumption  in  the  salt  marsh  is  that  the 
decomposer  community  (aerobic  and  anaer- 
obic) is  large,  diverse,  and  extremely 
active,  consuming  about  50%  of  the  total 
energy  flowing  through  the  ecosystem,  as 
shown  in  Figure  4  (Teal  1962).  The  decom- 
poser community  of  the  estuarine  ecosystem 
can  be  divided  conveniently  into  two 
groups:  (1)  aerobic  heterotrophs  (bacteria 
and  fungi)  which  utilize  inorganic  matter 
in  standing  dead  grass  stalks,  the  water 
column,  and  aerobic  sediments;  and  (2)  an- 
aerobic bacteria  in  anoxic  (oxygen-poor) 
sediments.  The  activity  of  the  aerobic 
group  enhances  the  nutritive  quality  of 
both  particulate  and  dissolved  organic 
matter  for  the  larger  consumers.  Particu- 
late organic  matter  is  colonized  by  the 
aerobic  heterotrophs  as  it  is  gradually 
fragmented  into  detritus.  Its  nutritional 
value  is  enhanced  by  increasing  the  rela- 
tive nitrogen  composition  of  the  particu- 
late organic  carbon  (POC),  as  shown  by 
Odum  and  de  la  Cruz  (1967).  This  can  be 
symbolized  as  follows:  POC  +  O2  +  NH4+  -^ 
bacterial  POC-N  +  CO2.  Dissolved  organic 
carbon  (plant  leachates,  etc.)  can  be  as- 
similated by  micro-heterotrophs  and  also 
converted  into  POC-N.  Some  aerobic  bac- 
teria are  also  critical  elements  of  the 
nitrogen  cycle,  as  discussed  below. 

Anaerobic  decomposers  function  in  a 
variety  of  roles  in  the  salt  marsh  ecosys- 
tem. They  are  essential  to  the  geochemical 
cycles  that  release  plant  nutrients  in  a 
continuous  stream  to  the  primary  produc- 
ers. The  nitrogen  cycle  is  especially  im- 
portant because  evidence  to  date  indicates 


that  nitrogen  is  the  limiting  nutrient  in 
the  salt  marsh  (Valiela  and  Teal  1979). 
Since  the  decomposition  of  cellulose  is 
nitrogen-limited  (Pomeroy  and  Wiegert 
1980),  the  decomposition  of  the  large 
standing  stock  of  organic  matter  in  the 
system  results  in  a  competition  for  nitro- 
gen between  decomposers  and  primary  pro- 
ducers. 

Four  groups  of  bacteria  are  involved 
in  the  nitrogen  cycle.  One  group  in  the 
sediments  (nitrogen  fixers)  converts  atmo- 
spheric nitrogen  to  nitrate  and  nitrite 
(N2  -*  NC3 -+  NO2-);  another  group  (the  de- 
nitrifiers)  reduces  nitrates  and  nitrites 
to  atmospheric  nitrogen.  A  third  group 
(ammonifiers)  converts  dead  tissue  into 
ammonia.  A  fourth  group  (nitrifiers)  oc- 
cupies the  thin,  oxidized  layer  around 
Spartina  roots  and  converts  ammonia  from 
anaerobic  sediments  into  nitrates  directly 
usable  by  the  plant. 

The  anaerobic  zone  in  salt  marsh- 
estuarine  sediments  extends  upward  almost 
to  the  sediment  surface  because  of  the 
enormous  oxygen-depleting  capacity  (chemi- 
cal oxygen  demand)  of  these  sediments.  The 
metabolic  activity  of  anaerobic  bacteria 
is  responsible  for  this  oxygen  demand. 

Benthic  Infauna 

Other  organisms  in  estuarine  sedi- 
ments include  metazoan  animals  larger  than 
bacteria  but  so  small  that  studying  them 
and  documenting  their  functions  are  diffi- 
cult. This  group  is  called  the  meiofauna, 
and  although  it  contains  various  phyloge- 
netic  groups  and  trophic  positions,  its 
overall  role  apparently  is  that  of  a  tro- 
phic intermediary  between  bacteria  and 
macroconsumers.  Nematodes  and  other  meio- 
fauna appear  to  be  major  processors  of 
bacterial  tissue,  and  they  are  an  impor- 
tant component  of  the  food  of  many  so- 
called  deposit  feeders  (Sikora  1977;  Bell 
and  Coull  1979).  Intertidal  biomass  of 
nematodes  in  creek  banks  in  the  study  area 
has  been  measured  at  6.4  g  ash  free  dry 
weight  (afdw)/  m^  (Sikora  et  al.  1977). 

Larger  benthic  organisms  (macroben- 
thos)  in  salt  marsh  estuaries  are  usually 
divided  into  epibenthos  and  macro-infauna. 
Because  oysters  are  epibenthos,  we  will 
omit  further  discussion  of  epibenthos 


11 


RECYCLING 


Kcal/m2/vr 


Figure  4.  Model  of  energy  flow  in  the  Georgia  salt  marshes  (adapted  from  Teal 
1962). 


12 


until  later.  Macro-infauna  are  often  di- 
vided into  two  functional  groups,  deposit 
feeders  and  suspension  feeders.  Theories 
have  been  developed  to  explain  why  often 
the  two  groups  appear  to  be  mutually  ex- 
clusive in  local  areas  (Rhoads  and  Young 
1970;  Levinton  1972). 

Suspension  feeders  include  clams  and 
sone  tube-dwelling  polychaetes  (worms). 
Deposit  feeders  are  often  more  motile,  and 
some  workers  even  include  in  the  group 
those  quasidemersal  nektonic  organisms 
that  burrow  into  the  bottom  to  feed,  e.g., 
grass  shrimp.  Kany  polychaetes  and  gas- 
tropods (snails)  are  also  deposit  feeders. 
Another  major  category  of  the  estuarine 
macrobenthic  community  is  the  predators, 
including  some  gastropods,  turbellarians 
(flatworms),  nemertines  (round  worms),  and 
echinoderms  (starfish). 

Deposit  feeders  and  demersal  nekton 
are  important  in  reworking  the  sediments 
by  burrowing  and  plowing  (bioturbation). 
This  activity  redistributes  organic  matter 
and  other  nutrients  to  the  water  column 
and  introduces  oxygen  into  the  sediments. 
For  example,  one  mullet  can  rework  45  m^ 
of  bottom  area  per  year  (Pomeroy  and 
Wiegert  198G). 

Conversely,  suspension  feeders  (in- 
cluding oysters)  filter  particles  from  the 
water  column  and  then  deposit  organic  miat- 
ter  in  the  form  of  feces  on  the  sediment 
where  it  becomes  available  to  the  decom- 
posers. Krauter  (1976)  estimated  that  salt 
marsh  macrobenthic  organisms  (in  the  marsh 
proper)  deposit  1,709  g  dry  wt/m-^/  yr, 
which  is  455  g  of  organic  matter.  He  also 
calculated  that  53%  of  the  marsh's  annual 
primary  production  could  be  processed 
through  the  feeding  mechanisms  of  these 
organisms. 

Zooplankton 

Estuarine  animals  living  suspended  in 
the  water  column  generally  are  classified 
as  zooplankton  if  they  are  either  so  small 
or  such  weak  swimmers  that  they  are  trans- 
ported passively  by  water  currents.  The 
mobility  of  zooplankton  typically  is  lim- 
ited to  vertical  migrations  in  the  water 
column;  for  example,  a  daily  migration 
from  the  surface  to  bottom  waters  and  back 
again  is  a  commonly  observed  pattern  among 


many  forms.  By  altering  their  vertical 
elevation  in  the  water  column,  zooplank- 
ters  can  use  variations  in  food  supply  and 
use  water  movements  in  estuaries  for  dis- 
persion by  "riding"  parcels  of  water  mas- 
ses as  the  latter  traverse  an  estuary. 
For  example,  some  species  of  zooplankton 
follow  the  salinity  wedge  on  the  bottom  as 
the  wedge  progresses  landward,  or  the  sur- 
face layers  of  freshwater  as  they  move 
seaward. 

Zooplankton  often  are  divided  into 
holoplankton  and  meroplankton.  Holoplank- 
ton  spend  their  entire  life  cycles  in  the 
water  column  while  meroplankton  spend  only 
their  larval  stages  above  the  bottom. 
Holoplankton  include  microzooplankton, 
such  as  copepods  and  rotifers;  and  macro- 
zooplankton,  like  euphausiids,  ctenophora 
and  other  jellyfish.  Meroplankton  include 
larval  finfish  and  decapods;  and  a  large 
contingent  of  the  larvae  of  many  macroben- 
thic animals,  including  many  polychaetes, 
barnacles,  clars  and  mussels,  and,  of 
course,  oyster  larvae. 

The  functional  importance  of  zoo- 
plankton in  the  estuarine  ecosystem  is 
partly  expressed  by  a  high  turnover  rate 
of  planktonic  species,  by  large  popula- 
tions, and  by  the  very  small  average  size 
of  individual  members.  These  three  fac- 
tors ensure  that  zooplankton  process  a 
large  amount  of  the  organic  materials 
available  in  some  estuaries,  much  of  which 
represent  the  conversion  of  phytoplankton 
into  the  tissue  of  higher  consumers.  The 
zooplankton  community  as  a  group  largely 
depends  on  phytoplankton  as  a  carbon 
source,  and  thus  tends  to  be  more  impor- 
tant (abundant)  in  estuaries  dominated  by 
phytoplankton,  rather  than  in  those  where 
macrophyte  production  is  of  primary  impor- 
tance. The  carbon  pathway  from  phytoplank- 
ton to  zooplankton  to  higher  consumers  is 
a  significant  trophic  link  in  all  estua- 
ries, however,  including  those  in  the 
study  area. 

Although  the  oyster  larvae  are  mem- 
bers of  the  zooplankton  community,  they 
are  extremely  vulnerable  to  predation  by 
plankton  feeders,  including  members  of  the 
macroplankton  group,  such  as  ctenophores. 
Some  years  ago  in  the  New  Jersey  oyster 
grounds,  oyster  spatfall  (larval  recruit- 
ment) was  reduced  during  years  of  large 


13 


ctenophore  populations  (Nelson  1925).  He- 
roplankton  usually  compose  a  greater  por- 
tion of  the  zooplankton  community  during 
summertime  when  many  clams,  mussels,  oys- 
ters, barnacles,  crabs,  polychaetes,  and 
other  benthic  organisms  are  spawning.  This 
input  of  living  protein  from  the  bottom 
into  the  water  significantly  increases  the 
food  supply  of  filter-feeding  animals, 
both  nektonic  and  epibenthic  suspension 
feeders.  Many  of  the  latter  probably  can- 
nibalize larvae  of  their  own  kind. 

Nekton 

The  active  swimmers  in  the  estuary 
are  divided  into  pelagic  and  demersal  nek- 
ton. The  pelagic  nekton  feeds  in  the 
water  column,  either  on  phytoplankton  and 
detritus,  on  zooplankton  (including  oyster 
larvae),  or  on  other  nektonic  forms.  The 
bottom  feeders  or  demersal  nekton  feed  on 
adult  benthos,  including  oysters  and  their 
associates.   Darnell  (1961)  reported  the 


feeding  habits  of  some  typical  estuarine 
nekton  (Figure  5). 

Terrestrial  Consumers 

The  other  major  group  of  consumers 
characteristic  of  the  marsh-estuarine  sys- 
tem is  the  large,  diverse  collection  of 
"terrestrial"  or  land-based  consumers. 
This  group  comprises  insects  and  other 
small  arthropods,  including  some  fiddler 
crabs;  pulmonate  gastropods,  especially 
Littorina  irrorata;  birds;  reptiles  (even 
alligators);  and  mammals,  such  as  the  rice 
rat,  mink,  otter,  and  raccoon.  The  spe- 
cific members  of  this  group  that  directly 
impinge  on  the  oyster  reef  community  are 
discussed  in  Chapter  3.  The  total  biomass 
of  terrestrial  consumers,  including  the 
active  primary  consumers  (plant  hoppers 
and  grasshoppers)  that  graze  Spartina  di- 
rectly, was  estimated  at  1  g  C/m2  (Pomeroy 
and  Wiegert  1980). 


-  ••----  M,  *^ 


--'-^.T-v^-*;'' 


h^'W 


.^^•-"W^. 


^  %==■ 


^c;  -- 


i-L^   -^  »:^  ** 


'2: 


•fM- 


A  view  of  the  estuarine  environment  in  which  oyster  reefs  occur  in  coastal  South 
Carolina.  Photo  by  South  Carolina  Wildlife  and  Marine  Resources  Department. 


14 


FOOD 
CATEGORIES 


TROPHIC  SPECTRUM  OF  ESTUARIME  COMMUNITY 


CONSUMER  SPECIES  (NEKTON! 


I  <  £ 


li| 


5o  £5  ::. 

_)    UJ     uj    CL     "J    - 

m  o:   o.  (A  >  < 


2    o  <  5  o 
<    _i  I  <  < 

w     u.   «   o    - 


— ■  -■■■■■ 


MACROBOTTOM 
ANIMALS 


MICROBOTTOM 
ANIMALS 


-    ^■-  -I 

"il"+|-|H' 


IMIHII 
ll-l-hl  — 

-I  -  -  -  - 


ZOOPLANKTON 


PHYTOPLANKTON 


M 


VASCULAR  PLANT 
MATERIAL 


l-l 


ORGANIC  DETRITUS 
AND  UNDETERMINED 
ORGANIC  MATERIAL 


Mlim- 


— -■ — " 


Figure  5.  Trophic  spectrum  of  an  estuarine  community  (Lake  Pontchartrain 
Estuary,  Louisiana)  (adapted  from  Odum  1971  after  Darnell  1961). 


15 


This  photograph  depicts  individual  reef  oysters  occurring  at  the  mean 
low  water  mark.  Note  the  vertical  orientation.  Photo  by  Rhett  Talbert, 
University  of  South  Carolina. 


16 


CHAPTER  2 
FUNCTIONAL  OYSTER  BIOLOGY  AND  AUTECOLOGY 


This  chapter  summarizes  the  salient 
information  on  oyster  biology,  especially 
that  relating  to  the  functional  position 
of  the  oyster  in  the  estuarine  ecosystem. 
Each  aspect  of  oyster  biology  discussed 
here  is  presented  as  an  aid  to  understand- 
ing this  functional  role. 

A  number  of  excellent  treatises  on 
oyster  biology,  including  the  monograph  on 
the  American  oyster  by  Galtsoff  (1964), 
preclude  the  necessity  for  another  exten- 
sive treatment.  Readers  interested  in 
more  detail  on  subjects  discussed  here 
should  refer  to  Galtsoff  (1964)  or  other 
references  cited  in  the  chapter. 


2.1  EVOLUTION  AND  TAXONOMY 

The  oyster  evolved  long  ago  from  an 
ancestral  mollusk  into  a  highly  reorga- 
nized and  in  some  ways  simplified  form. 
The  major  evolutionary  steps  involved  are 
summarized  below  as  they  were  recon- 
structed by  Yonge  (1960): 

(1)  lateral  compression  of  the  body 

(2)  extension  of  the  mantle  to  the 
margins  of  the  shell 

(3)  division  of  the  shell  into 
halves  (valves)  separated  by 
a  noncalcareous  ligament 

(4)  fusion  of  pallial  muscles  to 
form  paired  adductor  muscles 

(5)  reduction  of  head  and  develop- 
ment of  labial  palps 

(6)  development  of  cilia  on  (paired) 
gills,  and  development  of  a  gill 
feeding  function  in  addition  to 
their  respiratory  role 

(7)  probable  reduction  of  metabolic 
requirements  over  that  of  ances- 
tral forms 

(8)  loss  of  foot  and  byssus  in  the 
adult  life  stage 

(9)  loss  of  anterior  adductor  muscle 

(10)  rounding  of  the  body 

(11)  development  of  a  horizontal 
orientation  with  the  left  valve 
down  in  the  adult  form 


The  currently  accepted  formal  classi- 
fication of  the  American  oyster  is  pre- 
sented below,  accompanied  by  the  major 
morphological  and  ecological  characteris- 
tics that  apply  to  each  category.  There 
is  currently  some  controversy  about  the 
classification  of  some  oyster  genera  and 
species,  and  descriptors  are  not  standard- 
ized, so  that  different  workers  have  used 
shell  morphology,  geographical  range, 
reproductive  behavior,  and  larval  shell 
morphology  to  classify  oysters.  New  tools 
of  biochemical  genetics  offer  hope  of 
resolving  some  of  the  controversial  ques- 
tions in  oyster  taxonomy. 

For  this  report,  the  American  oyster 
will  be  classified  according  to  Abbott 
(1974)  as  follows: 

Class  Bivalvia  (Pelecypoda) 
Order  Pterioida 
Family  Ostreidae 
Genus  Crassostrea 
Species  virginica 


Each  taxon  will  now  be  described 
briefly. 

Class  Bivalvia 

This  class  includes  clams,  mussels, 
scallops  and  oysters.  Some  general  char- 
acteristics are  (1)  a  shell  divided  into 
two  valves  hinged  dorsal ly  by  a  ligament 
of  conchiolin  and  connected  by  one  or  two 
adductor  muscles;  (2)  a  shell  usually 
consisting  of  three  layers:  an  outer 
organic  horny  matrix  (conchiolin),  a  mid- 
dle prismatic  layer,  and  an  inner  nacreous 
or  pearly  layer;  (3)  a  laterally  com- 
pressed body;  (4)  either  a  small  head  or 
none  at  all;  (5)  a  wedge-  or  hatchet- 
shaped  foot  (if  present),  (6)  a  mantle 
extending  to  the  margins  of  the  shell  and 
forming  a  large  mantle  cavity,  containing 
ciliated  gills  (ctenidia)  that  function  in 
feeding,  pumping,  and  respiration;  (7)  a 
crystalline  style  that  releases  amylase 
for  starch  digestion. 


17 


Order  Pterioida 

The  order  Pterioida  is  one  of  four 
orders  of  bivalves,  each  distinguished  by 
the  structure  of  its  gills,  and  includes 
pearly  and  winged  oysters,  scallops,  and 
the  true  oysters.  These  orders  are  char- 
acterized by  paired  gills  that  are  greatly 
lengthened  (compared  to  the  ancestral 
type)  and  folded  back  on  themselves  to 
form  four  demibranchs  interconnected  by 
tufts  of  cilia.  The  mantles  in  these  tnol- 
lusks  have  taken  over  the  sensory  function 
of  the  molluskan  head,  including  some 
visual  or  light-sensing  capacity. 

Family  Ostreidae 

This  family  includes  a  large  number 
of  edible  and  nonedible  oysters.  They  are 
generally  restricted  to  shallow  coastal 
waters  between  44°  S  and  64°  N  (Galtsoff 
1964).  Oysters  have  unequal  valves  with 
no  hinge  teeth  except  in  the  prodissoconch 
or  larval  shell.  In  all  but  their  larval 
stages,  oysters  have  completely  lost  their 
byssus  (attachment  filaments)  and  foot  and 
have  retained  only  the  posterior  adductor 
muscle,  which  is  kidney-  or  crescent- 
shaped. 

Genus  Crassostrea 

The  oysters  included  in  this  genus 
are  characterized  by  extremely  variably 
shaped  (ecomorphic)  shells,  depending  on 
the  substrate  and  current  regimes  of  the 
habitat  in  which  the  oysters  grow.  Mem- 
bers of  the  genus  Crassostrea  are  anatom- 
ically distinct  from  their  counterparts  in 
the  genus  Ostrea,  in  that  Crassostrea  are 
somewhat  larger  at  maturity,  with  a  deeper 
cupped  left  valve  on  which  they  ordinarily 
rest.  ^  They  also  possess  a  distinctive 
asymmetrical  space  between  the  right 
mantle  and  gill  plates,  known  as  the 
promyal  chamber.  The  promyal  chamber  is 
important  because  it  probably  permits 
greater  pumping  rates,  an  advantage  in 
silt-laden  water  (Ahmed  1975).  This  cham- 
ber also  functions  in  the  reproductive 
success  of  this  genus.  Eggs  of  Crassos- 
trea species  are  small  ( -x.  40  y)  and  are 
released  directly  into  the  water,  rather 

Note,  however,  that  reef  oysters  are  usu- 
ally oriented  vertically  with  both  left 
and  right  valves  pointed  upward. 


than  being  incubated  within  the  mantle 
cavity,  as  those  of  the  genus  Ostrea.  The 
promyal  chamber  allows  for  higher  release 
velocity  for  eggs  and  is  important  for  egg 
dispersal. 

The  production  of  free-living  plank- 
tonic  larvae  is  critical  to  members  of  the 
genus  Crassostrea  because  it  promotes  ge- 
netic exchange  over  wide  areas.  Oyster 
larvae  have  been  documented  to  travel  at 
least  50  km  (30  mi).  Quayle  (1969)  and 
Stenzel  (1971)  estimated  that  they  could 
disperse  up  to  1,300  km  (800  mi). 

Probably  the  most  important  charac- 
teristic of  the  genus  Crassostrea,  which 
has  permitted  almost  worldwide  distribu- 
tion, is  its  ability  to  tolerate  wide 
ranges  of  salinity,  turbidity,  tempera- 
ture, and  oxygen  tension. 

The  morphology  has  changed  little 
since  the  oyster  arose  during  the  Triassic 
period  about  190  million  years  ago.  The 
genus  Crassostrea  arose  during  the  Creta- 
ceous period  (T35  million  years  ago) 
(Stenzel  1971).  Representatives  of  this 
genus  characteristically  occur  in  turbid 
estuaries  with  soft  ■  bottoms  in  the  Indo 
Pacific  area,  Eurasia,  Africa,  and  North 
and  South  America. 

Crassostrea  virqinica 

The  Eastern  or  American  oyster  (£. 
virginica)  is  the  species  that  builds  the 
intertidal  reefs  focused  on  in  this  re- 
port. This  species  is  distributed  along 
the  entire  east  coast  of  North  America, 
from  the  Gulf  of  St.  Lawrence  in  Canada  to 
Key  Biscayne,  Florida,  to  the  Yucatan  and 
the  West  Indies;  and  it  has  been  reported 
even  in  Brazil  (Gunter  1951).  Figure  6 
(from  Ahmed  1975)  illustrates  this  8,050- 
km  (5,000-mi)  range.  Crassotrea  virqinica 
prevails  over  this  immense  range  because 
of  its  tolerance  to  low  temperature  (Sten- 
zel 1971). 

Physiological,  ecological,  and  bio- 
chemical data  indicate  that  £.  virqinica 
has  several  distinct  physiological  races 
(Loosanoff  and  Nomejko  1955;  Menzel  1955; 
Hillman  1964;  Li  and  Fleming  1967;  Ahmed 
1975).  On  the  other  hand,  Buroker  et  al. 
(1979)  concluded  that  significant  genetic 
distinctions  occur  only  between  popula- 
tions of  C.   virqinica  from  Nova  Scotia  and 


18 


140 


120 


60-' 


40 


20 


0° 


l?S^ 


PACIFIC  OCEAN 


40 


Figure  6.  The  distribution  of  Crassostrea  virginica  is  indicated  by  the  shaded 
line.  Note  the  distribution  of  the  other  major  North  American  species,  Ostrea 
Turida,  shown  by  the  solid  line  on  the  west  coast  (adapted  from  Ahmed  1975) . 


19 


west  Florida.  These  researchers  concluded 
that  the  two  populations  were  only  82% 
genetically  similar,  approximately  the 
level  of  similarity  between  £.  virginica 
and  £.  rhizophorae.  The  latter  two  spe- 
cies are  genetically  close  enough  to  have 
been  successfully  hybridized  in  the  labor- 
atory (Menzel  1968).  Stauber  (1950)  pos- 
tulated that  C.  virginica  was  discontin- 
uously  distributed  on  the  east  coast  dur- 
ing prehistoric  times,  and  that  speciation 
was  occurring  before  oyster  culture  activ- 
ities by  man  removed  the  barriers  to  gene 
transfer. 

N.  E.  Buroker  (University  of  Maryland 
Marine  Products  Laboratory,  Crisfield, 
Maryland;  pers.  comm. )  indicates  a  single, 
large  panmictic  (genetically  homogeneous) 
population  exists  between  Cape  Cod,  Mary- 
land, and  Corpus  Christi,  Texas,  with  96% 
to  99%  genetic  similarity.  Levinton  (1973) 
reported  that  six  species  of  bivalve  mol- 
lusks  (not  including  oysters)  showed  an 
increase  in  genetic  variability  with  an 
increase  in  intertidal  elevation,  corre- 
sponding to  increasing  environmental  vari- 
ability. This  would  be  an  interesting 
parameter  to  study  in  intertidal  reef  oys- 
ter populations. 

Without  further  consideration  of  the 
evolutionary  origins  of  the  oyster,  we 
will  concentrate  on  the  functional  (eco- 
logical) classification  of  C.  virginica 
between  Cape  Fear,  North  Carolina,  and 
Cape  Canaveral,  Florida.  From  this  point 
on,  the  generic  term  "oyster"  will  mean  £. 
virginica,  and  "oyster  reef"  will  refer  to 
oyster  reefs  in  the  study  area  unless  spe- 
cified otherwise. 

The  general  anatomy  of  the  adult  oys- 
ter appears  in  Figures  7  and  8  (adapted 
from  Galtsoff  1964).  Note  the  insert  dia- 
gram in  Figure  7  showing  the  proper  way  to 
describe  oyster  size. 


2.2  OYSTER  REPRODUCTION  AND  DEVELOPMENT 


oyster  is  dioecious  (with  sepa- 
0.  but  once  a  year  some  members 


The 
rate  sexes] 

of  a  given  local  population  change  their 
gender  from  male  to  female  (protandry)  or 
female  to  male  (protogyny).  This  sexual 
lability  is  possible  partly  because  of  the 
simplicity  of  the  oyster  reproductive 


system,  which  lacks  ducts,  glands,  or  sec- 
ondary sexual  structures  (Yonge  1960). 
Oysters  develop  functional  gonads  at  a 
young  age  (2  to  3  months)  and  small  size 
(less  than  1  cm  in  height).  Usually  they 
tend  to  develop  as  males  during  their 
first  season,  with  subsequent  protandric 
change  (to  females)  in  following  seasons 
(Menzel  1955).  A  small  percentage  of  any 
given  population  (  <1%)  functions  as  true 
hermaphrodites  (Kennedy  and  Battle  1963), 
and  this  pattern  seems  to  hold  for  other 
species  in  the  genus  Crassostrea  (Asif 
1979). 

Some  preliminary  evidence  indicates 
that  populations  of  oysters  under  certain 
kinds  of  stress  tend  to  develop  a  higher 
proportion  of  males  than  females,  but  this 
remains  to  be  conclusively  demonstrated 
(Amemiya  1936;  Loosanoff  and  Nomejko  1955; 
Kennedy  and  Battle  1963;  Bahr  and  Hillman 
1964).  It  is  interesting  to  speculate, 
however,  that  the  stress  encountered  in 
the  higher  portions  of  the  oysters'  verti- 
cal range  in  the  intertidal  zone  (the 
upper  reef  zone)  could  produce  androgenous 
(predominantly  male)  colonies  that  would 
contribute  little  to  the  reproductive  suc- 
cess of  the  population. 

After  oyster  gonads  reach  maturity  in 
a  local  population,  a  temperature  (or  sa- 
linity) shock  triggers  the  emission  of 
sperm  from  one  or  more  males.  The  temper- 
ature at  which  oyster  populations  in  dif- 
ferent regions  begin  to  spawn  has  been 
used  in  the  past  to  distinguish  physiolog- 
ical races.  Atlantic  coast  and  gulf  coast 
oysters  have  thus  been  separated  into 
17°  C,  20°  C,  and  25°  C  spawners  (Yonge 
1960).  Reef  oysters  subject  to  very  high 
summer  temperatures  are  probably  members 
of  the  last  group. 

The  emission  of  sperm  from  male  oys- 
ters occurs  via  the  exhalent  chamber  of 
the  mantle.  A  chemical  constituent  of  the 
sperm  (a  protein  pheromone)  stimulates  the 
females  in  the  area  to  release  eggs,  and  a 
spawning  chain  reaction  can  sweep  dramati- 
cally over  a  dense  population,  turning  the 
water  white.  Females  expel  eggs  from  the 
inhalent  chamber  rather  than  through  the 
exhalent  chamber.  This  process  involves  a 
preparatory  contraction  in  portions  of  the 
mantle  margins  to  reduce  the  size  of  the 
exhalent  opening.  Eggs  then  pass  through 


20 


MOUTH 


LABIAL  PALPS 
CEREBRAL  GANGLION 


LEFT  MANTLE 


PERICARDIUM 

RIGHT  MANTLE 

GILLS 

ADDUCTOR  MUSCLE 

SHELL 
TENTACLES 


FUSION  OF  TWO  MANTLE 
LOBES  AND  GILLS 


X 


_L 


_L 


CENTIMETERS 


Figure  7.  Anatomy  of  the  ovster  (Crassostrea  virginica)  and  diagram  showing  the 
correct  method  of  measuring  the  height,  length,  and  width  of  oyster  shells  (from 
Galtsoff  1964). 


21 


CONNECTIVE  TISSUE 

INTESTINE 

DIGESTIV 
DIVERTICULA 

GONAD 


KIDNEY 

EPIBRANCHIAL 
CHAMBER 

LATERAL 
AFFERENTV 

BRANC 
EFFERENT  VEIN 


GILL  ROD 


LEFT  MANTLE 


GILLS 


WATER  TUBE 
OF  THE  GILLS 


GONAD 

INTESTINE 
BLOOD  SINUS 

STOMACH 


DIGESTIVE 
DIVERTICULA 

KIDNEY 

PROMYAL 
CHAMBER 


COMMON 
AFFERENT  VEIN 


i<yj  AFFERENT  VEIN 


BRANCHIAL 
EFFERENT  VEIN 

GILL  MUSCLES 
RIGHT  MANTLE 


WATER  TUBE 
OF  THE  GILLS 

GILLS 


EDGE  OF 
LEFT  MANTLE 


EDGE  OF 
RIGHT  MANTLE 


MANTLE  CAVITY 


0  0.5 

I 1 1 

CENTIMETERS 


Figure  8.  Transverse  section  of  the  dorsal  part  of  an  adult  Crassostrea  virginica 
(adapted  from  Galtsoff  1964). 


22 


the  gill  filaments  (against  the  normal 
feeding  current)  and  accumulate  near  the 
inhalent  chamber.  Rapid  and  repeated  con- 
tractions of  the  adductor  muscle  then 
forcefully  eject  the  eggs  a  considerable 
distance.  The  latter  mechanism  is  also 
used  to  expel  unwanted  particulate  mate- 
rial (pseudofeces)  from  the  mantle  cavity. 

Fertilization  occurs  in  the  water 
column  via  chance  encounters  of  eggs  and 
sperm,  and  larval  development  ensues. 
Thus  begins  the  free  living  phase  of  oys- 
ter larvae.  These  larvae  function  as  zoo- 
plankters  (meroplankton)  in  the  water  col- 
umn, and  probably  are  significant  as  a 
food  source  for  planktivores  in  local 
areas. 

After  passing  through  blastula  and 
gastrula  stages,  the  young  oyster  develops 
into  a  trochophore  larva  characterized  by 
a  band  of  locomotory  cilia  called  the  pro- 
totroch.  As  development  continues,  the 
larval  oyster  secretes  a  pair  of  shells, 
and  the  prototroch  becomes  the  larval 
velum,  a  ring  of  locomotory  and  feeding 
cilia  characterizing  the  veliger  larva. 
The  first  shelled  larval  stage  is  also 
termed  the  straight-hinge  (veliger)  stage. 

The  straight-hinge  stage  is  succeeded 
by  the  umbo  (veliger)  stage,  in  which  the 
larval  "beak"  on  the  left  valve  overhangs 
the  hinge  line.  During  the  latter  part  of 
this  stage,  the  larval  oyster  develops  a 
foot  and  a  byssus  gland  with  which  it  will 
eventually  attach  itself  to  the  substra- 
tum. With  the  development  of  the  foot  the 
larvae  becomes  known  as  a  pedi veliger. 
During  the  latter  part  of  the  pedi veliger 
stage,  the  larval  oyster  develops  a  pair 
of  darkly  pigmented  eyes.  The  presence  of 
these  eyes  indicates  that  the  free-swim- 
ming oyster  is  ready  to  attach  and  meta- 
morphose into  the  adult  form.  At  that 
time  the  larva  is  termed  an  eyed  pedive- 
liger. 

Depending  on  water  temperature  and 
food  availability,  the  larval  life  stage 
of  C^.  virginica  will  last  approximately  7 
to  10  days.  However,  some  larvae  will 
remain  planktonic  for  up  to  2  months  dur- 
ing cooler  periods  or  in  the  absence  of 
sufficient  food.  Early  winter  sets  of 
oyster  larvae  in  the  northern  Gulf  of  Mex- 
ico may  be  attributed  to  this  phenomenon 


(Edwin  W.  Cake,  Gulf  Coast  Research  Lab., 
Ocean  Springs,  Mississippi;  pers.  comm. ). 

Feeding  activities  in  larval  oysters 
are  generally  well  understood  due  to 
recent  advances  in  commercial  oyster  cul- 
ture. In  the  artificial  conditions  of  an 
oyster  hatchery,  mixed  cultures  of  various 
small  "naked"  flagellates  (algae)  produce 
adequate  nutrition  for  the  growing  oys- 
ters. It  is  important  to  emphasize  the 
value  of  mixed  cultures,  as  opposed  to 
monocultures,  for  oyster  food  sources. 
There  are  apparently  synergistic  reactions 
among  various  food  items  that  are  as  yet 
unknown  but  that  are  very  important  to 
oyster  growth  (Epifanio  1979).  This  is 
hardly  suprising  because  the  diet  of 
oyster  larvae  in  the  natural  state  is 
obviously  far  from  a  pure  culture  and 
probably  includes  bacteria  and  small  de- 
trital  particles  as  well  as  algae  and  pro- 
tozoa. The  diet  could  also  include  dis- 
solved organic  matter. 

After  a  variable  planktonic  period 
(about  2  weeks)  from  initial  fertiliza- 
tion, the  surviving  oyster  larvae  prepare 
for  settlement  and  metamorphosis.  At  this 
stage  the  "mature"  larvae  are  signifi- 
cantly larger  than  the  younger  straight- 
hinge,  early  umbo,  and  late  umbo  stages; 
and  they  are  experimentally  separable  by  a 
160-y  mesh  sieve  that  retains  the  mature 
stages  but  not  the  immature  (Hidu  and 
Haskin  1971). 

Several  environmental  factors  influ- 
ence the  settlement  of  larval  oysters, 
including  the  physico-chemical  and  biolog- 
ical factors  discussed  by  Hidu  and  Haskins 
(1971).  They  maintained  that  light,  sa- 
linity, temperature,  and  current  velocity 
all  affect  "prospective"  spat  (newly  set- 
tled oysters).  Thorson  (1964)  proposed 
that  the  settling  response  of  marine 
invertebrates  is  often  cued  by  light.  For 
example,  oyster  larvae  tend  to  be  photo- 
positive  throughout  their  larval  life  span 
but  may  become  photonegative  in  response 
to  a  temperature  increase.  Late  settling 
oyster  larvae  also  tend  to  be  more  demer- 
sally  distributed  than  earlier  larvae, 
possibly  because  of  their  heavier  shells. 

Along  the  Atlantic  coastal  regions 
south  of  Virginia,  spatfall  appears  to  be 
denser  in  intertidal  areas.   Hidu  and 


23 


Haskin  (1971)  related  this  phenomenon  to  a 
water  temperature  increase  during  flood 
tides  over  intertidal  mudflats.  The  slack 
water  areas  of  eddy  currents  also  seem  to 
favor  heavier  than  average  spatfall  pat- 
terns (Roughley  1933).  Spatfall  will  be 
discussed  again  in  Chapter  4  when  the  dis- 
tribution of  reefs  in  an  estuary  is  con- 
sidered. 

The  biological  cues  to  oyster  larval 
settling  are  related  to  the  fact  that  oys- 
ter larvae  are  gregarious  and  apparently 
respond  to  a  waterborne  pheromone  or  me- 
tabolite released  by  oysters  that  have  al- 
ready metamorphosed  (Hidu  and  Haskin 
1971).  Larvae  also  seem  to  respond  posi- 
tively to  a  protein  on  the  surface  of  oys- 
ter shells.  This  gregarious  tendency  is 
important  to  a  reef-building  (colonial) 
organism  such  as  the  oyster,  which  re- 
quires settlement  in  proximity  for  suc- 
cessful fertilization  (Crisp  and  Meadows 
1962,  1963).  See  Chapter  3  for  additional 
details  of  gregarious  behavior. 


2.3  OYSTER  FEEDING,  DIGESTION, 
AND  ASSIMILATION 

The  feeding  organs  of  oysters  are  (1) 
the  ciliated  gills  that  provide  the  water 
currents  (with  the  assistance  of  the  man- 
tle) and  sort  particles;  (2)  the  palps 
surrounding  the  mouth  that  also  play  a 
role  in  the  particle-sorting  process;  (3) 
the  crystalline  style,  a  semirigid  clear 
rod  composed  of  digestive  enzymes  that 
function  in  the  mechanical  breakdown  of 
food  particles;  (4)  the  gastric  shield 
against  which  the  style  rotates  to  grind 
food  particles;  (5)  the  stomach,  in  which 
food  and  digestive  enzymes  are  mixed;  and 
(6)  the  digestive  diverticula  surrounding 
the  stomach,  a  group  of  blind-ending  tu- 
bules with  ducts  leading  to  the  stomach. 
The  latter  function  in  intracellular  di- 
gestion. 

The  feeding  of  all  filter-feeding  bi- 
valves (including  oysters)  had  been  as- 
sumed to  be  a  continuous  process  in  those 
organisms  that  are  always  submerged.  The 
ciliary  feeding  currents  and  the  produc- 
tion and  erosion  (dissolution)  of  the  re- 
volving crystalline  style  have  been 
thought  to  occur  continuously  in  undis- 
turbed animals.  This  view  was  challenged 


by  Morton  (1973,  1977),  who  presented  per- 
suasive evidence  that  even  in  many  sub- 
tidal  bivalves,  the  feeding  process  is 
cyclic  and  discontinuous,  affected  by 
tidal  and  seasonal  factors. 

It  is  obvious  that  an  intertidal  oys- 
ter cannot  feed  when  exposed  during  ebb 
tides,  but  an  interesting  aspect  of  Mor- 
ton's hypothesis  is  that  the  feeding  pro- 
cess is  necessarily  cyclic  in  subtidal  as 
well  as  in  intertidal  bivalves.  The  impli- 
cation of  discontinuous  ciliary  suspension 
feeding  with  a  tidal  rhythym  is  that  tidal 
and  seasonal  cycles  were  incorporated  by 
ancestral  bivalves  in  the  evolution  of 
their  feeding  process. 

According  to  Morton  (1977),  the  feed- 
ing of  intertidal  oysters  occurs  in  three 
cyclic  stages:  (1)  a  feeding  stage  during 
which  the  oyster  pumps  water  with  ciliary 
currents  produced  by  the  gills;  (2)  an  ex- 
tracellular digestive  stage,  during  which 
the  crystalline  style  acts  on  ingested 
food  that  has  been  rolled  into  mucous 
strings;  and  (3)  an  intracellular  diges- 
tive stage,  during  which  small  particles 
of  food  are  further  digested,  absorbed, 
and  assimilated  within  the  digestive 
diverticula  of  the  stomach.  The  three 
stages  are  illustrated  in  Figure  9.  Note 
that  the  production  of  pseudofeces  (con- 
solidated particulate  matter  that  is 
expelled  without  undergoing  the  digestive 
process)  occurs  during  the  active  feeding 
cycle  when  rejected  particles  accumulate 
in  the  inhalent  chambers.  Fecal  produc- 
tion results  from  the  extracellular  diges- 
tive and  intracellular  digestive  process- 
es, but  feces  and  pseudofeces  cannot  be 
released  except  during  inundation.  Morton 
concluded  that  the  three  feeding  cycles 
occur  during  two  alternate  phases:  (1) 
food  is  collected,  filtered,  selected,  and 
passed  to  the  stomach;  (2)  food  collection 
ceases  and  the  accumulated  material  is 
digested. 

The  specific  diet  of  intertidal  oys- 
ters, like  that  of  most  estuarine  consum- 
ers, is  not  clearly  understood.  The  gills 
of  the  adult  oyster  have  been  reported  to 
retain  diatoms,  dinoflagellates,  and 
graphite  particles  from  2\i  to  3p  but  to 
pass  70%  to  90%  of  Escherichia  coli  and 
80%  of  graphite  particles  from  Ip  to  2p  . 
On  the  other  hand,  Loosanoff  and  Engle 


24 


FEEDING 


FECES  AFTER 

INTRA-CELLUlAR 

DIGESTON 


STYLE 
DISSOLVES 

EXTRA-CELLULAR 

DIGESTIVE 

CYCLE 


BREAKDOWN  OF 
DIGESTIVE  DIVERTICULA 
FRAGMENTATION 
SPHERULES 


INTRA-CELLULAR 

DIGESTIVE 

CYCLE 


ASSIMILATION 


ABSORPTION 
IN  DIGESTIVE 
DIVERTICULA 


FECES  AFTER 

EXTRA-CELLULAR 

DIGESTION 


Figure  9.  A  schematic  representation  of  the  rhythmic  nature  of  the  feeding  process 
and  extracellular  and  intracellular  digestive  mechanisms  in  oysters  (adapted  from 
Morton  1973). 


25 


(1946)  found  ambiguous  and  variable  re- 
sults when  examining  the  relation  between 
particle  size  and  retention  on  the  gill  in 
oysters.  These  results  suggest  that  the 
filtering  efficiency  of  oysters  is  not 
necessarily  related  to  their  pumping  rate. 
The  role  of  mucous  in  actually  trapping 
food  particles  in  oysters  is  unclear,  as 
is  the  importance  of  dissolved  organic 
material  to  the  overall  energy  intake. 

The  assimilation  of  significant  lev- 
els of  dissolved  organic  matter  (DOM)  in 
oysters  was  documented  by  Collier  et  al. 
(1953),  although  the  methods  were  criti- 
cized by  Galtsoff  (1964).  Oysters  probably 
"leak"  some  organic  carbon  (Johannes  et 
al.  1969).  Some  workers  feel  that  hetero- 
trophic microorganisms  (bacteria)  repre- 
sent the  only  significant  consumers  (and 
packagers)  of  DOM  (Sottile  1973). 

Feeding  activity  in  oysters  is  high- 
est at  low  concentrations  of  food;  there 
is  a  negative  correlation  between  pumping 
rate  and  turbidity  (Loosanoff  1962).  The 
effect  of  turbidity  on  the  pumping  rate  is 
illustrated  in  Figure  10  (Loosanoff  and 
Tommers  1948).  Some  ambiguity  between 
laboratory  and  field  studies  exists  how- 
ever; for  example,  oysters  held  above  the 
bottom,  in  the  so-called  maximum  turbidity 
zone,  grew  more  rapidly  than  those  on  the 
bottom  in  commercial  beds  in  Buzzards  Bay, 
Massachusetts  (Rhoads  1973).  Reef  oysters 
may  have  a  similar  advantage  in  the  study 
area.  The  average  suspended  load  of  par- 
ticulate organic  matter  (POM)  in  a  typical 
estuary  in  Georgia  ranges  between  4.6  and 
15.8  mg/liter  afdw  (Odum  and  de  la  Cruz 
1967).  Hanson  and  Snyder  (1979)  reported 
extraordinarily  high  levels  of  suspended 
particulate  organic  carbon  (POC)  in  the 
study  area  (0.02  to  0.1  gC/liter),  equiva- 
lent to  approximately  40  to  200  mg  POM  and 
much  higher  than  the  1967  estimate  of  Odum 
and  de  la  Cruz.  High  levels  of  suspended 
organic  matter  could  reflect  strong  tidal 
currents. 

Particulate  organic  matter  is  a  mix- 
ture of  marsh  plant  detritus,  phytoplank- 
ton,  benthic  algae,  bacteria,  zooplankton 
(incuding  oyster  larvae),  and  DOM  adsorbed 
onto  clay  particles.  An  intertidal  oyster 
diet  is  a  mixture  of  these  items,  some  of 
which  are  not  incorporated  into  oyster 
tissue  while  others  are  more  assimilable. 


The  presence  of  cellulolytic  activity  in 
the  crystalline  style  of  the  oyster  has 
been  reported  (Newell  1953),  but  the 
amount  and  kind  of  cellulose  that  can  be 
used  by  the  animal  are  unknown.  Because 
the  diet  of  the  oyster  includes  dinofla- 
gellates  and  other  algae  with  cellulose 
tests  (outer  covering),  the  ability  to 
digest  such  structural  polysaccharides 
appears  to  be  advantageous. 

Results  from  laboratory  experiments 
on  oyster  feeding  are  sometimes  ambiguous 
or  at  least  not  directly  applicable  to 
oysters  in  their  natural  setting.  For 
example,  a  study  by  Epifanio  (1979)  indi- 
cated that  the  gross  chemical  composition 
of  experimental  algal  cultures  fed  to  oys- 
ters (protein,  lipid,  carbohydrates,  and 
ash)  was  less  important  to  subsequent  oys- 
ter growth  than  was  the  specific  type  of 
algae  used.  Oysters  have  even  been  shown 
to  grow  on  cornstarch-supplenented  diets 
(Ingle  1967). 

A  final  note  on  the  specific  diet  of 
intertidal  oysters:  in  the  only  analyses 
of  6^^C  (stable  carbon  isotope  ratio  test) 
of  oyster  tissue  from  the  Duplin  River, 
Georgia,  Haines  (1976)  and  Haines  and 
Montague  (1979)  found  the  stable  carbon 
isotopic  ratio  ranged  from  -21°/oo  to 
-24°/oo,  typical  of  organic  matter  pro- 
duced by  phytoplankton.  The  interpreta- 
tion indicates  that  oysters,  even  in  small 
tidal  creeks  surrounded  by  Spartina,  feed 
only  on  algae.  We  think  this  interpreta- 
tion should  be  accepted  cautiously  due  to 
discrepancies  found  in  different  tissues 
of  shrimp.  (Brian  Frye,  University  of 
Texas  Marine  Science  Institute,  Port 
Aransas;  pers.  comm. ) 

The  rate  at  which  intertidal  oysters 
ingest  particulate  matter  is  the  product 
of  four  factors:  (1)  the  average  rate 
(volume/time)  at  which  they  can  clear  the 
water  of  POM  of  a  favorable  size  range; 
(2)  the  concentration  of  suspended  food  in 
this  size  range;  (3)  the  total  time  that  a 
given  oyster  (or  reef)  is  inundated;  and 
(4)  the  percentage  of  inundation  time  that 
oysters  filter  water.  Any  significant  up- 
take of  DOM  would  add  to  this  total  rate. 
An  energy  budget  for  individual  oysters  is 
included  in  the  Appendix  and  summarized  in 
Section  2.5;  energy  requirements  of  a  unit 
area  of  reef  are  discussed  in  Chapter  3. 


26 


z 
o 

(- 
o 

o 


Z 


o 


90 


80- 


70 


60 


50- 


40- 


30- 

0 


• 


.''^o-  ■ 


SILT  •• ■• 

KAOLIN  D D 

CA  CO,    o-  — -o 


I  I  I — 

1  2  3 

CONCENTRATION    g/liter 


Figure  10.  Effects  of  turbidity  on  pumping  rate  (adapted  from  Loosanoff  and 
Tommers  1948). 


27 


2.4  STRESSES   ON   OYSTER   POPULATIONS: 
NATURAL  AND  CULTURAL 

Natural  Stress 

Much  oyster  literature  concerns  the 
variety  of  microscopic  organisms  that 
cause  oyster  mortalities.  These  pathogens 
have  caused  massive  oyster  die-offs  in 
local  areas  and  sometimes  in  broad  re- 
gions, e.g.,  the  infamous  outbreak  of  the 
bacterium  "MSX"  (f^inchinia  nelsoni )  in  New 
Jersey,  Delaware,  and  Vircinia  during  the 
late  1950's  and  early  1960's.  "Disease 
organisms"  is  an  anthropomorphic  and  a 
pejorative  phrase  typically  applied  to 
organisms  that  appear  to  be  harmful  to 
animals  and  plants  valued  by  man,  and  it 
often  stands  in  the  way  of  an  objective 
functional  approach  to  ecosystems.  Oys- 
ters are  ancient  mollusks  that  undoubtedly 
have  been  competitive  with,  preyed  upon, 
and  parasitized  by  many  species.  Their 
survival  to  the  present  attests  to  the 
fact  that  they  have  maintained  a  comple- 
mentary functional  role  within  the  estua- 
rine  ecosystem.  As  such,  they  have  been 
subject  to  various  ecosystem  feedback  reg- 
ulators, including  so-called  "disease 
organisms"  that  maintain  an  oscillating 
stability  in  oyster  population  density.  In 
the  context  of  the  present  discussion, 
protozoan,  fungal,  bacterial,  and  other 
oyster  parasites,  comniensals,  and  preda- 
tors, such  as  oyster  drills  and  oyster 
catchers,  are  considered  oyster  associ- 
ates, or  ecosystem  regulators.  These 
function  under  natural  conditions  to  con- 
trol excessive  populations  and  regulate 
the  distribution  and  density  of  oyster 
reefs  themselves.  It  appears,  however, 
that  man-induced  stresses  on  oysters  miay 
sometimes  shift  the  balance  in  favor  of 
the  oyster  regulator  by  creating  subtle 
changes  of  temperature,  oxygen,  salinity, 
or   pollution   levels   (Galtsoff   1964). 

We  are  unaware  of  any  studies  at- 
tempting to  distinguish  between  oyster 
vulnerability  to  "disease"  in  subtidal  vs. 
intertidal  habitats.  Since  oyster  disease 
is  often  density-dependent,  extremely 
dense  intertidal  reef  populations  may  be 
more  vulnerable  than  sparse  communities. 
Reefs,  however,  persist  in  some  areas  for 
long  periods  (see  Chapter  4),  and  oysters 
apparently  have  adapted  better  to  the 
stress  of  intertidal  existence  than  have 
the  pathogens. 

28 


Oyster-associated  organisms,  includ- 
ing common  oyster  commensals,  are  dis- 
cussed in  Section  3.2.  Usually,  the  oc- 
currence and  density  of  commiensals  are 
less  in  intertidal  reef  oyster  populations 
than  in  subtidal  oysters.  Common  commen- 
sals include  the  boring  sponge  (Cliona  ce- 
lata),  the  polychaete  mud  worm  (PoTydora 
websteri ),  and  the  pea  crab  (Pinnotheres 
ostreum).  None  of  these  organisms  actual- 
ly kills  the  oyster,  but  they  do  produce 
a  stress.  The  boring  sponge  and  the  mud 
worm  induce  additional  shell  deposition; 
the  pea  crab  lives  within  the  oyster's 
mantle  cavity  and  steals  food  and  mucous 
from  the  gills,  and  perhaps  even  feeds  on 
developing  gametes  (Galtsoff  1964). 

Other  natural  stresses  include  low 
oxygen  concentration,  high  temperature, 
excessive  turbidity  (sedimentation),  ei- 
ther overabundance  or  shortage  of  appro- 
priate food,  crowding,  and  high  wave  ener- 
gy or  strong  water  currents.  Oysters  Are 
remarkably  tolerant  of  all  these  condi- 
tions, however.  For  example,  a  subtidal 
oyster  population  in  the  James  River, 
Virginia,  was  relatively  resistant  to  a 
severe  freshet  (flooding)  associated  with 
the  1972  tropical  storm  Agnes  (Larsen 
1974).  They  close  tightly  and  respire 
anaerobical  ly  when  exposed  to  the  air  or 
during  low  oxygen  conditions  (Hochacka 
and  Mustafa  1972).  Temperatures  up  to 
40°  C  or  more  can  be  tolerated  for  short 
periods  (see  Section  3.1).  Reef  growth 
can  accommodate  slow,  steady  sedimentation 
but  not  sudden  pulses  of  sediment.  Oysters 
can  withstand  crowding,  and  as  shown  in 
Chapter  3,  population  density  is  important 
to  their  intertidal  survival.  Typically, 
intertidal  reef  oysters  are  not  robust  and 
fat,  and  do  not  contain  high  levels  of 
glycogen.  The  natural  stresses  of  their 
environment  are  reflected  by  the  long  nar- 
row valves  and  watery  tissue  texture  char- 
acteristic of  "coon"  oysters. 

Man-related  stress 

Man-induced  perturbations  on  oysters 
can  conveniently  be  divided  into  eight 
classes  (Table  1)  as  follows:  (1)  physi- 
cal disturbances,  especially  sedimentation 
resulting  from  dredging  and  excessive  boat 
traffic;  (2)  salinity  changes  due  to 
freshwater  diversion  or  local  hydrologic 
alteration;  (3)  eutrophication  or  over- 
enrichment  of  water  from  organic  matter, 


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29 


sewage,  and/or  fertilizer;  (4)  toxins. 
Including  pulp  mill  sulfites,  heavy  met- 
als, chlorinated  hydrocarbons,  organophos- 
phates,  radionuclides,  and  petroleum 
hydrocarbons;  (5)  physical  impairment  of 
feeding  structures  by  oil;  (6)  thermal 
loading,  primarily  from  power  plants;  (7) 
overharvesting;  and  (8)  wetland  loss  due 
to  development. 

These  perturbations  can  be  lethal  or 
sublethal  for  oysters,  but  even  when  sub- 
lethal, the  oysters  may  be  unfit  for  con- 
sumption either  by  humans  or  by  other 
predators.  Oysters,  like  most  suspension 
feeders,  efficiently  concentrate  suspended 
and  dissolved  constituents  of  the  water 
column  to  levels  several  orders  of  magni- 
tude above  background  concentrations  (bio- 
accumulation).  Human  pathogens,  pesti- 
cides, and  heavy  metals  are  prime  exam- 
ples. Greig  and  Wenzloff  (1978)  reported 
that  oysters  with  high  levels  of  heavy 
metals  in  their  tissues  did  not  purge  or 
lose  these  metals  rapidly  when  transferred 
to  clean  water. 

Quantifying  sublethal  effects  on  oys- 
ters is  complicated  by  the  fact  that  oys- 
ters live  at  the  water  sediment  interface, 
and  most  pollutant  concentrations  in  sed- 
iments are  different  from  those  in  water. 
While  very  low  concentrations  of  some 
toxins  in  oysters,  like  dioxin,  may  be 
significant,  the  capability  to  detect 
these  pollutants  has  been  achieved  only 
recently,  so  that  much  recent  literature 
on  pesticide  residues  in  oysters  and  other 
organisms  may  be  misleading  (e.g.,  Buqq 
et  al.  1967). 

The  effect  of  crude  oil  extracts  on 
the  carbon  budget  of  Mytilus  edulis,  the 
edible  mussel,  is  illustrated  in  Figure 
11.  As  shown,  carbon  ingested  and  assimi- 
lated declines  with  increasing  oil  concen- 
tration. Comparable  effects  could  be 
expected  for  the  oyster. 

The  estuaries  in  the  study  area  are 
presently  not  as  severely  impacted  by  man- 
induced  (cultural)  change  as  are  some 
other  oyster-producing  areas,  such  as  sec- 
tions of  the  Louisiana  and  Florida  coasts, 
Chesapeake  Bay,  and  Long  Island  Sound.  In 
addition,  intertidal  reefs  are  in  some 
ways  more  resistant  to  man-induced  pertur- 
bations  (e.g.,  salinity  intrusion  and 


resultant  susceptibility  to  predation) 
because  of  the  periodic  exposure  due  to 
tides.  Conversely,  intertidal  reef  exis- 
tence is  already  stressful,  and  added 
stress  may  inhibit  reef  formation. 

Effects  of  marsh  alteration  in  Texas 
have  decreased  local  oyster  production 
(Moore  and  Trent  1971).  Changes  in  hy- 
drology and  pollution  have  probably  con- 
tributed to  local  declines  in  oyster  reef 
density  in  the  Savannah,  Georgia,  area. 
Historical  change  in  intertidal  oyster 
reefs  in  the  study  area,  caused  by  both 
natural  and  cultural  perturbations,  is 
discussed  in  Section  4.2. 


Harvest  of  Intertidal  Oysters 

Because  this  paper's  overall  objec- 
tive is  to  describe  the  ecological  func- 
tion and  importance  of  the  oyster  reef  as 
a  component  of  the  coastal  ecosystem  in 
the  study  area,  we  include  here  only  a 
brief  discussion  of  several  aspects  of 
exploitation  of  reef  oysters  by  man.  More 
information  on  the  present  commercial  har- 
vest and  potential  for  future  exploitation 
may  be  found  in  Gracy  and  Keith  (1972), 
Keith  and  Gracy  (1972),  and  Gracy  et  al. 
(1978).  These  references  are  for  South 
Carolina,  where  commercial  harvest  is  con- 
centrated in  the  study  area. 

(1)  Oyster  harvest  by  man  has  been 
an  important  cultural  activity  since  long 
before  recorded  history  (at  least  as  early 
as  2000  B.C.,  Keith  and  Gracy  1972).  Nu- 
merous oyster  shell  middens  and  shell 
rings  of  apparent  ceremonial  significance 
in  the  study  area  attest  to  the  importance 
of  the  oyster  in  the  diet  of  early  coastal 
residents.  Many  oyster  shells  found  in 
these  artifacts  are  large  and  thick, 
which,  when  considered  in  light  of  the 
presence  of  many  whelk  and  oyster  drill 
shells,  indicate  that  a  significant  por- 
tion of  the  prehistorically  harvested  oys- 
ters were  of  subtidal  origin. 

(2)  Recent  oyster  harvest  in  the 
study  area,  however,  is  primarily  concen- 
trated on  intertidal  oyster  populations. 
This  harvest,  both  recreational  and  com- 
mercial, involves  the  very  labor-intensive 
and  time-consuming  removal  of  clumps  of 
oysters  from  exposed  mud  flats,  an  effort 


30 


300- 


200 


%     100- 


-100 


A, 


CARBON  CONSUMED 

CARBON  ASSIMILATED 

NET  CARBON  FLUX 

CARBON  RESPIRED 


\ 


V 


\ 


\ 
\ 

\ 

■v.: 


\ 


01 


10  50 

%OIL  EXTRACT 


EM 


Figure  11.  Effects  of  crude  oil  extract  on  Mytilus  edulis  carbon  budgets  calcu- 
lated for  100-mg  mussels  held  at  3lo/oo  salinity  under  summer  conditions  (15°  C, 
21!3Mg  C/liter)    (adapted  from  Gilfillan  1975). 


31 


conducted  done  from  small,  flat-bottomed 
skiffs  (bateaus). 

(3)  The  majority  of  the  (clumped) 
oysters  collected  today  are  of  a  quality 
that  makes  them  less  suitable  for  the  raw 
bar  trade  than  for  canned  oysters.  Thus 
the  oyster  industry  in  the  study  area 
traditionally  has  been  an  oyster  steam- 
canning  industry. 

(4)  Of  the  intertidal  oysters  har- 
vested, the  most  valuable,  in  terms  of 
their  shape,  size,  and  condition,  are 
found  low  in  the  intertidal  zone  rather 
than  in  mature  reefs,  or  oyster  rocks,  as 
they  are  called  locally. 

(5)  Oyster  production  or  total  har- 
vest apparently  peaked  in  the  early  1900's 
and  has  steadily  declined  for  numerous 
reasons  as  follows:  over-harvesting  and 
generally  poor  management;  pollution,  re- 
sulting in  closing  many  local  areas  to 
oystering;  labor  problems,  i.e.,  a  dwin- 
dling number  of  people  willing  to  work  in 
the  labor-intensive  oyster  industry;  and 
changes  in  the  hydrology  of  local  area. 

(6)  Total  oyster  production  from  the 
study  area  (principally  South  Carolina) 
accounts  for  about  8%  of  total  U.  S.  pro- 
duction (Lee  and  Sanford  1963).  Table  2 
from  Gracy  et  al.  (1978)  summarizes  recent 
oyster  production  from  the  study  area  and 
includes  both  subtidal  and  intertidal  oys- 
ters. Presently  it  is  unclear  if  the  de- 
cline in  intertidal  oyster  harvest  indi- 
cates a  decline  in  mature  oyster  reef  den- 
sity. For  example,  the  closure  of  coastal 
areas  to  oystering  because  of  pollution  by 
human  pathogens  is  in  some  respects  bene- 
ficial to  natural  oyster  reef  populations 
that  are  thereby  assured  of  nonexploita- 
tion.  On  the  other  hand,  hydrologic 
changes  accompanying  marsh  alteration  and 
increased  coastal  activities  are  likely  to 
be  extremely  damaging  to  the  somewhat 
fragile  reefs.  In  Section  4.2  we  discuss 
the  historical  change  in  reef  density  in 
the  study  area. 

In  summary,  the  true  mature  oyster 
reef  subunit  of  the  coastal  ecosystem  in 


the  study  area  is  not  of  commercial  inter- 
est because  the  reef  oysters  are  of  poor 
market  quality.  The  exception  to  this  is 
that  high  reef  oysters  can  be  removed  and 
replanted  lower  in  the  intertidal  zone. 
The  increased  efforts  at  oyster  management 
in  the  study  area  could  benefit  natural 
reefs  in  that  additional  sources  of  oyster 
larvae  could  be  created.  The  commercial 
exploitation  of  intertidal  oysters  ulti- 
mately will  depend  on  the  study  area's 
economic  climate.  Increased  mechanization 
that  would  solve  the  labor  problem  (Hixson 
1975)  is  constrained  by  continual  rise  in 
energy  costs. 


2.5  ENERGY  SUMMARY 

A  summary  of  estimates  of  energy  flow 
in  oyster  reefs  in  the  study  area  appears 
in  Figure  12.  These  estimates  were  based 
on  the  most  reliable  available  information 
(see  the  Appendix  for  details  and  ration- 
ale). The  numbers  shown  in  Figure  12  are 
the  values  for  standing  oyster  biomass  and 
for  oyster  respiration  rate.  The  respira- 
tion estimate  is  particularly  important  as 
an  index  of  oyster  function  because  it 
represents  the  energy  "tax"  paid  by  reef 
oysters  to  support  their  other  activities. 
The  ratio  between  average  biomass  (kcal/ 
m2)  and  respiration  (kcal/m^/yr)  gives  the 
turnover  time  of  the  oyster  portion  of  the 
reef  as  0.38  yr  (or  2.6  times/yr).  This 
is  the  average  time  that  any  given  organic 
carbon  molecule  "survives"  as  a  constitu- 
ent of  oyster  tissue  before  becoming  oxi- 
dized to  CO2  and  recycled.  Gamete  produc- 
tion represents  another  high  energy  expen- 
diture, and  the  typical  watery  tissue  of 
"coon"  oysters  in  reefs  is  symptomatic  of 
oysters  that  are  continually  spawned  out 


(or  subjected  to  a  po 


itinual  ly 
or  diet). 


The  extremely  high  ingestion  and 
egestion  (biodeposition)  estimates  are  ap- 
proximate but  indicate  the  qualitative  im- 
portance of  reef  oysters  in  the  study  area 
for  transferring  suspended  organic  matter 
to  the  reef  surface.  This  process  supports 
the  high  bacterial  metabolism  noted  in 
Section  3.3,  which  in  turn  accelerates  the 
rate  of  carbon  flux  through  the  ecosystem. 


32 


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Close-up  view  of  oyster  shell  debris  characteristic  of  the  high  energy 
beach  shores  at  the  mouth  of  large  intertidal  creeks.  This  shell  pro- 
vides a  substrate  for  oyster  spat  settlement.  Photo  by  Rhett  Talbert, 
University  of  South  Carolina. 


34 


NET  PRODUCTION  1000 
(3.3%  OF  MAX.ASSIMILATION) 


WORK 


INGESTION 
2.0-5.0  X  iC 


PREDATION    2000 
(0.6%  OF  MAX. 
ASSIMILATION) 


BIODEPOSITION 
1.8-4.9  X  106 


GAMETE  PRODUCTION 
7500  -  13,000 


RESPIRATION 

13000 

(43%  OF  MAX.ASSIMILATION) 


Fioure  12.  Summary  of  energy  flow  throuqh  intertidal  reef  oysters, 
expressed  in  kiloca lories  per  meter  square  per  year. 


Values  ai-e 


35 


Intertidal  reefs  in  coastal  South  Carolina.  Note  the  relatively  flat  top  of  the 
reefs  in  the  background,  a  contnon  feature  indicative  of  the  upper  survival  limit 
of  the  oysters  in  the  intertidal  zone.  Photo  by  Rhett  Talbert,  University  of 
South  Carolina. 

36 


CHAPTER  3 
OYSTER  REEF  DESCRIPTION  AND  SYNECOLOGY 


The  objective  of  this  chapter  is  to 
detail  the  intertidal  oyster  reef  commun- 
ity in  the  study  area.  The  following  sec- 
tions will  describe  the  reef,  physically 
and  biologically,  to  set  the  stage  for 
Chapter  4  in  which  we  discuss  the  rela- 
tionship of  the  reef  subsystem  to  the 
entire  estuarine  ecosystem. 

Much  of  the  material  in  this  chapter 
was  taken  from  Bahr  (1974),  the  only 
available  study  that  treats  the  entire 
reef  community  (in  Georgia)  quantitative- 
ly. Extrapolations  of  the  results  from 
Bahr  (1974)  to  the  entire  study  area 
should  be  made  cautiously,  and  with  the 
understanding  that  in  South  Carolina  estu- 
aries, oysters  in  reefs  are  less  dense 
and  net  growth  is  more  significant  than 
is  the  case  in  Georgia  (S.  Stevens,  Uni- 
versity of  Georgia,  Sapelo  Island;  pers. 
comm. ) . 


3.1  GENERAL  REEF  DESCRIPTION 

Intertidal  oyster  reefs  range  in  size 
from  small  scattered  clumps  to  massive 
solid  mounds  of  living  oysters  and  dead 
shells.  Reefs  are  limited  to  the  middle 
portion  of  the  intertidal  zone,  where  min- 
imum inundation  time  determines  the  maxi- 
mum elevation  of  reef  growth.  Predation 
and  siltation  limit  oyster  populations  in 
the  lower  intertidal  and  subtidal  zones  to 
scattered  individuals. 

The  following  passage  by  Dean  (1892) 
describes  intertidal  oyster  reefs  or 
"ledges"  in  South  Carolina  at  the  turn  of 
the  century. 

Often  at  low  tide  the  oyster  ledges 
appear  to  the  eye  curiously  like  a 
low  hedge  of  frosted  herbage,  gray- 
ish-green in  color.  A  nearer  view 
discloses  branching  clusters  or 
clumps  of  oysters,  densely  packed 
together,  whose  crowded  individuals 
now  become  modified  or  distorted 
according  to  their  position  on  the 


cluster.  The  individuals  that  cap 
the  cluster  project  upward  like  flat- 
tipped  fingers,  slender,  narrow,  and 
long,  whose  shape  has  given  them 
throughout  the  South  the  names  "cat 
tongues,"  "raccoon  paws,"  or  "rac- 
coons." In  many  localities,  as 
throughout  the  region  of  Skull  Creek, 
the  raccoon  ledges,  continuing  for 
ages  to  encroach  upon  the  stream  bed, 
have  formed  vast  oyster  flats,  acres, 
sometimes  miles,  in  extent. 

During  exposure  to  the  atmosphere 
(ebb  tide),  the  surface  of  a  reef  dries 
and  turns  gray,  but  upon  wetting,  a  living 
reef  appears  greenish-brown  due  to  a  thin 
film  of  algae.  In  contrast,  piles  of  dead 
shells  in  the  intertidal  zone  (wet  or  dry) 
generally  are  less  colorful  than  are  liv- 
ing reefs. 

A  section  through  a  typical  reef  is 
depicted  in  Figure  13.  The  uppermost  por- 
tion is  level  but  slopes  steeply  at  the 
edges.  The  living  portion  of  a  reef  is 
thicker  at  the  perimeter  than  in  the  cen- 
ter, where  mud  trapped  by  biodeposition 
and  sedimentation  smothers  the  oysters. 
This  sedimentation  results  from  suspended 
matter  settling  out  as  turbid  water  slows 
down  while  passing  over  a  reef. 

Often  the  surface  of  a  reef  is  uni- 
formly covered  with  oysters  closely  wedged 
together,  so  that  it  is  difficult  to  re- 
move an  individual  clump.  Once  a  hole  is 
made  in  a  reef,  however,  adjacent  oysters, 
lacking  support,  tend  to  fall  toward  the 
cavity  and  are  readily  removed.  Most  ma- 
ture oysters  are  long  and  narrow,  and  vir- 
tually all  are  oriented  with  their  growing 
edges  facing  upwards  (Figure  14).  These 
are  the  typical  "coon  oysters"  described 
in  Galtsoff  (1964).  They  seem  to  grow 
toward  the  least  disturbed  water,  like 
branches  on  a  tree  seeking  light,  and  away 
from  encroaching  sediment  beneath.  A  sim- 
ilar growth  pattern  on  a  much  smaller 
scale  was  proposed  for  colonies  of  the 
freshwater  bryozoan,  Lophopodella  carteri. 


37 


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38 


Figure  14.  Several  generations  of  oysters  (C.  virginica)  growing  vertically 
on  muddy  bottom  of  Altamaha  Sound,  Georgia  (adapted  from  Galtsoff  1964).  This 
growth  pattern  results  in  oyster  clusters  termed  "coon"  oysters  as  depicted  in 
photo.  Photo  by  Wiley  M.  Kitchens,  U.S.  Fish  and  Wildlife  Service. 


39 


by  Bishop  and  Bahr  (1973).  Bahr  (1974) 
reported  no  evidence  of  orientation  of 
individual  oysters  with  respect  to  cur- 
rents in  reefs  in  Doboy  Sound  (in  contrast 
to  the  studies  by  Lawrence  1971);  but  R. 
Frey  (University  of  Georgia  Marine  Insti- 
tute, Sapelo  Island;  pers.  comm. )  detected 
such  orientation  among  oysters  in  reefs 
located  in  Blackbeard  Creek,  which  is 
characterized  by  strong  bidirectional  cur- 
rents. 

All  reefs  studied  at  Sapelo  Island, 
Georgia,  were  identical  in  height,  150  cm 
above  mean  low  water  (MLW),  except  the 
lower  immature  reefs,  which  presumably 
were  still  growing.  Peak  height  appears 
to  represent  maximum  equilibrium  eleva- 
tion, given  present  sea  level  and  the 
local  tidal  amplitude.  Generally  flooding 
tides  reach  the  lowest  portion  of  the 
reefs  approximately  2  hours  following 
slack  ebb,  completely  covering  the  upper- 
most oysters  approximately  2  hours  before 
peak  flood.  On  an  ebbing  tide,  the  tops 
of  the  reefs  become  visible  about  2  hours 
following  peak  flood,  with  the  result  that 
the  tops  of  the  reefs  are  inundated  only 
4  hours  per  tide,  or  8  hours  per  day.  The 
relationship  between  reef  elevation  and 
tidal  amplitude  is  unknown  for  other 
areas. 

Exposure  to  air  during  ebb  tides 
allows  the  visible  portion  of  a  reef  to 
dry.  Only  the  upper  layer  (5-10  cm)  of 
oysters  and  dead  shells  actually  dries 
out,  however.  The  underlying  shell  layer 
remains  moist  and  appears  reddish-brown 
when  the  dry  shells  are  removed  because  a 
thin  layer  of  detritus  covers  each  shell. 
This  lower  layer  of  shells  and  living  oys- 
ters appears  to  lack  the  film  of  algae 
characterizing  the  upper  layer.  A  reef 
can  thus  be  considered  as  consisting  of 
three  "horizons,"  one  pale  greenish-gray, 
one  reddish-brown,  and  one  silver-black, 
color  characteristic  of  shells  buried  in 
an  anaerobic  environment  high  in  ferrous 
sulfide  (Wiedemann  1971).  Fine  scrape 
marks  appear  on  many  shells  from  the  green 
and  brown  horizons,  indicating  that  the 
organic  film  is  constantly  grazed.  Mud 
crabs  ( Panopeus  herbstii  and  Eurypanopeus 
depressus)  graze  these  films  on  partially 
inundated  reefs  (Bahr  1974). 


Oysters  in  the  upper  (green)  horizon 
have  sharper  growing  edges  than  those  in 


the  brown  layer,  indicating  faster  growth. 
Presumably  this  is  a  function  of  extreme 
crowding  and  sediment  encroachment  on  the 
lower  oysters.  Many  dead  oysters  are 
found  in  the  black  and  lower  brown  hori- 
zons, with  the  valves  still  together,  but 
ful 1  of  silt  and  clay. 

Approximately  61%  (by  volume)  of  the 
reef  material  collected  from  the  upper 
surface  down  to  the  black  horizon  consists 
of  living  oysters,  21%  consists  of  dead 
shells,  and  the  remaining  18%  consists  of 
silt,   clay,  and  nonoyster  macrofauna. 

Vertical  Zonation 

Although  the  three  horizons  described 
for  the  oyster  reef  are  somewhat  arbitrar- 
ily defined,  there  is  a  definite  vertical 
change  in  reef  macrofaunal  composition. 
This  is  a  result  of  interspecific  toler- 
ance to  desiccation  (drying)  rather  than  a 
feeding  limitation  resulting  from  reduced 
inundation  time.  The  pattern  of  zonation 
in  the  study  area  (Figure  13)  is  typified 
by  the  zonation  pattern  on  dock  pilings 
from  the  lower  Duplin  River  examined  after 
years  of  exposure  to  fouling  organisms. 
From  these  pilings  one  can  extrapolate  the 
optimal  elevations  for  oysters  and  other 
epifauna  of  the  reefs. 

At  Sapelo  Island,  Georgia,  oysters  on 
pilings  are  virtually  limited  to  an  eleva- 
tion (1.5m  above  MLW)  corresponding  to 
the  maximum  elevation  of  reefs.  One  could 
assume  that  this  pattern  of  vertical  zona- 
tion would  be  compressed  in  areas  of  lower 
tidal  amplitude.  Oyster  growth  is  maximal 
from  about  60  to  70  cm  above  MLW,  the  ele- 
vation corresponding  to  the  level  of  the 
sediment  surface  on  which  these  reefs  were 
located.  Dean  (1892),  observing  growth 
patterns  on  pilings,  reported  that  oyster 
growth  in  South  Carolina  was  maximal  in 
the  mid  intertidal  zone. 

Populations  of  the  barnacle  Chthama- 
lus  fragilis  dominate  the  upper  60  cm  of 
tidal  range.  Other  barnacles  (Balanus  ) 
and  two  mussels  (Ischadium  and  Guekensia) 
representative  of  the  reef  community  oc- 
cupy the  lower  intertidal  and  upper  sub- 
tidal  ranges  on  the  pilings,  which  repre- 
sent a  zone  extending  beyond  the  lower 
limits  of  the  reef.  In  fact,  optimum  ele- 
vation for  these  species  appears  to  be  be- 
low the  limits  of  the  reef  zone.  Wiedemann 


40 


(1971)  remarked  on  the  paradoxical  re- 
striction of  the  barnacle,  £.  fragilis,  to 
the  uppermost  oysters  in  a  reef  or  to 
blades  of  marsh  grass  above  the  maximum 
height  of  oyster  reefs.  Of  the  three  spe- 
cies of  barnacles  in  the  reef  community, 
C^.  fragilis  is  restricted  to  the  upper,  or 
green,  horizon.  Another  related  barna- 
cle, C^.  stel  latus,  has  been  described  as 
an  obligate  intertidal  form  for  reasons  of 
competition  rather  than  physiology  (Con- 
nell  1961;  Barnes  and  Barnes  1969).  The 
restriction  of  Chthamalus  to  the  mid  to 
upper  intertidal  zone  was  demonstrated  by 
Connell  to  result,  not  from  intolerance  to 
constant  inundation,  but  rather  from  com- 
petitive exclusion  by  Balanus  spp.  In  the 
oyster  reef  community,  where  barnacle  den- 
sity is  not  as  great  as  in  Connell 's 
study,  oysters  seem  to  assume  the  role  of 
"squeezing  out"  all  but  the  uppermost 
individuals  of  C.  fragilis.  Many  well- 
preserved  individuals  of  the  latter  spe- 
cies are  found  trapped  and  overgrown 
between  adjoining  oysters.  Chthamalus 
fragilis  represents  the  most  obvious  exam- 
ple of  vertical  zonation  in  the  reefs,  but 
other  evidence  of  similar  restrictions  can 
be  observed;  e.g.,  anemones  occur  almost 
exclusively  in  the  brown  horizon. 

Green  and  Hobson  (1970)  stated  that  a 
difference  in  elevation  of  6  cm  in  the 
intertidal  zone  results  in  a  significant 
effect  on  rates  of  mortality;  however, 
they  were  describing  an  infaunal  assem- 
blage dominated  by  the  little  gem  clam 
(Gemma  gemma).  The  oyster  reef  displays  a 
similar  sensitivity  at  the  upper  limit  of 
its  intertidal  range.  At  slightly  lower 
elevations,  however,  these  effects  are 
buffered  by  the  physical  complexity  and 
density  of  the  reef,  which  trap  and  hold 
moisture  above  the  level  of  the  surround- 
ing sediment. 

Temperature  Effects  on  Oyster  Reefs 

Oysters  adjacent  to  a  hole  in  a  reef 
made  by  sampling  often  die  after  being 
dislodged  from  their  normal  position  in 
the  reef.  Undisturbed  oysters  are  normally 
oriented  vertically,  (with  the  ventral 
side  upward)  and  those  which  collapse  into 
a  sampling  site  are  usually  horizontally 
oriented.  The  latter  position  results  in 
exposure  of  a  greater  proportion  of  sur- 
face area  to  direct  solar  radiation,  with 


little  chance  for  mutual  shading.  The  tem- 
perature of  sediment  within  a  reef  varies 
widely  with  depth;  e.g.,  temperatures  were 
35°C  at  the  surface  and  28°C  at  6  cm  depth 
during  one  measurement  in  October  (Bahr 
1974). 

More  critical  than  sediment  tempera- 
ture is  the  fact  that  the  internal  temper- 
ature of  an  oyster  is  a  function  of  the 
orientation  of  the  oyster  with  respect  to 
direct  solar  radiation.  For  example,  the 
internal  temperature  of  a  reef  oyster  in 
Georgia  varied  (in  the  same  October  obser- 
vation) from  34°C  to  over  38°C,  according 
to  whether  it  was  oriented  vertically  or 
horizontally  (Bahr  1974).  In  full  shade 
the  temperature  dropped  to  31.5°C.  This 
implies  that  mutual  shading  of  crowded 
reef  oysters  is  beneficial  and  important 
to  the  maintenance  of  temperatures  within 
the  tolerance  limits  of  the  oyster.  In  the 
summer  when  the  angle  (azimuth)  of  the  sun 
is  highest,  significantly  higher  tempera- 
tures result  on  incident  surfaces;  there- 
fore, high  mortalities  could  easily  result 
from  the  disruption  of  the  angular  orien- 
tation of  reef  oysters  which  provides  the 
shading  to  protect  the  oysters.  Copeland 
and  Hoese  (1966)  reported  mass  mortalities 
of  intertidal  oysters  in  Texas  during  the 
summer.  Hodgkin  (1959)  concluded  that  an- 
nual high  mortalities  of  littoral  fauna 
and  flora  near  Fremantle,  Australia,  re- 
sulted from  high  temperature,  which  was  a 
major  factor  in  the  maintenance  of  charac- 
teristic shore  zonation.  Thus,  it  appears 
that  oyster  reefs  grow  to  elevations  above 
that  at  which  individual  oysters  could 
survive  the  rigors  of  temperature  stress 
and  minimal  inundation  time. 

Lehman  (1974)  examined  the  effects  of 
thermal  loading  from  the  discharge  water 
of  a  local  power  plant  on  the  oyster  reef 
community  at  Crystal  River,  Florida.  He 
concluded  that  an  average  annual  increase 
of  4°  C  in  the  water  surrounding  experi- 
mental reefs  (relative  to  unaffected 
reefs)  caused  an  increase  in  oyster  bio- 
mass,  metabolic  rate,  and  turnover  rate, 
but  a  decrease  in  the  diversity  of  the 
reef  community. 

Salinity  Effects  on  Oyster  Reefs 

Although  oysters  are  euryhaline  and 
can  tolerate  low  salinities,  reefs  are 


41 


limited  to  areas  with  significant  tidal 
amplitudes  ordinarily  associated  with  rel- 
atively high  salinity  coastal  environ- 
ments. The  effect  of  long-term  salinity 
changes  on  oyster  reefs  has  not  been 
reported  (see  Section  4.2).  The  reef  life 
style  allows  oysters  to  invade  the  preda- 
tor-rich, high  salinity  zones  of  estua- 
ries. Predators  are  excluded  because  of 
the  reef's  daily  exposure  to  the  atmos- 
phere resulting  from  the  ebb  and  flood  of 
the  tides. 

Reef  Surface  Area 

The  surface  area  of  oysters  and  dead 
shells  in  a  series  of  reef  samples  was 
measured  by  Bahr  (1974).  He  calculated 
that  at  least  50  m^  of  surface  area  is 
available  for  habitation  by  epifauna  for 
every  square  meter  of  overall  reef  area. 
The  production  of  this  large,  highly  ir- 
regular surface  area  is  an  important 
aspect  of  the  functional  role  of  the  oys- 
ter. In  the  marsh-estuarine  ecosystem  that 
is  relatively  devoid  of  hard  substrate, 
the  oyster  provides  this  limited  resource 
for  other  oysters  and  for  the  associated 
macrofauna  that  will  be  described  in  the 
next  section. 


3.2  REEF-ASSOCIATED  MACROFAUNA 

A  total  of  42  species  of  macrofauna 
(or  groups  of  related  species)  represent- 
ing seven  phyla  are  associated  with  the 
oyster  reef  community  in  Georgia  (Table 
3).  This  is  only  a  fraction  of  the  303 
species  listed  by  Wells  (1961)  in  his 
monograph  on  the  fauna  of  subtidal  and 
intertidal  oyster  beds,  but  slightly  more 
than  the  37  species  found  by  Dame  (1979) 
in  South  Carolina  reef  samples.  Rarely 
present  and  thus  not  shown  in  Table  3  are 
unidentified  species  of  boring  sponges, 
bryozoans,  hydroids,  and  mites;  all  of 
these,  except  mites,  occur  abundantly  on 
subtidal  oysters  but  only  incidentally  in 
the  intertidal  reef  community.  Probably  a 
maximum  of  50  macrofaunal  species,  includ- 
ing those  not  readily  separable,  occur  in 
the  community  samples  on  which  these  num- 
bers are  based  (Bahr  1974).  Twenty-one 
species  occurred  in  the  majority  of  the 
samples;  17  occurred  in  93%  or  more  sam- 
ples; 8  species  occurred  in  every  sample. 
Mean  frequencies  for  each  reef  species 


over  the  entire  sampling  period  and  rela- 
tive frequency  of  each  species  are  listed 
in  Table  4.  The  biomass  and  relative  bio- 
mass  of  each  major  species  or  group  of 
species  are  given  in  Table  5.  No  relation- 
ship between  the  size  of  reefs  and  the 
macrofaunal  community  was  observed  by  Bahr 
(1974)  although  a  theory  exists  that  indi- 
cates a  direct  (positive)  relationship 
between  reef  size  and  species  richness 
(Simberloff  1974;  Jackson  1977). 

A  comparison  of  the  results  of  Dame's 
reef  survey  with  the  reef  macrofauna  data 
reported  by  Bahr  (1974)  indicates  that 
Dame  found  slightly  fewer  species  or 
groups  of  related  species  (Table  3).  Dame 
also  found  a  lower  density  of  macrofauna, 
by  an  order  of  magnitude  (about  3,300  in- 
dividuals/m^  compared  to  about  38,000/m2 
reported  by  Bahr).  Some  of  these  differ- 
ences may  result  from  differences  in  sam- 
pling technique  since  Dame  sieved  his  oys- 
ter reef  sediment  samples  through  a  1.0-mm 
screen,  whereas  Bahr  used  a  0.5-irjTi  mesh 
screen. 

Lehman  (1974)  reported  31  species  of 
invertebrate  organisms  or  groups  of  relat- 
ed organisms  from  oyster  reefs  in  Crystal 
River,  Florida.  Of  these,  only  nine  spe- 
cies were  also  found  by  Bahr  (1974)  to  be 
associated  with  Georgia  reefs.  Lehman  re- 
ported the  total  abundance  of  reef-associ- 
ated organisms  to  be  about  6,200/m2  and 
oyster  density  to  be  about  3,800/m2  in  his 
control  area.  His  estimate  of  biomass  of 
oyster  reef  associated  organisms  was  135g/ 
m^  dry  wt. 

Specific  groups  of  organisms  that 
reside  in  oyster  reefs  in  the  study  area 
will  be  discussed  below. 

Oyster  Commensals 

The  relationship  between  the  oyster 
pea  crab  (Pinnotheres)  and  the  oyster 
represents  inquilinism,  an  association 
slightly  detrimental  to  the  host  species 
(Nicol  1960).  Beach  (1969)  reported  that 
Pinnotheres  becomes  increasingly  rare  in 
oysters  in  the  higher  portions  of  the 
intertidal  zone.  Dame  (1970)  found  only 
about  1%  incidence  of  pea  crabs  among 
intertidal  oysters  in  South  Carolina; 
likewise,  Bahr  (1974)  found  only  a  3% 
incidence. 


42 


Table  3.  Nacrofauna  found  in  Georgia  oyster  reefs  (adapted  from  Bahr  1974). 


Taxa 


Mollusca 

Pelecypoda  ^  , 

Crassostrea  virqinica  (Gmelin)  '  ' 
Guekensia  deniissa  (Dil  lwyn)^'° 
Ischadium  recurvum  (Rafinesque)^'^'*- 
Mya  arenaria  (Linnaeus ) 
Gemma  gemma  (Totten)^ 
Petricola  pholadiformis  (Lamarck)^ 

Gastropoda  . 

Odostomia  impressa  (Say)  ' 
Arthropoda 

Insecta  . 

Anurida  maritima  (Guerin)^'  '^ 

Cirripedia 

Balanus  improvisus  (Darwin)  ' 
Balanus  eburneus~rGou1d)^'"'^ 
Chthamalus  fragilis  (Dan-n'n)^ 

Decapoda  a  b  c 

Eurypanopeus  depressus  (Smith)  '  ' 
Panopeus  herbstii  (Milne-Edwards)^'^'^ 
Pinnotheres  ostreum  (Say)^'° 
Sesarma  cinereum  (Say ) ^ 
Clibanarius  vittatus  (Bosc) 

Amphipoda  ,  h 

Melita  nitida  (Smith)"''^ 

Parhyale  hawaiiensis 

Gamma rus  pa1ustris^>*^ 
Isopoda 

Cassidinidea  lunifrons  (Richardson) 

Edotea  motosa  (Stimpson) 
Annelida 

Polychaeta  a  b  c 

Neanthes  succinea  (Prey  and  Leuckart)  '  ' 

Nereiphyllis  fragilis  (Webster)^''^ 

Streblospio  benedicti  (Webster)^'^ 

Heteromastus  filiformis  (Claparede)^''^ 

Polydora  websteri  <^^^^^ 

Tharyx  setigera(Hartman)^ 

Spirorbis  sp. 

Sabel laria  megaris^ 

Amphitrite  ornata  (Leidy)^'" 

Marphysa  sanguinea  (Montagu)^'" 

Lysidice  ninetta 

Syllidae  (unidentified) 

Dodecaceria  sp. 


Continued 
43 


Table  3.  (Concluded) 


Taxa 


Annelida  (continued) 

Polychaeta  (continued) 

Lepidonotus  sublevis( Verri 1 1 ) 
Polychaete  (unidentified) 
Polychaete  (unidentified) 
Polychaete  (unidentified) 

Nemertea  ^ 

Nemertina  (unidentified) 

Coelenterata 

Anthozoa  (unidentified) 

Platyhelminthes 
Turbellaria 

Polyclad  (unidentified) 

Sipuncul ida 

Sipunculid  (unidentified) 


?Genus  reported  by  Wells  (1961). 
Species  reported  by  Dame  (1979). 
Species  reported  by  Lehman  (1974), 


44 


Table  4.  Mean  annual  frequency  distribution  of  reef  macrofauna. 


Mean  freq.     Variance    Standard 
Species  (#/ni2)         S      deviation    %  of  total 

s- 

X 


Crassostrea  virginica^  14666.9  4811.3  717.2  38.65 

Guekensia  demissa  514.8  459.5  68.5  1.36 

Ischadium  recuryum  5028.0  4051.0  603.7  13.25 

Mya  arenarlja  852.8  1577.7  235.2  2.25 

Gemma  gemma 

Petricola  pholadiformis 

Odostomia  impressaa  1643.5  1792.5  264.3  4.33 

Anurida  maritimaa  ^  5453.7  3626.4  1300.5  14.37 

fBalanus  ?ip?o7Tsus^  1063.9  1063.1  158.5  2.80 

1  Balanus  iburneus'^  ,  16.9  58.8  8.7  0.04 

Lchthamalus  fragilis^  166.3  387.4  57.7  0.44 

Eurypanopeus  depressus  1037.1  430.5  64.2  2.73 

Panopeus  herbItiT5  103.1  75.0  11.2  0.27 

Pinnotheres  ostreum  24.5  33.5  5.0  0.06 
Sesarma  cinereum 
Clibanarius  vittatus 

Melita  nitidis            ,  334.2  455.2  67.9  0.88 

ParhyalFTa^iiensis^  966.2  1278.4  190.6  2.55 

Gammarus  palustris"^  5.2  -  - 

Cassidinidea   lunifrons'^  323.5  171.6  25.6  0.85 


{ 
{ 


Edotea  montosa 


14666.9 

4811.3 

717.2 

514.8 

459.5 

68.5 

5028.0 

4051.0 

603.7 

852.8 

1577.7 

235.2 

1.3 

- 

- 

0.4 

- 

- 

1643.5 

1792.5 

264.3 

5453.7 

3626.4 

1300.5 

1063.9 

1063.1 

158.5 

16.9 

58.8 

8.7 

166.3 

387.4 

57.7 

1037.1 

430.5 

64.2 

103.1 

75.0 

11.2 

24.5 

33.5 

5.0 

0.1 

- 

- 

0.4 

- 

- 

334.2 

455.2 

67.9 

966.2 

1278.4 

190.6 

5.2 

- 

- 

323.5 

171.6 

25.6 

1.3 

- 

- 

1739.1 

1778.3 

268.1 

78.0 

60.9 

9.0 

1362.4 

1723.4 

259.8 

519.8 

314.7 

46.9 

359.3 

436.4 

65.1 

0.3 

- 

- 

1.1 

- 

- 

1.7 

- 

- 

4.3 

- 

- 

5.2 

- 

- 

1.3 

- 

- 

0.4 

- 

- 

0.9 

- 

- 

0.4 

- 

- 

0.9 

- 

- 

8.7 

- 

- 

4.8 

- 

- 

204.0 

194.8 

29.0 

1442.5 

1376.6 

205.5 

7.8 

- 

- 

0.4 

■" 

^ 

4.58 


Neanthes  succinea 

Nereiphyllis  fragilis!  78.0  60.9  9.0  0.21 

Streblo'spio  benedicti^  ,  1362.4  1723.4  259.8  3.59 

Heteromastus  filiformis^  519.8  314.7  46.9  1.37 

Polydora  websterT^  359.3  436.4  65.1  0.95 

Tharyx  setigera 

Spirorbis  sp. 

Sabellaria  megaris 

Amphi trite  ornata 

Marphysa  sanguinea 

Lysidice  ninetta 

Syllidae  (unidentified) 

Dodecaceria  sp. 

Lepidonotus  sublevis 

Polychaete  (unidentified) 

Polychaete  (unidentified) 

Polychaete  (unidentified) 

Nemertina  (unidentified)^      204.0      194.8     29.0        0.54 

Anthozoa  (unidentified)^      1442.5     1376.6    205.5        3.80 

Polyclad  (unidentified) 

Sipunculid  (unidentified) 

Total:     37,947.4 


^Twenty-two  species  found  in  93%  of  all  samples  and  considered  dominant. 
Brackets  enclose  groups  of  "similar"  species  that  reduce  major  macrofauna 

members  of  the  reef  community  to  16. 

45 


Table  5.  Ranked  biomass  of  16  major  oyster  reef  species  or 
groups  of  species  and  proportion  of  total  macrofaunal  biomass. 

~  Mean  biomass 

Species  or  group  of  species     (g/m^  +  2  s-)  %  of  total 


Crassostrea  virginica  969.6+93.4  87.534 

Guekensia  demissa  83.7+26.9  7.554 

Ischadium  recurvum  24.4+13.0  2.200 

Eurypanopeus  depressus  13.5  jH  2.3  1.220 

Panopeus  herbstii  7.3+  4.4  0.656 

Neanthes  succinea  3.4  +  1.6  0.304 

Anthozoa  (unidentified)  1.5+  0.5  0.131 

3  Cirripedia  species  T-^l  0.6  0.130 

3  Amphipoda  species  1'2  ji  0.5  0.106 

Nereiphyllis  fragilis  0.8+  0.5  0.069 

Mya  arenaria  0.3+  0.5  0.024 

Odostomia  impressa  0.3+  0.1  0.024 

Nemertea  (unidentified)  0.1  +  0.0  0.013 

Anurida  maritima  0.1  ±    0.1  0.013 

3  Polychaeta  species  0.1  +  0.1  0.013 

Cassidinidea  lunifrons  0.0  +  0.0  0.001 


Total  1,107.7 


46 


Other  inhabitants  of  shells  of  sub- 
tidal  oysters  were  virtually  nonexistent 
within  reef  oysters  examined  in  the  Geor- 
gia study,  e.g.,  worms  (Polydora  spp. ) 
were  found  free  in  the  samples  but  not 
inside  oysters.  Boring  sponges  (Cliona 
spp.)  were  absent  on  intertidal  oysters 
but  abundant  on  subtidal  oysters  and  dead 
shells.  Infestation  (with  Cliona)  results 
in  shell  deterioration  in  subtidal  oysters 
due  to  shell  erosion  by  Cliona.  Infested 
(with  Cliona)  oysters  are  particularly 
vulnerable  to  predation,  and  the  shells 
are  fragmented  into  pieces  which  tend  to 
be  washed  away  rather  than  remaining  in 
situ  as  substrate  for  further  coloniza- 
tion. This  is  one  of  the  principal  reasons 
that  subtidal  reefs  are  absent  in  the 
study  area.  Guida  (1976)  discussed  the 
abundance  of  Cliona  spp.  in  subtidal  oys- 
ters and  oyster  shells.  No  oyster  drills 
or  starfish  were  ever  seen  on  the  reefs 
examined.  Parasitic  gastropod,  Odostomia 
impressa,  was  abundant,  (up  to  5,460/rr,2). 

Insects 


An  interesting  organism  occurring  in 
abundance  on  oyster  reefs  in  the  study 
area  is  a  collembolan  insect,  Anurida  mar- 
itima,  a  true  marine  insect  (Miner  1951 ). 
The  trophic  role  of  a  similar  intertidal 
collembolan  (Oudemansia  esakii )  in  Hong 
Kong  has  been  described  as  saprophagic  on 
recently  dead  macrofauna,  including  oys- 
ters (Chan  and  Trott  1972).  Anurida 
appears  to  be  a  true  oyster  associate 
since  it  is  only  observed  on  mud  flats 
near  oysters.  The  greatest  concentrations 
are  inside  dead  pairs  of  oyster  shells, 
which  often  house  masses  of  live  insects 
along  with  large  numbers  of  exuviae  (shed 
exoskeletons).  Small  and  covered  with  a 
nonwettable  cuticle,  Anurida  is  extremely 
buoyant  and  would  be  washed  away  during 
flood  tides  were  it  not  for  crevices  in 
oyster  shells  which  allow  masses  of  them 
to  cling  together.  As  in  the  case  of 
Oudemansia,  Anurida  probably  emerges  to 
the  reef  surface  during  ebb  tide  and 
retreats  before  flood  tide.  Dame  (1979) 
reported  a  few  Anurida  ('^6/m2)  present  in 
South  Carolina  reefs  and  Lehman  (1974) 
reported   Anurida   from  Florida   reefs. 

Barnacles 


has  been  noted  in  previous  sections  (see 
Section  3.1).  Dame  (1979)  did  not  report 
C^.  fragilis  on  South  Carolina  reefs,  which 
may  indicate  that  these  reefs  were  lower 
in  the  intertidal  zone.  Since  total  bar- 
nacle density  on  oyster  reefs  does  not 
approach  the  density  observed  on  pilings 
(Bahr  1974),  it  appears  that  unknown  fac- 
tors limit  barnacle  survival  on  intertidal 
reefs.  It  has  been  reported  that  Balanus 
eburneus  reaches  maximum  density  at  a 
elevation  of  9  to  14  m  below  sea  level 
(Relini  and  Giordano  1969). 

Mud  Crabs 

Two  of  the  most  characteristic  mem- 
bers of  the  reef  community  are  the  common 
mud  crabs  Eurypanopeus  depressus  and  Pano- 
peus  herbstii.  observed  by  Bahr  (1974)  at 
mean  densities  of  l,037/m2  and  lOS/m^, 
respectively.  They  seem  to  remain  quies- 
cent in  the  brown  horizon  during  exposure 
of  the  reefs  but  begin  active  feeding  with 
tidal  inundation.  Feeding  consists  of  us- 
ing one  or  both  chelae  to  scrape  the  film 
of  algae  and  detritus  from  shells  in  the 
brown  and  green  horizons.  The  "grazed" 
appearance  of  shells  and  the  fact  that 
neither  algae  nor  detritus  accumulates  on 
shells  indicate  the  proficiency  of  graz- 
ing. These  two  crabs  are  undoubtedly 
omnivorous,  and  Bahr  (1974)  noted  Panopeus 
predation  on  small  oysters  on  reefs  and 
Eurypanopeus  predation  on  amphipods  in  the 
laboratory.  Dame  (1979)  reported  much  low- 
er densities  of  mud  crabs  on  South  Caro- 
lina reefs;  he  found  the  two  species  in 
approximately  equal  densities. 

Soft  Shelled  Clams 

Common  occurrence  of  small  soft  shell 
clams  in  the  reef  samples  was  noted  by 
Bahr  (1974)  at  densities  ranging  up  to 
6,460/m2.  No  adult  clams  have  been  ob- 
served in  reef  samples.  It  appears  that 
clam  spat  (juveniles)  settle  on  the  reefs 
and  survive  only  temporarily.  Mya  arenaria 
has  not  been  reported  to  range  success- 
fully as  far  south  as  Georgia,  although 
adult  specimens  have  been  found  at  Sapelo 
Island.  Dame  (1979)  did  not  report  find- 
ing Mya  arenaria  in  South  Carolina  reefs. 

Mussels 


A  marked  vertical  zonation  of  Chtha-  Kuenzler's  (1961)  study  of  the  ribbed 
malus  fragilis,  one  of  three  barnacle  spe-  mussel  Guekensia  demissus  (formerly  called 
cies  identified  from  the  reef  community,   Modiolus)  demonstrates  that  this  animal's 

47 


functional  importance  in  the  marsh  system 
resides  more  in  terms  of  nutrient  (phos- 
phorus) cycling  than  in  energy  flow.  He 
estimated  the  mean  density  of  Guekensia  in 
the  entire  marsh  at  7.82  animals/m^, 
whereas  in  oyster  reefs  in  Georgia,  this 
mussel  averaged  over  SOO/m^.  Ischadium 
recurvum  was  found  to  be  10  times  more 
numerous  in  reefs  than  was  Guekensia  (see 
Table  4),  and  together  these  two  species 
contributed  9.5%  of  total  macrofaunal  bio- 
mass  (112.08  g/m2).  Dame  (1979)  reported 
about  7  Guekensia/m^  in  South  Carolina 
reefs  and  about  71)0  Ischadium/m^,  or  two 
orders  of  magnitude  greater  than  Gueken- 
sia. 

Anemones 

Anemones  are  sessile  epibenthic  sus- 
pension feeders  that  have  soft  bodies  and 
are  extremely  vulnerable  to  dessication. 
Thus,  they  are  not  normally  considered 
intertidal  organisms.  Their  common  occur- 
rence in  reef  samples  in  Georgia  (Bahr 
1974)  attests  to  the  capacity  of  oyster 
reefs  to  retain  water  above  MLW  and  to 
extend  the  vertical  distribution  of  such 
creatures.  Dame  (1979)  did  not  report  any 
anthozoans  in  South  Carolina  reefs,  but 
this  group  could  have  been  overlooked  in 
preserved  samples. 

Polychaetes 

Polychaetes  are  generally  one  of  the 
dominant  groups  in  benthic  systems  because 
of  their  contribution  to  total  biomass  or 
to  numbers,  or  both;  but  they  are  usually 
considered  infauna,  with  some  obvious 
exceptions  such  as  the  serpulids,  which 
produce  encrusting  calcareous  tubes.  Smith 
(1971)  found  that  polychaetes  constitute 
the  major  portion  of  macrofauna  in  a  sub- 
littoral  community  near  Sapelo  Island.  In 
the  oyster  reef  community,  polychaetes 
accounted  for  only  0A%  of  the  total  bio- 
mass, most  of  which  was  contributed  by  one 
species,  Neanthes  succinea,  which  averaged 
1,739  animal s/m^,  compared  to  281 /m^  in 
Long  Island  Sound  (Sanders  1958). 

The  three  most  abundant  small  poly- 
chaetes, Polydora  websteri,  Heteromastus 
filiformis.  and  Streblospio  benedicti, 
together  compri sed  only  about  0.01%  of 
total  macrofaunal  biomass.  There  is  a 
relative  dearth  of  polychaetes  in  this 


reef  system  compared  with  other  communi- 
ties. This  is  perhaps  related  to  the  pre- 
dominantly epibenthic  nature  of  the  reef 
community  and  to  the  absence  of  a  substan- 
tial layer  of  aerobic  sediment.  Dame 
(1979)  found  significant  numbers  of  Heter- 
omastus in  South  Carolina  reef  samples, 
but  he  did  not  find  many  of  the  other  two 
small  polychaetes,  probably  because  of  the 
large  mesh  size  used  to  screen  his  benthic 
samples. 

Amphipoda 

Amphipods  are  more  numerous  and  di- 
verse in  sublittoral  oyster  beds  than  on 
intertidal  reefs  since,  in  the  latter  sit- 
uation, tidal  pools  are  not  available  to 
sustain  them  during  ebb  tides.  Grackles 
were  observed  feeding  on  oyster  reefs, 
probably  preying  on  amphipods  and  mud 
crabs  (Bahr  1974).  Dame  (1979)  found  rel- 
ativity few  amphipods  in  South  Carolina 
oyster  reef  samples,  and  only  one  species, 
Melita  nitida,  was  reported. 

Accidentals 

Hydroids,  bryozoans,  flatworms,  and 
sponges,  all  commonly  associated  with  sub- 
tidal  oysters  (Guida  1976),  were  so  rarely 
encountered  in  Georgia  oyster  reefs  as  to 
be  considered  "accidentals"  in  the  reef 
community. 


3.3  REEF  COMMUNITY  ENERGETICS 

The  energy  requirements,  expenditures 
and  an  overall  energy  budget  for  reef  oys- 
ters are  discussed  in  the  Appendix.  The 
additional  energy  requirements  of  nonoys- 
ter  members  of  the  reef  community  are  ad- 
dressed in  the  following  section.  The  data 
used  are  primarily  those  reported  by  Bahr 
(1974). 

The  best  available  estimate  of  total 
energy  requirements  of  the  reef  community 
is  the  rate  at  which  a  unit  area  of  reef 
consumes  oxygen  (community  respiration 
rate).  A  sine  curve  fitted  to  oxygen  con- 
sumption of  the  total  reef  community  in 
Georgia  for  a  1-year  period  is  depicted  in 
Figure  15.  The  variation  in  community  oxy- 
gen uptake  ranged  from  approximately  6  to 
50  g02/m2/day  over  a  temperature  range  of 
9°  to  30°  C. 


48 


o 
O 


E 
E 
o 
o 


Jan  12 


Apr  13       May  18        Jun  22  Jul  22 
Apr  20        May  26 

Sampling  dates 


Aug  30  Sep  29  Oct  19   Nov  18 

Oct  29    Nov  29 


Figure  15.  Seasonal  oxygen  consumption  (QO2)  of  reef  community.  Data  points 
are  average  values  for  four  samples  with  95%  confidence  intervals  (Bahr  1976), 


49 


The  area  beneath  the  curve  in  Figure 
15  was  integrated  over  a  1-year  period  to 
yield  a  total  of  8,168  gOa/m^/yr  consumed 
by  the  oyster  reef  community,  equivalent 
to  27,036  kcal/m2/yr,  assuming  a  respira- 
tory quotient  of  0.85.  This  estimate  of 
the  community  metabolic  energy  demand  by 
the  reef  community  is  conservative  in  that 
it  is  derived  by  multiplying  hourly  rates 
by  12  hours,  with  the  assumption  that 
little  respiratory  activity  occurs  during 
reef  exposure  at  ebb  tide.  However,  Lehman 
(1974)  reported  a  significant  metabolic 
rate  of  exposed  oyster  reefs  by  using  an 
infrared  gas  analyzer  to  detect  CO2  re- 
leased from  enclosed  reef  samples.  This 
measured  rate  was  about  20%  of  the  rate 
measured  by  oxygen  changes  during  inunda- 
tion. Total  community  metabolism  in  the 
Georgia  reefs  is  partitioned  among  oys- 
ters, other  macrofauna,  small  organisms, 
and  chemical  oxygen  demand. 

Macrofaunal  Respiration 

The  contribution  of  each  species  of 
macrofauna  to  total  community  oxygen  con- 
sumption at  a  given  temperature  is  a  func- 
tion of  its  proportion  to  the  total  bio- 
mass,  its  size-frequency  distribution,  and 
the  relationship  between  rate  of  respira- 
tion and  size  of  an  individual.  Small  rare 
species  contribute  little  to  total  biomass 
and  cannot  contribute  significantly  to 
total  Qxygen  uptake  (QO2);  large  rare  spe- 
cies, on  the  other  hand,  can  often  alter 
total  oxygen  uptake  (Smith  1971).  Banse 
et  al.  (1969)  and  Pamatmat  (1968)  con- 
cluded that  the  most  reliable  method  of 
estimating  relative  importance  of  various 
macrofaunal  species  in  terms  of  total  com- 
munity respiration  is  to  multiply  mean  ash 
free  dry  weight  (afdw)  per  species  by  the 
density  of  that  species  in  the  community. 
By  this  criterion,  the  oyster  reef  commu- 
nity members  were  ranked  in  terms  of 
macrofaunal  metabolic  importance,  as  shown 
in  Table  6.  The  two  species  that  comprised 
95%  of  total  biomass,  Crassostrea  virqin- 
ica  and  Guekensia  demissa.  contributed 
87.5%  and  7.5%  of  total  community  biomass, 
respectively. 

The  respiration  of  oysters  accounts 
for  approximately  50%  (48.1%),  or  about 
13,000  kcal/mVyr  of  the  total  reef  com- 
munity respiration.  Total  oxygen  require- 
ments (hence  energy  requirements)  of  non- 
oyster  macrofauna  was  thus  estimated  to 


account  for  only  10%  of  the  total  reef 
requirements,  about  800  g02/m^/yr  or  about 
2,700  kcal/m^/yr.  This  latter  figure  is 
similar  to  the  total  oxygen  uptake  rate  of 
the  subtidal  soft  bottom  community  near 
Sapelo  Island  (Smith  1971). 

Nonoyster  macrofauna  were  divided 
into  14  species  or  groups  of  related  spe- 
cies, and  estimates  of  the  annual  oxygen 
consumption  rates  were  derived  experimen- 
tally (Bahr  1974),  as  shown  in  Table  6. 

Microbial  and  Meiofaunal  Respiration 

The  metabolism  of  small  consumer 
organisms  represents  22%  of  the  total  reef 
community  metabolism  (Bahr  1974).  This 
estimate  is  approximate  since  it  is  based 
on  the  difference  between  total  community 
oxygen  consumption  and  the  sum  of  esti- 
mated macrofaunal  and  chemical  oxidation 
rates. 

The  large  surface  area  of  an  oyster 
reef  (at  least  50  times  the  area  of  a 
plane  surface)  provides  a  large  surface 
for  aerobic  bacteria  as  well  as  for  epi- 
fauna  (see  Section  3.1),  and  thus  this 
estimated  large  energy  requirement,  1,600 
g02/m2/yr  (5,400  kcal/mVyi"),  is  not  too 
improbable. 

Chemical  Oxidation 

Bahr  (1974)  estimated  that  the  pro- 
portion of  total  reef  community  oxygen 
uptake  accounted  for  by  the  chemical  oxi- 
dation of  reduced  compounds  (20%)  was  only 
slightly  lower  than  microbial  metabolism. 
This  estimate  reflects  the  continual 
release  of  reduced  compounds  from  the 
anaerobic  decomposition  of  reef-derived 
organic  matter. 

Summary 

The  seasonal  energy  partitioning 
estimates  for  the  entire  reef  community 
are  depicted  in  Figure  16.  To  summarize, 
the  reef  community  converts  about  3  x  10** 
kcal/m2/yr  to  heat,  which  represents  the 
net  "cost"  to  the  ecosystem  of  supporting 
the  reef  community.  Systems  theory  would 
indicate  that  this  cost  is  repaid  by  the 
reef  community  in  the  form  of  feedback 
services.  For  example,  the  reefs  contin- 
ually release  plant  nutrients,  ammonia 
and  phosphorus-containing  compounds;  they 


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51 


^  Oyster  QO2 

^  Macrofaunal  QO2 


I     I  Chemical  QO2 
HI  Microbial  QO2 


Jan  12  Mario       Apr  13         May  18        Jun  22     Jul  22 

Apr  20       May  26 

Sampling  dates 


Aug  30   Sep  29  Oct  19    Nov  18 

Oct  29     Nov  29 


Figure  16.  Seasonal  energy  partitioning  estimates  for  the  entire  reef  community 
(Bahr  1974).     QOt  =  oxygen  consumption  rate. 


52 


significantly  increase  habitat  diversity 
and  provide  substrate  for  epifauna,  de- 
composers, and  small  nursery  species  (at 
least  during  flood  tides).  The  3  x  10"^ 
kcal/m2/yr  would  require  the  total  net 
production  of  about  5  m^  of  marsh  estuary 
for  each  square  meter  of  reef  if  total 
production  were  usable  by  the  community. 
If  only  phytoplankton  production  were 
usable,  the  reef  community  would  require 
at  least  50  m^  of  marsh  estuary  for  nutri- 
tional support  (see  Section  1.3). 

A  final  point  should  be  made  about 
oyster  reef  energy  requirements:  the  met- 
abolic rate  of  this  community  ranks  high 
among  the  values  measured  for  the  macro- 
fauna!  metabolism  of  benthic  communities, 
exceeding  even  such  systems  as  kelp  beds. 
Table  7  summarizes  the  results  of  some 
representative  benthic  community  metabolic 
measurements.  Of  particular  interest  is 
the  1974  study  by  Lehman,  in  which  total 
reef  community  metabolism  from  gulf  coast 
oyster  reefs  (Crystal  River,  Florida)  was 
measured  at  16  to  21  g  02m^/day  at  31.7°C. 
Lehman's  values  for  biomass  were  lower 
than  those  measured  from  Georgia  reefs 
(119.5  g/m^  dry  wt  vs.  970  g  afdw/m^),  and 
his  experimental  temperature  was  about  the 
same  as  the  maximum  experimental  tempera- 
ture used  by  Bahr  (1974). 

The  increasing  number  of  metabolic 
studies  in  which  partitioning  has  been  at- 
tempted have  well  established  that  macro- 
fauna  usually  play  a  relatively  minor  role 
in  total  benthic  community  energy  flow. 
Smith  (1971),  for  example,  determined  that 
the  proportion  of  total  respiration  rate 
attributable  to  macrofauna  of  a  sublitto- 
ral  community  was  equal  to  only  12.1%. 
Therefore,  the  oyster  reef  community  is 
unique  among  benthic  subsystems  in  that 
the  oysters  and  other  macrofauna  conspicu- 
ously dominate  community  metabolism  as 
well  as  community  structure.  Intertidal 
oyster  reefs  may  be  thought  of  as  hetero- 
trophic "hot  spots"  in  the  marsh-estuarine 
system. 


3.4  REEF  PREDATION 

No  quantitative  information  is  avail- 
able on  the  rate  at  which  salt  marsh  con- 
sumers prey  on  the  inhabitants  of  the 
intertidal  reef  community.  From  a  quali- 
tative standpoint,  the  predators  include 


53 


three  groups:  (1)  small  reef  residents 
such  as  mud  crabs;  (2)  strictly  aquatic 
forms  that  migrate  onto  the  reefs  to  feed 
during  flood  tides,  e.g.,  the  blue  crab 
(Cal linectes  sapidus)  and  the  sheepshead 
minnow  (Cyprinodon  variegatus);  and  (3) 
terrestrial  animals  that  prey  on  exposed 
reefs  during  ebb  tides,  e.g.,  raccoons  and 
wading  birds.  This  "time  sharing"  arrange- 
ment by  both  aquatic  and  terrestrial  pred- 
ators, representing  a  "coupling"  between 
the  reef  and  adjacent  ecosystems,  would 
appear  to  wreak  havoc  on  the  reefs;  but 
relatively  little  evidence  of  predation 
was  ever  detected  in  the  reefs  examined  by 
Bahr  (1974).  Blue  crabs  were  observed 
feeding  on  small  oysters  on  partially 
exposed  reefs;  raccoon  tracks  were  seen 
around  reefs;  and  the  most  commonly  ob- 
served reef  predators  were  boat-tailed 
grackles  (Cassidix  mexicanus),  seen  pick- 
ing unidentified  organisms  (probably  small 
crustaceans,  insects,  and  polychaetes) 
from  recently  exposed  reefs. 

Drinnan  (1957)  estimated  that  the 
European  oystercatcher  (Haematopus  ostra- 
lequs)  preyed  on  between  28.5  and  51  cock- 
les per  hour  during  active  feeding,  each 
cockle  being  between  23  and  30  mm  in 
length.  He  concluded  that  about  22%  of 
the  total  cockle  population  in  his  study 
area  in  Nova  Scotia  were  removed  as  a  re- 
sult of  this  predation. 

Butler  and  Kirbyson  (1979)  reported 
that  the  black  oystercatcher  {H.  bachmani ) 
can  eat  up  to  nine  large  oysters  per  hour, 
the  oysters  ranging  from  80  to  160  mm. 
These  birds  feed  primarily  on  single  oys- 
ters, however,  as  opposed  to  American  oys- 
tercatchers  (F[.  palliatus)  that  feed  on 
clumped  or  reef  oysters  (Tomkins  1947). 
The  latter  author  observed  predation  on 
Crassostrea  by  oystercatchers  on  reefs 
near  Savannah,  Georgia,  but  no  attempt  at 
quantification  was  made.  It  was  assumed 
from  Tomkins'  description  of  the  feeding 
behavior  of  f[.  palliatus  that  only  about  4 
hr/day  are  available  for  feeding  on  inter- 
tidal oysters  (2/hr/tide).  Observations 
on  the  density  of  oystercatchers  at  Sapelo 
Island  indicated  fewer  than  one  bird  per 
reef,  perhaps  one  per  eight  reefs,  result- 
ing in  an  estimated  maximum  of  25  oysters 
eaten  by  oystercatchers  per  reef  per  day 
(4  hr/  day  x  1/8  bird/reef  x  50  oysters/ 
hr/bird).  If  an  average  reef  were  approx- 
imately  25  m^,  a  total  loss  of  about 


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0.1  g/mVday,  or  40  g/mVyr  (200  kcal/m"/ 
yr)  could  be  estimated  as  sustained  by 
intertidal  oyster  reefs  from  predation  by 
birds.  This  estimate  is  obviously  clear- 
ly approximate. 

3.5  COLONIAL  ASPECTS  OF  THE  REEF  COMMUNITY 

The  gregarious  tendency  of  oyster  lar- 
vae has  obvious  adaptive  value  in  terms  of 
the  reproductive  success  of  subtidal  oys- 
ter populations.  It  is  also  of  great 
adaptive  value  for  intertidal  reef  oys- 
ters. Survival  in  the  upper  intertidal 
zone  in  the  study  area  may  depend  on  a 
crowded  colonial  life  style. 

The  only  single  oysters  (greater  than 
30  mm  in  height)  or  small  clusters  of  oys- 
ters normally  observed  in  the  intertidal 
zone  were  either  at  the  lower  level  of  the 
zone  (not  much  higher  than  60  cm  above 
MLW),  or  they  were  scattered  among  stalks 
of  cordgrass  (Spartina  alterniflora), 
where  they  were  shaded.  The  only  way  oys- 
ter reefs  attain  their  maximum  steady- 
state  elevation,  or  mature  stage,  is  via 
the  slow  process  of  reef  accretion  based 
on  mutual  support  and  self-shading. 

On  the  other  hand,  oysters  in  the 
study  area  in  the  low  intertidal  zone  or 


subtidal  zone  are  characteristically  heav- 
ily fouled  and  colonized  with  boring 
sponges,  i.e.,  Cliona  spp.  These  oys- 
ters dre  usually  thick-shelled,  with  the 
stunted  shape  characteristic  of  slow  grow- 
ing oysters,  particularly  in  high-salinity 
areas.  It  is  obvious  that  relatively  few 
oysters  survive  in  the  subtidal  zone  in 
these  marsh-estuaries  and  that  dead  shells 
are  rapidly  eroded  away  by  Cliona  spp. 

Oyster  spatfall  may  be  so  dense  in 
some  low  latitude  areas  that  it  consti- 
tutes a  "fouling"  situation.  This  condi- 
tion (dense  spatfall)  has  not  been  ob- 
served on  an  intertidal  reef,  however. 
Neither  the  density  of  barnacles  nor  oys- 
ter spat  appears  to  be  limited  by  space  on 
a  reef.  This  is  perhaps  not  attributable 
to  a  lack  of  prospective  spat  but  rather 
to  the  predatory  effects  of  adult  members 
of  the  reef  community,  especially  filter 
feeders  like  mussels,  barnacles,  and  oys- 
ters themselves.  The  vortices  set  up  by 
the  feeding  currents  of  reef  community 
filter  feeders  could  make  the  reef  surface 
a  somewhat  dangerous  place  to  settle.  This 
type  of  density-dependent  feedback  could 
explain  the  relatively  even  distribution 
of  oysters  in  the  mature  reefs  and  the 
symmetrical  form  of  the  reefs. 


American  oyster  catchers  "loafing"  on  an  oyster  reef  in  South  Carolina.  These  birds, 
rare  over  most  of  their  range,  are  concentrated  in  coastal  South  Carolina  and  Georgia, 
feeding  primarily  on  reef  oysters.  They  are  year-round  residents  and  represent  one  of 
the  major  predators  to  the  oysters.  Photo  by  Wiley  M.  Kitchens,  U.S.  Fish  and  Wild- 
life Service. 


55 


"Fringing"  reefs  typical  of  those  lining  the  shores  of  tidal  creeks  of  high 
salinity  estuaries  in  coastal  South  Carolina.  Photo  by  Rhett  Talbert,  Univer- 
sity of  South  Carolina 


'^^•i:*S^^r 


Oyster  reefs  interspersed  in  channels  dissecting  an  intertidal  mud  flat, 
by  Rhett  Talbert,  University  of  South  Carolina. 

56 


Photo 


CHAPTER  4.  OYSTER  REEF  DEVELOPMENT,  DISTRIBUTION, 
PHYSICAL  EFFECTS,  AND  AREAL  EXTENT 


4.1  REEF  DEVELOPMENT 

From  a  physical  standpoint,  a  reef  is 
a  biologically  constructed,  wave-resistant 
or  potentially  wave-resistant  structure. 
Worldwide,  reefs  range  from  mounds  less 
than  1  m  in  height  and  diameter  to  massive 
structures  1,000  m  across  and  100  m  thick 
(Pettijohn  1975).  In  general,  reef  mor- 
phology is  a  function  of  the  constituent 
organisms  and  organism  byproducts  of  which 
it  is  built,  whether  these  organisms  are 
corals,  encrusting  or  sediment-binding 
algae,  tube-building  polychaetes,  or  oys- 
ters. 

The  thesis  presented  here  is  that  the 
location  of  oyster  reefs  in  the  salt 
marsh-estuarine  ecosystem  is  not  acciden- 
tal; rather,  it  is  the  result  of  interact- 
ing physical  and  biological  processes 
that,  if  fully  understood,  would  explain 
the  natural  distribution  of  reefs  in  a 
given  area.  Marshall  (1954)  concluded  from 
a  study  of  the  distribution  of  oyster  bars 
in  Alligator  Harbor,  Florida,  that  physio- 
graphic conditions  and  predation  were  the 
most  important  factors. 

In  terms  of  physical  conditions,  a 
minimum  stability  is  undoubtedly  required; 
that  is,  a  water  current  or  wave  energy 
regime  above  a  certain  threshold  level 
will  prevent  the  development  of  an  inter- 
tidal  oyster  reef.  At  the  same  time,  the 
development  of  a  reef  presumably  affects 
the  physical  stability  of  an  area  by  damp- 
ening current  velocity  and  wave  energy.  To 
be  viable,  a  reef  also  needs  a  minimum 
current  velocity  for  the  input  of  food  and 
the  export  of  waste  products.  The  local 
reef  area  could  thus  be  self-limited  by 
its  dampening  influence  on  the  current 
regime. 

The  following  general  model  of  oyster 
reef  initiation,  "ontogeny,"  and  decline 
has  four  stages:  (1)  initial  colonization, 
(2)  clustering  phase,  (3)  accretionary 
phase,  and  (4)  maturation  and  senescence. 


Initial  Colonization 

Initial  reef  formation  begins  with 
the  settlement  and  growth  of  single  oys- 
ters and  small  scattered  oyster  clusters 
within  the  lower  intertidal  zone.  A  suit- 
able substrate  must  be  present  for  the 
settlement  of  oyster  spat  and  initial  oys- 
ter growth  in  an  area  where  water  flow  is 
sufficient  to  prevent  stagnation  (Galtsoff 
and  Luce  1930).  Suitable  substrates  may 
consist  of  either  sand,  firm  mud,  or  clay. 
Shifting  sand  and  extremely  soft  mud  are 
the  only  bottom  types  totally  unsuitable 
for  oyster  communities  (Galtsoff  1964). 
Oyster  larvae  will  attach  to  any  hard 
object,  such  as  fallen  trees,  driftwood, 
bushes,  branches,  old  shell  material,  or 
discarded  solid  waste  (bottles,  cans, 
plastic,  etc.)  exposed  in  the  intertidal 
zone.  It  is  important  that  the  areas  be 
subject  to  little  sediment  deposition. 

Clustering  Phase 

With  time,  additional  generations  of 
oyster  larvae  will  settle  in  the  area  of 
the  new  reef  and  attach  themselves  to 
other  live  oysters  and  dead  shell  sur- 
faces. This  process  results  in  the  forma- 
tion of  distinct  oyster  clusters.  A  clus- 
ter is  a  small  colony  of  three  to  seven 
generations  of  oysters,  the  majority  of 
which  are  dead  (Grave  1905).  The  oldest 
and  lowest  oysters  in  the  cluster  die  from 
overcrowding  and  suffocation,  but  their 
shells  remain  to  support  the  upward  and 
outward  growth  of  the  cluster.  This  sup- 
port is  aided  by  the  relatively  flat  shape 
and  low  specific  gravity  of  oyster  shells. 

Accretionary  Stage 

Small  oyster  clusters  increase  in 
size  through  the  settlement  of  additional 
spat  and  eventually  coalesce,  forming 
larger,  massed  oyster  clusters  (Grinnell 
1971)  that  comprise  the  true  construc- 
tional nucleus  of  the  intertidal  oyster 
reef.   If  environmental  conditions  remain 


57 


stable,  the  newly  formed  reef  accretes 
laterally  and  vertically  within  the  inter- 
tidal  zone.  Dead  shell  material  scattered 
around  the  reef  aids  in  building  up  the 
channel  floor  or  reef  platform,  paving  the 
substrate  for  the  reef  to  spread  laterally 
(Wiedemann  1972).  Lateral  reef  accretion 
generally  occurs  in  a  direction  perpendic- 
ular to  tidal  currents  so  that  the  effec- 
tiveness of  currents  in  transporting 
nutrients  and  removing  fecal  material  is 
exploited  (Grave  1905;  Grinnell  1971). 

On  a  still  smaller  scale,  individual 
oysters  on  the  reef  surface  tend  to  orient 
themselves  so  that  their  planes  of  commis- 
sure (i.e.,  opening  between  the  valves) 
are  alined  roughly  parallel  to  the  current 
direction  (Lawrence  1971).  Lawrence  (1971) 
found  that  either  the  anterior  or  poste- 
rior shell  margin  may  face  the  oncoming 
current  direction,  a  fact  suggesting  that 
this  alinement  is  necessary  for  the  hydro- 
dynamic  stability  of  the  individual  oys- 
ters. The  macro-orientation  of  a  reef  with 
respect  to  the  local  current  regime  and 
the  micro-orientation  of  its  constituent 
oysters  are  only  demonstrable  where  the 
currents  are  uni-  or  bi-directional.  For 
example,  most  of  the  reefs  examined  by 
Bahr  (1974)  were  located  at  the  southern 
edge  of  Sapelo  Island  in  Doboy  Sound,  an 
area  with  multidirectional  currents,  and 
no  definite  macro-  or  micro-orientation 
was  observed. 

Vertical  accretion  continues  as  long 
as  the  upper  (living)  layer  of  the  oyster 
reef  remains  within  the  portion  of  the  in- 
tertidal  zone  in  which  oysters  are  viable. 
Bahr  (1976)  found  the  maximal  reef  height 
for  oysters  to  be  a  constant  feature  of 
the  intertidal  oyster  reefs  in  Doboy 
Sound.  No  reefs  in  this  area  exceeded 
72  cm  above  the  surrounding  mud  surface  or 
1.5m  above  mean  low  water. 

At  this  stage  of  development,  the 
reef  consists  of  an  approximately  1-m 
thick  accumulation  of  live  oysters,  dead 
shell,  and  mixed  shell  and  mud  (Figure 
13).  The  uppermost  portion  of  the  reef  is 
level,  sloping  off  steeply  at  the  edges. 
The  living  portion  of  the  reef  is  thicker 
at  the  edge  than  in  the  center  because  of 
mud  trapped  by  the  reef.  The  central  core 
of  the  reef  is  corposed  of  mixed  dead 
shell  and  mud.   If,  for  example,  the  reef 


is  formed  on  a  soft  mud  substrate,  its 
weight  will  cause  the  entire  structure  to 
slowly  subside  or  sink.  Vertical  upbuild- 
ing in  a  viable  reef  keeps  pace  with  grad- 
ual subsidence,  and  the  upper  reef  surface 
remains  at  a  steady  state  with  respect  to 
mean  water  level.  The  reefs  examined  by 
Bahr  (1974)  were  typical  of  this  stage  of 
development. 

Senescent  Stage 

A  senescent  stage  of  intertidal  oys- 
ter reef  development  is  reached  when  the 
upper  surface  of  the  reef  can  no  longer 
accrete  vertically  and  the  majority  of 
live  oysters  populate  only  the  flanks  of 
the  reef.  The  mature  reef  will  have  a  bar- 
ren central  zone,  or  ridge  in  the  case  of 
long  linear  reefs,  comprising  dead  shell 
and  various  sized  fragments  of  shell.  The 
barren  central  region  has  been  referred  to 
as  a  "hogback"  (Gunter  1979)  or  flatland 
surface  (Grinnell  1971).  Gunter  (1979) 
suggests  that  for  gulf  coast  reefs  the 
constant  motion  or  saltation  of  fine  shell 
"grit"  in  the  central  zone  prevents  the 
survival  of  new  oyster  spat,  so  that  this 
area  remains  void  of  organisms.  This 
"grit  theory"  would  not  hold,  however,  for 
the  smaller,  relatively  sheltered  reefs  in 
the  environment  of  the  salt  marsh  estuary. 

An  extension  to  the  senescent  stage 
of  reef  development  was  proposed  by  Grave 
(1905).  He  suggested  that  with  time,  the 
barren  central  "flatland"  surface  would  be 
built  up  with  thicker  accumulations  of 
sand,  mud,  and  shell  debris,  and  would  be 
colonized  by  Spartina.  The  reef  would  then 
become  an  oyster  marsh  island,  with  a 
length  and  width  greater  than  that  of  the 
original  oyster  reef,  and  surrounded  by  a 
thin  band  of  intertidal  oysters.  Little 
Egg  Island  in  the  mouth  of  the  Altamaha 
River  in  Georgia  may  be  an  example  of  such 
an  oyster-formed  island. 


4.2   DISTRIBUTION  OF  OYSTER  REEFS  IN  THE 
MARSH-ESTUARINE  ECOSYSTEM 

This  section  includes  some  specula- 
tive material  that  remains  to  be  confirmed 
by  scientific  study.  There  is,  however, 
ongoing  research  at  Sapelo  Island,  Geor- 
gia, that  should  help  explain  the  observed 
distribution  of  reefs  in  the  ecosystem 


58 


(S.  Stevens,  University  of  Georgia  Marine 
Institue,  Sapelo  Island,  Georgia;  pers. 
comm. ) . 

Present  Distribution 

Current  speed  and  bottom  roughness 
have  been  theorized  as  controlling  the 
distribution  of  estuarine  suspension-feed- 
ing macrobenthos  (Wildish  and  Kristmanson 
1979).  The  distribution  of  intertidal  oys- 
ter reefs  in  the  study  area  is  described 
in  terms  of  the  three  hydrographic  zones 
of  the  estuary  (see  Section  1.2).  The 
zones  are  (1)  the  lower  sound  and  inlet 
areas  between  barrier  islands;  (2)  the 
middle  region  of  the  estuary,  including 
the  major  rivers  feeding  the  sounds;  and 
(3)  smaller  tidal  creeks  draining  the 
marshes  (Figure  2). 

The  typically  high  energy  regime  and 
sedimentary  instability  of  the  lower  sound 
region  render  this  zone  the  least  favor- 
able for  reef  development.  Where  reefs 
are  found  in  the  lower  sound  areas,  they 
presumably  indicate  local  pockets  of  shel- 
ter from  storm  surges. 

From  the  lower  to  middle  estuarine 
zone,  wave  energy  is  probably  the  control- 
ling factor.  The  middle  zone  is  charac- 
terized by  an  optimum  current  regime  for 
reefs;  the  regime  of  the  lower  zone  is  too 
turbulent,  and  the  upper  zone  is  too  slug- 
gish. Oyster  reefs,  sometimes  exceeding  a 
kilometer  in  length,  in  the  middle  estua- 
rine zone  are  predominantly  (but  not  ex- 
clusively) oriented  along  the  banks  of 
rivers.  Circular  reefs  and  oyster  reef 
islands  also  occur  infrequently  in  this 
zone.  Many  reefs  in  the  middle  estuarine 
zone  are  near  the  entrances  to  small  tidal 
creeks  that  feed  the  larger  rivers.  This 
orientation  is  not  accidental  and  may 
indicate  the  importance  of  slight  differ- 
ences in  current  regimes,  which  are  en- 
hanced at  the  confluence  of  water  bodies. 

The  complex  network  of  tidal  creeks 
and  small  rivers  that  drain  the  marshes  is 
also  an  area  of  significant  oyster  reef 
development.  The  distribution  of  inter- 
tidal oyster  reefs  within  this  zone  is 
perhaps  the  most  consistent  and  predict- 
able of  the  three  estuarine  subdivisions. 
The  pattern  of  oyster  reef  development  and 
tidal  creek  meander  systems  are  strongly 


correlated.  Oyster  reefs  are  likely  to 
occur  in  three  zones  within  a  tidal  creek 
system  (Figure  17):  (1)  on  the  concave 
outer  banks  of  meander  loops,  (2)  in  areas 
immediately  adjacent  to  smaller  tidal 
tributaries,  and  (3)  at  points  of  tidal 
stream  confluence. 

The  oyster  reef  tendency  to  develop 
on  the  concave  outer  banks  of  tidal  creeks 
is  predictable  from  the  hydrography  of 
stream  meanders.  The  outer  or  cut-bank  of 
the  meander  loop  is  the  zone  of  highest 
current  velocity  within  the  channel.  The 
sediment  substrate,  therefore,  tends  to 
consist  of  firm,  consolidated  mud,  swept 
clean  of  soft  mud  and  slime  unsuitable  as 
a  spat  settlement  surface.  Once  the  reef 
colony  is  established,  these  higher  veloc- 
ity currents  provide  nutrients  and  remove 
fecal  matter.  Keck  et  al.  (1973)  discussed 
this  same  relationship  between  meander 
morphology  and  oyster  distribution  in  the 
Murderkill  River,  Delaware.  Reefs  in  that 
region  tend  to  form  in  areas  adjacent  to 
smaller  tidal  tributaries  where  important 
marsh-derived  nutrients  are.  Oyster  reefs 
at  points  of  tidal  stream  confluence  are 
also  influenced  by  hydrographic  factors. 
During  flood  tide,  the  confluence  of  flow 
between  the  two  tidal  creeks  results  in  a 
zone  of  circular  back-eddy  formation  lo- 
cated at  the  point  bar  (Figure  17).  The 
turbulence  associated  with  this  process 
provides  nutrients  to  the  reef.  During 
flood  tide,  the  point  bar  is  an  area  of 
relatively  higher  current  velocity  and 
little  deposition. 

Historical  Changes  in  Reef  Distribution 

Four  surveys  of  intertidal  oyster 
reefs  along  the  Georgia  coast  demonstrate 
changes  in  oyster  distribution  from  1889 
to  1977.  These  are  Drake  (1891),  Galtsoff 
and  Luce  (1930),  Linton  (1968),  and  Harris 
(1980).  The  survey  results  reveal  two 
aspects  of  the  change  in  oyster  reef  dis- 
tribution over  time:  a  change  in  total 
reef  area,  and  local  changes  (increases  or 
decreases)  in  specific  areas. 

Galtsoff  and  Luce  (1930)  reported  few 
significant  changes  occurring  in  the  dis- 
tribution and  extent  of  natural  oyster 
beds  between  the  years  1889  (Drake  1891) 
and  1925.  They  reported,  however,  a  de- 
cline in  the  health  of  many  intertidal 


59 


Figure  17.  Typical  distribution  of  oyster  reefs  in  small  tidal  creeks. 
Zones  are  (1)  concave  outer  banks  of  meander  loops,  (2)  areas  adjacent 
to  tidal  tributaries,  and  (3)  tidal  stream  confluence. 


60 


oyster  communities,  noting  in  a  number  of 
cases,  nothing  but  silt-covered,  dead  oys- 
ter shells  remained  of  once-productive 
oyster  reefs.  This  historical  decline  in 
the  welfare  of  the  intertidal  oyster  com- 
munity is  further  supported  by  the  most 
recent  survey  of  Harris  (1980),  Total 
acreage  of  the  intertidal  oysters  has 
decreased  dramatically  from  approximately 
688  ha  (1,700  acres)  in  1889  to  less  than 
121  ha  (300  acres)  in  1977  (Harris  1980). 
Large  areas  of  dead  oyster  shell  were  also 
reported  in  the  1977  survey.  Harris  re- 
lated the  steady  decline  of  the  Georgia 
commercial  oystering  industry  to  the  de- 
crease of  total  oyster  acreage.  In  addi- 
tion, there  is  reason  to  believe  that  the 
acreage  figures  reported  by  Harris  (1980) 
are  somewhat  exaggerated,  perhaps  because 
they  were  partly  based  on  aerial  imagery 
that  did  not  permit  easy  distinction  be- 
tween living  reefs  and  dead  shells.  For 
example,  Harris  reported  a  total  reef  area 
of  9,632  m^  in  the  Duplin  River;  Bahr 
(1974)  reported  6,040  m^  of  living  oyster 
reefs  in  the  same  river  based  on  a  ground- 
level  survey. 

Intertidal  oyster  populations  in 
South  Carolina  have  apparently  also  de- 
clined during  the  same  period.  We  are  un- 
able at  present  to  attribute  this  decline 
to  any  specific  factor.  It  may  be  the 
result  of  a  slow  shifting  of  ecological 
conditions  that  reflect  a  natural  succes- 
sional  pattern  in  the  marsh-estuarine  eco- 
system (e.g.,  sea  level  change).  Puffer 
and  Emerson  (1953)  cited  natural  cyclic 
changes  in  environmental  conditions — pri- 
marily temperature  and  salinity--as  the 
cause  of  oyster  reef  death  and  subsequent 
repopulation  in  Aransas  Bay,  Texas.  Alter- 
natively, this  decline  may  be  the  result 
of  a  man-induced  perturbation  of  the 
marsh-estuarine  ecosystem,  such  as  dredg- 
ing, waterway  construction,  pollution,  or 
overharvesting. 

It  is  easy  to  explain  a  decline  in 
oyster  reefs  near  population  and  indus- 
trial centers  such  as  Savannah,  Georgia, 
but  it  is  much  more  difficult  to  account 
for  a  decline  of  reef  area  in  the  more 
pristine  part  of  the  Georgia  coast  near 
Sapelo  Island. 

The  salinity  of  the  Duplin  River  at 
Sapelo  Island,  Georgia,  appears  to  have 


increased  recently  (B.  J.  Kjerfve,  Univer- 
sity of  South  Carolina,  Columbia;  pers. 
comm. ).  This  salinity  increase  could  be 
caused  by  a  reduction  in  ground  water 
inputs  due  to  consumptive  losses  resulting 
from  pumping  for  agricultural  irrigation. 
This  change  could  partly  explain  the  grad- 
ual decline  in  viable  oyster  reef  area  in 
the  Duplin  River  and  in  other  parts  of  the 
study  area,  although,  it  is  not  clear  how 
a  salinity  increase  up  to  25  /oo  or  30  /oo 
would  affect  the  reef  community. 

With  respect  to  local  changes  in  reef 
distribution,  it  is  possible  to  find  exam- 
ples of  reef  area  increases  in  some  spe- 
cific portions  of  the  Georgia  coast.  For 
example,  in  Altamaha  Sound,  Georgia,  oys- 
ter reefs  have  developed  in  areas  farther 
inland  in  the  lower  sound  than  they  oc- 
curred in  1889  (Figure  21).  Associated 
with  this  shift  in  reef  distribution  is 
the  accretion  of  marsh  islands  in  south- 
ern Altamaha  Sound.  The  accretion  of 
marsh  and  marsh  islands  may  relate  to  the 
sediment-trapping  capacity  of  intertidal 
oyster  reefs  (Grave  1905;  Wiedemann  1972; 
Stephens  et  al.  1976).  The  growth  of 
intertidal  oyster  reefs  farther  inland  of 
the  lower  sound  may  relate  to  shifting 
salinity  conditions  in  Altamaha  Sound. 

In  summary,  reef  distribution  along 
the  Georgia  coast  surprisingly  has  changed 
little  over  the  last  90  years.  Oyster 
reefs  occur  (in  general)  today  in  approx- 
imately the  same  locations  where  they 
occurred  in  1889  (see  Figure  21).  The 
living  oyster  reef  area,  however,  signifi- 
cantly has  declined  in  the  same  period. 


4.3  THE  PHYSICAL  EFFECTS  OF  OYSTER  REEFS 
ON  THE  MARSH-ESTUARINE  ECOSYSTEM 

Hypothetical ly,  reefs  affect  the 
physiography  and  hydrologic  regime  of  salt 
marsh  estuaries  three  ways:  by  modifying 
current  velocities,  both  positively  and 
negatively;  by  passively  changing  sedimen- 
tation patterns;  and  by  actively  augment- 
ing sedimentation  through  biodeposition. 

Interpretation  of  reef  effects  on  the 
ecosystem  over  time  from  analyses  of  sur- 
vey data  of  the  last  century  is  difficult 
because,  although  90  years  is  a  long  bio- 
logical time,  it  is  short  geologically. 


61 


For  example,  the  average  sediment  deposi- 
tion rate  in  the  study  area  is  less  than  4 
mm/yr  (Letsch  and  Frey  1980).  This  means 
that  from  the  years  since  the  first  reef 
survey  in  1889,  theoretically  only  about 
one-third  of  a  meter  of  sediment  has 
accumulated. 

Oyster  reefs  undoubtedly  dampen  tidal 
current  velocities  over  the  entire  ecosys- 
tem because  of  friction,  but  the  magnitude 
of  the  drag  coefficient  of  a  unit  area  of 
reef  is  unknown,  as  is  the  overall  effect. 
Reefs  also  augment  current  velocity  in  lo- 
cal areas  by  constricting  tidal  streams, 
but  no  quantitative  data  are  available  to 
detail  the  specific  effects. 

Grave  (1905)  noted  that  oyster  reefs 
are  wave-  and  current-resistant  structures 
that  exert  a  physical  influence  over  the 
marsh  system.  He  observed  that  small  reefs 
originating  at  points  along  a  tidal  stream 
accrete  laterally  across  the  stream  (into 
the  current),  and  by  displacing  and  con- 
stricting the  current  cause  erosion  of  the 
opposite  marsh  bank.  This  process  may  re- 
sult in  the  formation  of  marsh  islands. 

Passive  sedimentation  due  to  the 
presence  of  reefs  is  qualitatively  obvious 
but  has  not  been  quantified.  The  magni- 
tude of  this  effect  would  be  related  to 
the  overall  reduction  in  tidal  current 
velocities  and  turbidity  levels.  Active 
sedimentation  through  biodeposition  can  be 
estimated  (see  Appendix).  The  biological 
process  of  aggradation  increases  the  size 
of  suspended  particles  and  increases  their 
effective  settling  rates.  The  dominant 
oyster  reef  zone's  coinciding  with  the 
maximum  turbidity  zone  in  estuaries  in 
the  study  area  indicates  that  this  effect 
may  be  significant.  Lund  (1957a)  reported 
that  oysters  biodeposited  or  "self-silted" 
eight  times  the  volume  of  sediment  in  test 
containers  than  would  have  deposited  in 
the  same  time  due  to  gravity  alone.  He 
calculated  that  a  uniform  single  layer  of 
oysters  in  a  natural  setting  with  rela- 
tively low  turbidity  water  could  biode- 
posit  sediment  at  a  rate  of  about  280 
tons/acre/yr  (6  x  lO**  g/m^/ yr). 

4.4  AREAL  EXTENT  OF  OYSTER  REEFS  IN  THE 
COASTAL  ECOSYSTEM 

The  most  obvious  criterion  by  which 
to  assess  the  importance  of  oyster  reefs 


on  the  marsh-estuarine  ecosystem  is  the 
relative  proportion  of  reef  surface  area 
to  the  total  surface  area  of  the  system. 
Planimetry  on  maps  of  the  Georgia  coastal 
zone  (Galtsoff  and  Luce  1930)  indicated 
that  the  total  intertidal  and  subtidal 
zones  of  the  entire  area  occupied  approxi- 
mately 1.8  X  lO^m^.  Of  this  area,  approxi- 
mately 75%  was  marsh  and  tidal  creeks,  and 
25%  was  open  water  (wider  than  about  350 
m).  The  linear  extent  of  the  oyster  reefs 
measured  about  403,000  m.  If  the  average 
reef  were  estimated  as  2  m  in  width,  the 
total  reef  area  in  1925  would  have  com- 
prised about  8  X  10^  m,  or  0.04%  of  the 
marsh-estuarine  area.  If  the  mean  reef 
width  were  3  m,  reef  area  would  increase 
to  1.2  X  106  m2^  or  0.06%,  Harris  (1980) 
estimated  that  the  total  viable  reef  area 
in  the  Georgia  coastal  zone  in  1977  was 
equal  to  102  ha,  or  about  0.05%  of  the 
marsh-estuarine  area.  This  presumably  rep- 
resents a  decline  from  1889,  when  Drake 
(1890)  estimated  that  6.8  x  10&  square 
meters  of  reefs  existed,  or  0.3%  of  the 
total  marsh  estuarine  zone  was  occupied  by 
oyster  reefs.  In  a  detailed  survey  of  the 
Duplin  River  drainage  basin,  Bahr  (1974) 
estimated  that  about  0.06%  of  the  marsh 
estuarine  zone  was  occupied  by  viable 
reefs. 

The  absence  of  quantitative  informa- 
tion about  the  areal  extent  of  intertidal 
oyster  reefs  in  South  Carolina  and  north- 
eastern Florida  does  not  allow  a  compari- 
son with  Georgia.  Apparently  oyster  reefs 
comprise  a  larger  percentage  of  the  marsh 
estuary  in  the  South  Carolina  area  than  in 
Georgia,  but  the  relative  difference  is 
unknown.  A  detailed  analysis  of  the  rela- 
tionship between  reef  area  and  tidal 
amplitude  in  the  study  area  would  be 
interesting.  A  small  area  of  the  Savannah 
River  basin  in  South  Carolina  surveyed  by 
McKenzie  and  Badger  (1969)  indicated  an 
extremely  high  oyster  reef  density  (9%), 
Lunz  (1943)  reported  an  extremely  high 
density  of  reefs  along  a  1-mi  wide  and 
40-mi  long  strip  surrounding  the  intra- 
coastal  waterway  in  South  Carolina  from 
Charleston  to  the  Santee  River,  He  report- 
ed that  33,6%  of  the  total  creek  banks  was 
lined  with  reefs,  Lunz  (1943)  also  report- 
ed that  these  reefs  were  populated  by 
about  136  oysters/yd^,  (or  about  114/m  ) 
of  2-inch  (50-mm)  or  larger  sized  oysters. 
This  represents  a  biomass  of  approximately 
50  g/m^  afdw,  much  lower  than  that  for  the 


62 


more  mature  reefs  described  in  Georgia. 
Lehman  (1974)  reported  that  oyster  reefs 
in  the  Crystal  River  estuarine  ecosystem 
in  West  Florida  occupied  about  3%  of  the 
total  surface  area. 

To  put  these  various  estimates  in 
perspective,  it  must  be  remembered  that 
different  survey  techniques  were  used,  and 
that  some  subjectivity  is  involved  in  dis- 
tinguishing viable  reefs  from  areas  of 
dead  shell.  Whether  or  not  a  major  de- 
cline in  oyster  reefs  has  occurred  since 


1899,  the  present  proportion  of  reef  area 
to  marsh-estuarine  area  throughout  the 
study  area  appears  to  be  between  0.04%  and 
0.06%,  with  some  local  variation.  The  reef 
community's  occupying  such  a  small  propor- 
tion of  the  total  marsh-estuarine  area  may 
reflect  both  the  very  specific  physico- 
chemical  requirements  of  the  reef  commun- 
ity and  the  limited  productive  capacity  of 
the  total  system  in  supporting  the  high, 
heterotrophic  demands  of  the  oyster  com- 
munity. 


63 


An  example  of  Spartina  marsh  invading  the  top  levels  of  an  oyster  reef.  Photo  by 
Rhett  Talbert,  University  of  South  Carolina. 


64 


CHAPTER  5.  CONCEPTUAL  MODELS  OF  THE  INTERTIDAL 
OYSTER  REEF  COMMUNITY 


5.1   OBJECTIVES  AND  LEVELS  OF  RESOLUTION 

This  chapter  summarizes  some  conclu- 
sions, primarily  qualitative,  about  the 
significance  of  oyster  reefs  to  the  coast- 
al ecosystem  in  the  study  area.  The  sum- 
mary is  in  the  form  of  a  set  of  three  con- 
ceptual models  that  are  explicit  diagram- 
matic illustrations  of  the  interactions 
among  oyster  reefs  and  other  salt  marsh 
ecosystem  components.  Conceptual  models 
can  provide  succinct,  qualitative  expres- 
sions of  the  feedback  pathways,  forcing 
functions,  and  major  interconnections 
characterizing  a  particular  ecosystem. 
Conceptual  models  are  usually  over-simpli- 
fications of  the  real  world,  but  their 
formulation  may  indicate  deficiencies  of 
information  that  can  become  future  re- 
search goals.  Conceptual  models  take  a 
variety  of  forms,  from  simple  box  and  ar- 
row diagrams  to  detailed  and  complex  "spa- 
ghetti" diagrams  that  are  difficult  to 
interpret.  Figure  18  (from  Odum  1971) 
illustrates  one  conceptual  model  of  an 
oyster  reef  that  compares  it  in  functional 
terms  to  a  city. 

Oyster  reef  organization  and  function 
must  be  considered  at  different  levels  of 
space  and  time,  and  our  conceptual  models 
are  presented  at  three  (hierarchical)  lev- 
els of  resolution:  a  regional  level,  a 
drainage  unit  level,  and  a  reef  level 
(Figure  19).  The  regional  level  model 
treats  the  oyster  reef  system  over  the 
entire  study  area  or  a  large  portion  of 
the  study  area.  At  the  regional  level, 
detailed  reef  community  information  is 
relatively  unimportant  compared  with  that 
of  long-term  geological  processes  affect- 
ing regional  ecology.  The  relative  propor- 
tions of  salt  marsh,  open  water,  and  total 
reef  area  and  patterns  of  their  spatial 
distribution  are  particularly  significant 
at  the  regional  level  since  these  factors 
are  regulated  by  long-term  geological  pro- 
cesses. 

The  second  level  of  resolution  is  on 
a  smaller  and  more  detailed  scale — that  of 


a  single  marsh-estuarine  drainage  unit. 
For  example.  Figure  20  shows  the  oyster 
reef  distribution  in  the  Half  Moon  River 
estuary  on  Wilmington  Island,  Georgia. 
This  tidal  river  and  its  surrounding  salt 
marsh  watershed  exemplify  a  "typical"  lo- 
cal drainage  unit  in  which  oyster  reefs 
are  distributed  in  a  nonrandom  pattern. 
At  this  intermediate  scale  of  resolution, 
the  reef  community  is  more  visible  than  at 
the  regional  level  and  presumably  exerts  a 
more  profound  short-term  influence  on  the 
local  ecosystem.  Another  example  of  the 
resolution  achievable  at  this  level  may  be 
seen  in  Figure  21.  The  information  content 
at  this  scale  is  such  that  only  broad  spa- 
tial patterns  of  reef  distribution  within 
the  marsh-estuarine  ecosystem  are  discern- 
able.  The  perspective,  then,  is  an  "over- 
view." At  scales  smaller  than  this  (great- 
er resolution),  the  oyster  reef  system  is 
obscured. 

The  third  conceptual  level  of  resolu- 
tion is  of  a  discrete  reef  and  its  immedi- 
ate surroundings.  At  this  level,  a  reef 
can  be  considered  analogous  to  an  individ- 
ual in  a  "population"  of  reefs,  each  mem- 
ber being  influenced  by  local  forcing 
functions--hydrologic  forces,  short-term 
episodic  events,  and  biological  phenomena, 
such  as  spawning  events  and  predation. 
An  individual  reef  is  subject  to  local 
phenomena,  and  its  influence  is  primarily 
restricted  to  its  immediate  surroundings. 
The  purpose  of  the  third  level  conceptual 
model  is  to  summarize  the  specific  phenom- 
ena regulating  the  welfare  of  a  given 
reef.  The  cumulative  effects  of  the  "pop- 
ulation" of  reefs  in  a  drainage  basin  are 
addressed  at  the  drainage  unit  level. 

Some  important  differences  among  the 
above  three  conceptual  levels  of  organi- 
zation and  function  of  oyster  reefs  in  the 
study  area  are  summarized  in  Table  8.  The 
three  different  scales  of  resolution  are 
discussed  in  Sections  5.2,  5.3,  and  5.4. 

Symbols  used  in  the  models  were  de- 
veloped by  H.T.  Odum  (1971)  as  a  shorthand 


65 


Figure  18.  Comparison  of  two  systems  of  concentrated  consumers  whose  survival 
depends  on  strong  flows  that  bring  in  fuels  and  oxygen  and  outflow  wastes:  (a) 
reef  of  oysters  and  other  marine  animals  characteristic  of  many  estuaries;  (b) 
industrialized  city  (adapted  from  Odum  1971). 


66 


DUPLIN  RIVER 


DRAINAGE  BASIN  LEVEL 


INDIVIDUAL  REEF  LEVEL 


Figure  19.     Three  hierarchical   levels  of  oyster  reef  organization. 


57 


Figure  20.  Reef  distribution  in  a  single  drainage  basin,  the  Half  Moon  River 
Estuary,  Wilmington  Island,  Georgia.  Reefs  are  indicated  by  bold,  black  lines. 


68 


Figure  21.  Recent  and  historical  reef  distribution  in  the  Duplin  River  Estuary, 
Sapelo  Island,  Georgia  (adapted  from  Bahr  1974  and  Drake  1891). 


69 


Table  8.  Time  scales  relating  ecosystem  processes  and  components  at 
the  three  conceptual  levels  of  oyster  reef  organization  and  function. 


Regional  level 

Drainage  unit  level 

Reef  level 

Factors 

Approximate  time  scale 

1x10^  to  1x10^ 

yr 

1  to  100  +  yr 

<1  to  25  yr 

System 
components 


Intertidal  area 
Marsh  area 
Reef  area 
Mudflat  area 
Water  surface  area 


Wetland  area 
Water  area 

Phytoplankton  biomass 
Reef  area 
Reef  biomass 
Suspended  load 


High  (mature)  reef  area 
Low  (mature)  reef  area 
Suspended  load  (POC  and 
inorganic  carbon) 
Reef  biomass 
Predator  component 
Oyster  larvae 
Nutrient  pool 


Forcing  Sea  level   rise 

functions  Latitude-tidal 

pattern 

Lati  tude-temperature 

regime 

Riverine  sediment 

input 

Marine  input-salts 

Marine  inputs-storm 

energy 


Solar 

Tidal 

currents 

Sediment, 

riverine 


insolation 

and  wind-driven 


marine  or 


Local  tidal  regime 
(amplitude  and  period) 
Currents  (tidal  and  wind) 
Temperature  effects 
Sediment  input 
POC  input 


Important 
system 
processes 
related  to 
reefs 


Areal  trade  offs 
among  wetlands, 
waterbodies,  and 
reefs 


Physiographic  changes 
in  basin  caused  by 
reefs 


Reef  growth  (vertical) 

Reef  growth  (lateral ) 

Water  clearance  and 

biodeposition 

Mineralization  and  nutrient 

release 

Hydrologic  damping  by  reefs 


70 


for  expressing  the  functional  connections 
in  many  different  kinds  of  systems  to  com- 
pare these  systems  in  thermodynamic 
(energy  flow)  terms.  Odum  calls  the  short- 
hand "energese,"  and  it  is  becoming  more 
popular,  as  evidenced  by  its  increasing 
use  in  published  reports.  This  shorthand 
"language"  is  flexible  and  information- 
rich,  and  it  can  be  used  in  both  qualita- 
tive conceptual  models  and  in  quantitative 
"working"  models.  The  symbols  are  defined 
in  Figure  22,  taken  from  Odum  (1971). 


5.2    REGIONAL   LEVEL   CONCEPTUAL   MODEL 

The  regional  level  model  of  oyster 
reef  function  in  the  study  area  is  broad 
in  its  coverage  and  necessarily  quite 
simple.  At  this  level  of  resolution,  oys- 
ter reefs  were  probably  not  a  major  factor 
in  the  geomorphological  development  of  the 
area,  although  their  wide  surface  distri- 
bution and  largely  unknown  subsurface 
(fossil)  distribution  indicate  that  they 
indeed  may  have  played  a  geological  role. 
No  one  has  as  yet  quantified  the  physical 
importance  of  oyster  reefs  to  long-term 
coastal  processes. 

In  Figure  22  we  illustrate  the  theo- 
retical role  of  oyster  reefs  at  this  broad 
regional  scale.  As  indicated  in  Table  8, 
the  time-scale  of  change  at  the  regional 
level  is  in  the  geological  range,  outside 
the  realm  of  control  of  environmental 
managers  (although  not  immune  to  cultur- 
ally induced  alteration). 

The  major  process  symbolized  in  the 
regional  scale  conceptual  model  is  the 
dynamic  tradeoff  in  area  between  inter- 
tidal  and  subtidal  zones.  Oyster  reefs 
primarily  are  distributed  at  the  interface 
between  these  two  zones,  and  thus  the  reef 
"fringe"  partially  reflects  the  outline  of 
the  marsh-water  interface  throughout  the 
study  area.  Changes  in  the  position  of 
this  outline  are  a  function  of  such  long- 
term  processes  as  subsidence,  sea  level 
rise,  and  sedimentation  regimes.  For  all 
practical  purposes,  reef  distribution  at 
the  regional  level  can  be  considered  spa- 
tially homogeneous. 

Interactions  between  the  intertidal 
and  subtidal  zones  are  described  in  the 


order  of  the  work  gates  (1-4)  shown  in 
Figure  22. 

(1)  A  gradual  and  persistent  rise  in 
sea  level  (about  4  mm/yr)  has 
occurred  since  the  relative  sta- 
bilization of  mean  water  level 
(MWL)  following  the  last  ice 
age.  This  has  resulted  in  a  con- 
stant encroachment  upon  the  in- 
tertidal zone  by  open  water.  In 
the  absence  of  other  processes, 
the  intertidal  zone  would  even- 
tually become  open  water. 

(2)  The  loss  of  intertidal  area  is 
accelerated  by  erosion  from 
strong  tidal  currents  and  storm 
surges. 

(3)  Losses  of  intertidal  habitat  are 
offset  in  most  undisturbed  por- 
tions of  the  study  area  by  in- 
puts of  sediment  from  rivers 
and/or  from  the  marine  system. 
This  sedimentation  process  is 
augmented  by  increases  in  the 
volume  of  estuarine  basins  as  a 
function  of  sea  level  rise.  Mean 
water  current  velocities  decline 
as  volume  increases,  and  sedi- 
mentation is  enhanced. 

(4)  Latitude  determines  tidal  ampli- 
tude in  the  study  area,  which, 
in  conjunction  with  sediment 
sources,  regulates  the  deposi- 
tional  patterns. 


5.3  DRAINAGE  UNIT  LEVEL  CONCEPTUAL  MODEL 

The  components  and  interrelationships 
of  a  marsh  estuary  drainage  unit  including 
and  affected  by  oyster  reefs  are  shown  in 
Figure  23.  A  major  assumption  at  this  lev- 
el of  resolution  is  that  there  is  an  opti- 
mum ratio  of  wetlands  and  open  water 
which,  in  conjunction  with  tides,  support 
the  oyster  reef  area  in  a  given  drainage 
basin.  One  implication  of  this  assumption 
is  that  relative  reef  area  in  a  given 
drainage  unit  is  limited  by  ecosystem  lev- 
el processes,  (e.g.,  the  relationship  be- 
tween the  velocity  of  tidal  currents,  the 
cross-sectional  area  of  tidal  creeks,  and 
the  distribution  of  reefs).  This  thesis  is 


71 


WETLAND  AREA 
MUDFLAT  AREA 
OYSTER  REEF  AREA  1 


TOTAL 
INTERTIDAL 
AREA 


Driving  force  or  onergv  source  -  indicates  8  lource  of  energy  outtide  the  tyitem  under  consideration.    Example:  Staady 
flowing  source  -  rive rjvari able  source  -  sunlight. 

^  X     X^  ^    Energy  or  material  storage  tank  —  indicates  passive  storage  of  energy  or  matter  within  the  system.   Example:  energy  itorad 
^\^^        /        in  a  water  tank:  water  contained  with  an  estuarine  basin. 

Interaction  or  work  gate  -  indicates  the  interaction  of  two  or  more  types  of  energy  required  for  a  process.   Example:  fer- 
tilizer requirements  for  plant  growth. 

Production  unit  or  green  plants  —  indicates  the  processes,  interactions,  storage,  etc.,  involved  in  producing  high-quality 
energy  from  dilute  sources  like  sunlight.   Example:   biomass  of  green  plants. 


^ 


>l/  HMt  sink  —  energy  lottet  to  heat  according  to  the  Mcond  law  of  tharmodynamics. 


Figure  22.     Regional   level   conceptual  model   and  explanation  of  symbols, 


72 


Figure  23.  Drainage  unit  level  conceptual  model. 


73 


supported  by  the  relatively  static  distri- 
bution of  reefs  within  the  Duplin  River 
basin  over  time,  shown  in  Figure  21. 

Specific  interactions  shown  in  Figure 
23  are  described  below: 

(1)  The  local  tidal  regime  is  the 
primary  forcing  function  for 
oyster  reef  distribution  (and 
relative  area)  in  a  given  salt 
marsh  drainage  unit.  The  tidal 
effect  is  shown  interacting  si- 
multaneously with  water  area  and 
wetland  area.  These  respective 
components  (water  and  wetlands) 
have  a  1  to  2  ratio  in  the  Geor- 
gia marsh-estuarine  ecosystem 
(Pomeroy  and  Wiegert  1980).  The 
pattern  of  distribution  of  oys- 
ter reefs  in  the  Duplin  River, 
as  shown  in  Figure  21,  is  prob- 
ably not  a  chance  distribution. 
For  example,  oyster  reefs  are 
absent  from  the  upper  one-fourth 
of  the  basin,  probably  because 
of  ecosystem  level  processes 
(e.g.,  a  function  of  reduced 
current  velocities  in  the  upper 
reaches  of  the  river). 

(2)  Oyster  reef  area  in  a  given  lo- 
cale can  affect  local  turbidity 
levels  by  filtration  and  bi ode- 
position.  By  stabilizing  and 
elevating  sediment,  wetland  de- 
velopment can  be  enhanced.  Marsh 
grass  and  oyster  reefs  have  a 
reciprocal  functional  relation- 
ship in  that  reefs  develop  al- 
most exclusively  at  the  inter- 
face between  wetland  and  water. 
There  they  subsequently  grow  and 
trap  sediment,  eventually  becom- 
ing colonized  by  Spartina.  The 
marsh  invades  formerly  subtidal 
areas  in  this  leapfrog  fashion. 
For  example,  subsurface  (fossil) 
oyster  reefs  occur  in  a  pattern 
of  increasing  depth  extending 
from  an  existing  reef  into  the 
marsh.  (S.  Stevens,  University 
of  Georgia  Marine  Institute, 
Sapelo   Island;   pers.   comm. ). 

(3)  Suspended  materials  in  water 
column  inhibit  the  primary  pro- 
duction by  phytoplankton  as  a 


result  of  shading.  Therefore, 
oyster  reefs  theoretically  aug- 
ment phytoplankton  productivity 
by  actively  filtering  these 
materials  and  thereby  reducing 
turbidity. 

(4)  Oyster  reefs  in  local  areas  also 
contribute  to  primary  production 
(especially  of  phytoplankton  and 
benthic  algae)  by  rapidly  miner- 
alizing ingested  organic  matter 
into  usable  plant  nutrients. 
Kuenzler  (1961)  showed  that  the 
regeneration  of  phosphorus  by 
mussels  in  the  salt  marsh  was 
more  important  than  their  role 
in  energy  transformation.  Kit- 
chell  et  al.  (1979)  discussed 
the  roles  of  consumers  in  nutri- 
ent cycling.  Oyster  reefs  by 
Interactions  (3)  and  (4)  can 
increase  food  availability,  pro- 
viding feedback  in  keeping  with 
ecosystem  theory,  (e.g.,  Odum 
1971). 

(5)  Tidal  currents  maintain  extreme- 
ly high  suspended  sediment  loads 
in  some  study  area  estuaries, 
like  the  Duplin  River  (Hanson 
and  Snyder  1979).  The  conse- 
quences of  this  siltation  relate 
to  Interactions  (2)  and  (3). 


5.4  REEF  LEVEL  CONCEPTUAL  MODEL 

The  third  conceptual  model  is  shown 
in  Figure  24,  where  reef  development  is 
expressed  as  growth  in  three  dimensions: 
(1)  upward  toward  the  high  intertidal 
zone,  (2)  downward  toward  the  subtidal 
zone,  and  (3)  lateral  accretion. 

The  interactions  involved  in  such 
changes  are  described  below: 

(1)  Ingestion  by  oysters  and  other 
suspension-feeding  members  of 
the  reef  community  is  affected 
negatively  by  increased  water 
turbidity  (Section  2.3). 

(2)  Turbidity  of  estuaries  in  the 
study  area  is  usually  high  and 
closely  related  to  the  high 
tidal   current  regime.   Thus, 


74 


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Q. 
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u 

c 
o 
o 


N 
■r- 

(O 

en 
s- 
o 


CM 

0) 

S- 
=3 


75 


currents  indirectly  can  reduce 
oyster  feeding. 

(3)  Currents  have  been  shown,  how- 
ever, to  positively  affect  oys- 
ter ingestion  (Walne  1972). Thus, 
an  optimum  low-current  level 
probably  exists  to  stimulate 
oyster  feeding  with  a  minimum  of 
sediment  erosion. 

(4)  Eroded  sediments  in  the  water 
column  can  settle  out  on  a  reef 
and  bury  the  lower  level  oys- 
ters, causing  a  decline  in  reef 
viability.  Sediment  input  by 
currents,  coupled  with  a  high 
rate  of  biodeposition,  can  suf- 
focate all  but  the  uppermost 
oysters  in  a  reef. 

(5)  Oyster  reef  growth  in  a  positive 
vertical  direction  is  limited 
absolutely  by  the  local  tidal 
amplitude.  The  highest  portions 
of  the  reefs  examined  at  Sapelo 
Island  were  limited  to  1.5m 
above  MLW,  corresponding  to  a 
daily  inundation  tim.e  of  only 
8  hours,  or  conversely,  to  an 
exposure   time   of   16  hours, 

(6)  Lateral  extension  of  oyster 
reefs  apparently  occurs  at  a 
rate  limited  by  suitable  sub- 


strate at  the  proper  elevation 
in  the  intertidal  zone,  by  water 
currents,  and  by  available  food. 

(7)  In  addition  to  a  minimum  inunda- 
tion time,  vertical  reef  growth 
is  also  subject  to  temperature 
stress  in  the  study  area  (ex- 
tremely cold  spells  and  hot 
spells  during  reef  exposure). 

(8)  Reef  crowding  appears  to  buffer 
temperature  stress  and  to  allow 
vertical  reef  accretion  beyond 
the  maximum  level  at  which  indi- 
vidual oysters  survive. 

(9)  Downward  extension  of  oyster 
reefs  toward  the  subtidal  zone 
appears  limited  by  increased 
predation,  fouling,  and  shell 
erosion   by   boring   sponges. 

(10)  Predation  by  filter  feeding 
organisms,  nektonic,  and  epiben- 
thic,  reduces  the  available  pool 
of  oyster  larvae  and  perhaps 
prevents  overcrowding. 

(11)  The  gregarious  behavior  of  oys- 
ter larvae  ensures  a  new  crop  of 
spat  to  replenish  mortality 
losses  and  maintain  the  viabil- 
ity of  existing  reefs. 


76 


ir.\ 


'*>. 


n 


•ij 


4i>  ^ 


<\ 


rj 


'^: 


*:j^' 


m 


i^A 


3i^ 

V 

h 

^ 

w 

t  i^^k 

Wsk 

w%r*.    l«-^ 

Immature  reef  at  the  mouth  of  an  intertidal  creek.  Note  the 
mature  reefs  in  the  background.  Photo  by  Rhett  Talbert,  Uni- 
versity of  South  Carolina. 


77 


The  seeding  of  intertidal  oyster  beds  with  oyster  shell  to  induce  increased 
oyster  spat  settlement  in  areas  that  are  being  commercially  harvested.  Photo 
by  South  Carolina  Wildlife  and  Marine  Resources  Department. 


78 


CHAPTER  6.  SUMMARY  AND  MANAGEMENT  IMPLICATIONS  AND  GUIDELINES 


6.1  SUMMARY  AND  OYSTER  REEF  SIGNIFICANCE 

The  American  oyster  (Crassostrea  vir- 
qlnica)  is  not  only  an  extremely  valuable 
commodity  to  man  but  is  also  a  cosmopol- 
itan, physiologically  plastic,  and  ecolog- 
ically interesting  estuarine  organism. 
Its  natural  range  spans  the  Atlantic  coast 
and  much  of  the  gulf  coast,  and  its  ge- 
neric "brothers"  exist  in  coastal  systems 
worldwide. 

One  intriguing  aspect  of  oyster  be- 
havior is  its  propensity,  under  certain 
conditions,  to  form  massive,  discrete, 
intertidal  colonies,  or  reefs.  The  larg- 
est individual  oyster  reefs  formed  by  the 
American  oyster  occur  in  open  bays  along 
the  northern  gulf  coast.  Some  reefs  are 
many  kilometers  in  length;  they  consist 
mainly  of  dead  shells,  and  their  geometry 
is  partially  the  result  of  reworking  by 
storm  surges. 

In  the  South  Atlantic  Bight,  tidal 
amplitude  ranges  from  1  m  to  over  3  m  (3 
to  10  ft),  and  oyster  reefs  occur  in  close 
association  with  extensive  salt  marshes 
characteristic  of  the  area.  Oyster  reefs 
within  this  region  achieve  a  greater  ele- 
vation above  mean  sea  level  and  a  greater 
oyster  density  (in  terms  of  numbers  and 
biomass)  than  in  any  other  coastal  region. 
The  structure  and  ecological  function  of 
these  reefs  are  the  subjects  of  the  pre- 
vious five  chapters. 

Whereas  most  oyster  research  has  been 
carried  out  at  the  individual  or  popula- 
tion level  of  detail,  this  paper  has  em- 
phasized the  behavior  of  the  oyster  at  the 
ecosystem  level.  The  reef  community  de- 
scribed throughout  this  community  profile 
exhibits  characteristics  and  has  ecosystem 
importance  that  could  not  be  predicted 
from  even  "perfect"  knowledge  of  the  bio- 
logy of  individual  oysters.  Thus,  just  as 
a  termite  colony  is  more  than  a  collection 
of  termites,  so  an  oyster  reef  shows  emer- 
gent properties,  including  its  capability 
of  extending  the  intertidal  range  of  the 
reef  assemblage  upward  beyond  the  eleva- 
tion at  which  individual  oysters  normally 


could  survive.  Oyster  reefs  possess  the 
following  characteristic  properties:  (1) 
individual  oysters  in  a  reef  must  grow 
with  a  strong  vertical  orientation  to  sur- 
vive; (2)  individual  reefs  strictly  are 
limited  to  the  intertidal  zone,  and  the 
geometry  of  a  given  reef  is  strongly 
determined  by  mean  water  level,  sediment 
stability,  and  current  regime;  and  (3) 
patterns  of  reef  distribution  are  discern- 
able  within  drainage  basins,  such  that 
reef  density  is  usually  maximal  at  inter- 
mediate channel  widths  and  current  veloc- 
ities. In  other  words,  if  all  living  oys- 
ters in  a  drainage  basin  were  redistrib- 
uted either  randomly  or  homogeneously 
throughout  the  ecosystem,  a  large  portion 
of  the  function  (and  value)  of  the  oyster 
community  would  be  lost. 

One  primary  ecosystem  value  of  the 
oyster  reef  community  relates  to  its  phys- 
ical, rather  than  its  biological,  proper- 
ties. Mature  reefs  are  stabilizing  influ- 
ences on  erosional  processes  and  may  mod- 
ify long-term  changes  in  tidal  stream  flow 
and  overall  marsh  physiography,  although 
these  effects  have  not  been  quantified 
yet. 

The  extent  of  the  physical  influence 
of  reefs  on  the  marsh  system  is  a  function 
of  the  average  relative  proportion  of  reef 
area  to  total  intertidal  area  in  a  given 
drainage  basin.  The  available  estimates 
of  this  relationship  vary,  but  about  0.05% 
of  reef  area  to  total  intertidal  area 
(marsh  and  water)  may  be  a  reasonable 
estimate. 

Another  aspect  of  the  ecosystem  value 
of  oyster  reefs  relates,  in  natural  estua- 
rine areas,  to  reefs'  being  stable  islands 
of  hard  substrate  in  an  otherwise  unstable 
soft  muddy  environment.  These  islands  are 
essential  habitat  for  some  organisms, 
especially  the  sessile  suspension-feeding 
epifauna  usually  limited  by  the  available 
surface  area.  Reefs  also  provide  a  highly 
irregular  surface  with  crevices  that  serve 
as  havens  for  motile  invertebrates;  and 
some  small  fish  use  reefs  for  shelter 
during  flood  tides.   Oyster  reefs  are 


79 


Photo  indicates  the  "soupy"  nature  of  the  sediments  that  oft  times  support  oyster 
reefs.  The  reefs  represent  a  hard  substrate  "island"  habitat  in  an  otherwise  soft- 
bottomed  environment.  Photo  by  Leonard  Bahr,  Louisiana  State  University. 


80 


densely  populated  with  mussels,  mud  crabs, 
polychaetes,  barnacles,  and  other  macro- 
fauna,  and  countless  smaller  metazoa,  pro- 
tozoa and  bacteria. 

The  members  of  the  oyster  reef  com- 
munity are  limited  primarily  to  suspension 
and  deposit  feeding  macrofaunal  consumers. 
The  trophic  role  of  this  macrofaunal  com- 
munity as  a  whole  assimilates  carbon  de- 
rived from  phytoplankton  and  detrital 
sources  and  makes  it  available  to  higher 
consumers,  i.e,  terrestrial  and  aquatic 
animals.  Of  the  former,  raccoons  and 
birds  like  oyster  catchers  and  grackles 
are  predators  on  oyster  reefs.  Aquatic 
consumers  that  prey  on  healthy  living  oys- 
ters include  the  blue  crab  (Callinectes 
sapidus)  and  the  black  drum  (Poqonias 
cromis).  Many  other  aquatic  carnivores 
undoubtedly  visit  oyster  reefs  during 
flood  tides  and  prey  on  the  host  of  small 
invertebrates  residing  there. 

More  important  than  the  food  web 
roles  of  oyster  reef  inhabitants  in  the 
salt  marsh  estuarine  system  is  their  role 
in  mineralizing  organic  carbon  and  releas- 
ing nitrogen  and  phosphorus  in  forms 
usable  by  the  primary  producers.  The 
significance  of  the  energetic  roles  of  the 
reef  community  is  exemplified  by  the  meta- 
bolic rates  of  the  entire  community  being 
among  the  highest  measured  for  any  benthic 
community  (27,000  kcal/m2/yr).  This  rate 
is  partly  due  to  the  great  surface  area  in 
a  reef,  supporting  a  large  population  of 
aerobic  bacteria,  and  to  the  high  biomass 
of  the  resident  macrofauna  (up  to  1,100  g 
afdw/m2). 

Each  summer  the  reef  community  con- 
tributes a  stream  of  high  quality  protein 
to  the  water  column  in  the  form  of  gametes 
and  larvae  of  oysters  and  other  resident 
macrofauna.  These  meroplankton  (or  larvae) 
are  food  for  nektonic  filter  feeders,  food 
for  other  benthic  organisms,  and  recruits 
for  the  next  generation  of  reef  oysters 
and  associates.  Because  reefs  continually 
subside  into  the  mud,  new  generations  of 
oysters  at  the  top  are  necessary  to  main- 
tain the  steady  state  elevation  of  the 
upper  reef  surface. 

Oyster  growth  in  mature  reefs  appears 
extremely  slow,  and  some  of  the  larger 
resident  oysters  probably  are  5  to  10  or 


more  years  of  age.  They  are  typically  long 
and  narrow  and  usually  display  a  watery 
condition  with  little  glycogen  reserves,  a 
sign  of  stress  or  being  spawned  out. 

Because  oysters  in  reefs  apparently 
live  close  to  their  stress  tolerance 
threshold,  further  perturbation  by  man  can 
easily  destroy  the  entire  reef  community. 
Reefs  are  particularly  susceptible  to 
artificial  hydrologic  changes,  such  as 
those  that  follow  the  impoundment  or 
diversion  of  waterbodies  as  large  as 
coastal  rivers  or  as  small  as  individual 
tidal  streams.  Reefs  primarily  are  found 
at  the  interface  between  wetland  and  open 
water,  and  the  destruction  of  wetlands  for 
any  reason  results  in  a  decrease  in  this 
interface  zone.  Oysters  and  other  benthic 
macrofauna  are,  of  course,  also  connected 
to  and  depend  upon  wetland  macrophytes  via 
trophic  pathways  still  not  well  under- 
stood. 

Reef  oysters  are  susceptible  to  the 
increasing  array  of  man-made  chemicals  and 
heavy  metals  becoming  more  prevalent  in 
coastal  waters.  They  are  also  vulnerable 
to  the  eutrophic  effects  of  fertilizer- 
and  sewage-loading  in  coastal  waters 
through  the  potential  alteration  of  the 
composition  of  the  natural  phytoplankton 
community  in  a  manner  that  may  be  less 
desirable  or  even  toxic  to  oysters. 

Reef  oysters  have  evolved  to  tolerate 
high  levels  of  turbidity,  but  increased 
sedimentation  on  top  of  natural  levels  can 
smother  them.  Dredging  related  to  shell 
or  phosphate  mining,  navigation  or  pipe- 
line canals,  or  other  construction  activi- 
ties in  the  coastal  zone  can  drastically 
increase  the  natural  sediment  load  in 
local  areas.  In  addition,  the  artificial 
mixing  of  reduced  bottom  sediments  with 
water  above  the  bottom  can  deplete  the 
water  column  of  its  dissolved  oxygen. 

Direct  physical  alteration  of  mature 
oyster  reefs,  e.g.,  by  harvesting,  can 
destroy  an  entire  reef,  even  if  the  reef 
is  only  moderately  disturbed.  Harvest  of 
intertidal  oysters  is  productive  only  on 
immature  oyster  reefs  low  in  the  inter- 
tidal zone,  where  oysters  are  not  as 
crowded  as  in  mature  reefs  and  where 
growth  is  more  rapid.  Thus,  mature  reefs 
are  most  valuable  to  the  ecosystem  and  to 


81 


society  if 
rather  than 
food  value. 


they  are 
harvested 


left  undisturbed, 
for  their  limited 


6.2  MANAGEMENT  IMPLICATIONS  AND  GUIDELINES 

Clearly,  the  oyster  reef  component  of 
the  coastal  ecosystem  in  the  Southeastern 
United  States  depends  on  a  healthy  marsh- 
estuarine  environment.  Thus,  the  most 
logical  recommendation  for  reef  management 
is  to  mitigate  increasing  man-induced 
alterations  on  the  marsh  system  to  the 
extent  possible.  Changes  in  water  flow, 
both  surface  and  subsurface,  appear  to 
cause  the  most  far  reaching  and  cumulative 
damage  to  the  entire  system,  and  thus 
indirectly  to  the  reef  subsystem.  See 
Table  1  for  a  summary  of  cultural  stress 
on  oysters. 

The  maintenance  of  high  water  quality 
is,  of  course,  important  to  the  continued 
viability  of  oyster  reefs;  and  the  intro- 
duction of  urban,  industrial,  and  agricul- 
tural pollutants  from  both  point  and  non- 
point  sources  is  to  be  avoided.  Subtidal 
oysters  normally  can  tolerate  a  fair 
amount  of  insult  in  terms  of  poor  water 
quality  before  succumbing  to  many  of  the 
common  pollutants.  Such  oysters  usually 
become  dangerous  to  eat  before  they  die 
from  chemical  pollution.  Reef  oysters,  on 
the  other  hand,  are  already  stressed  and 
may  not  be  as  hardy.  At  present,  the  oys- 
ter reef  zone  of  the  South  Atlantic  Bight 
appears  relatively  free  of  toxic  chemicals 
and  excess  nutrients,  except  in  the  immed- 
iate vicinity  of  major  population  centers 
such  as  Savannah,  Georgia,  and  Charleston, 
South  Carolina. 


Long-term  effects  of  increasing 
freshwater  pumping  may  pose  a  problem  more 
serious  than  pollution  for  the  marsh  oys- 
ter reef  system.  Therefore,  future  urban, 
industrial,  and  agricultural  requirements 
for  freshwater  need  to  be  examined  and 
their  long-term  effects  on  salinity  dis- 
tribution predicted,  in  order  to  under- 
stand the  implications  of  development  for 
the  entire  coastal  ecosystem. 

There  have  been  several  proposals  and 
attempts  to  increase  oyster  reef  area 
locally  by  spreading  cultch  along  the 
fringe  between  marsh  and  water  to  induce 
oyster  settlement.  These  efforts  have 
been  largely  unsuccessful,  implying  that 
our  thesis  is  valid;  that  is,  the  distri- 
bution of  reefs  relates  to  a  specific  set 
of  conditions,  especially  with  respect  to 
water  flow,  and  the  proportion  of  a  marsh 
drainage  unit  occupied  by  oyster  reefs  is 
not  indefinitely  expandable.  The  guide- 
line derived  from  these  observations  is 
that  artificial  oyster  reef  development 
should  be  seriously  attempted  only  at 
former  reef  sites. 


In  conclusion,  the  intertidal  oyster 
reef  subunit  of  the  marsh  estuarine  eco- 
system is  an  important  component  of  the 
coastal  zone  in  the  Southeastern  United 
States,  and  this  subunit  has  declined  in 
total  area  during  the  last  90  years.  We 
can  only  guess  at  the  consequences  of  the 
continued  loss  of  reef  area,  but  these 
effects  could  be  both  obvious  and  subtle, 
and  could  definitely  result  in  an  ecosys- 
tem less  healthy,  rich,  and  productive, 
and  certainly  less  interesting  from  an 
aesthetic  point  of  view. 


82 


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Basin:  salinity  changes  and  oyster 
distribution.  Louisiana  State  Univer- 
sity, Center  for  Wetland  Resources, 
Baton  Rouge.  Sea  Grant  Publ.  No. 
LSU-T-76-002. 

Walne,  P.  R.  1972.  The  influence  of  cur- 
rent speed,  body  size,  and  tempera- 
ture on  the  filtration  rate  of  five 
species  of  bivalves.  J.  Mar.  Biol. 
Assoc.  U.  K.  52:345:374. 

Watling,  H.  1978.  Effect  of  cadmium  on 
larvae  and  spat  of  the  oyster  Cras- 
sostrea qigas  (Thunberg).  Trans.  R. 
Soc.  of  South  Africa  43(2):  125  - 
134. 

Wells,  H.  W.  1961.  The  fauna  of  oyster 
beds,  with  special  reference  to  the 
salinity  factor.  Ecol.  Monogr.  31: 
239-266. 

Wiedemann,  H.  V.  1971.  Shell  deposits 
and  shell  preservation  in  quaternary 
and  tertiary  estuarine  sediments  of 
Georgia,  U.S.A.  Sediment.  Geol. 
7(2):103-125. 

Wiegert,  R.  G.  1968.  Thermodynamic  con- 
siderations in  animal  nutrition.  Am. 
Zool.  8(1):71-81. 

Wildish,  D.  J.,  and  D.  D.  Kristmanson. 
1979.  Tidal  energy  and  sublittoral 
macrobenthic  animals  in  estuaries. 
J.  Fish.  Res.  Board  Can.  36:1197- 
1206. 

Yonge,  C.  M.  1960.  Oysters.  killmer 
Brothers  and  Haram,  Ltd.,  Birtenhead, 
London,  England.  209  pp. 

Zingmark,  R.  G.,  ed.  1978.  An  annotated 
checklist  of  the  biota  of  the  coestal 
zone  of  South  Carolina.  University 
of  South  Carolina  Press,  Columbia. 
364  pp. 


92 


APPENDIX:  OYSTER  BIOENERGETICS 


Oysters,  like  all  heterotrophic  orga- 
nisms, use  energy  in  proportion  to  their 
growth  rate,  their  reproductive  invest- 
ment, and  their  efforts  to  obtain  food, 
remove  waste,  defend  themselves  against 
parasites  and  predators,  and  maintain  a 
favorable  osmotic  balance.  This  section 
discusses  the  rates  and  partitioning  of 
energy  expenditures  for  individual  inter- 
tidal  oysters  and  the  oyster  population  as 
a  whole.  The  energy  requirements  of  the 
entire  reef,  a  prerequisite  for  under- 
standing the  dynamics  of  the  oyster  reef 
community,  are  estimated  in  Chapter  3. 

Ecologists  and  environmental  managers 
are  beginning  to  realize  the  value  of 
information  regarding  the  rates  and  path- 
ways of  energy  flow  in  communities  of 
organisms  and  entire  ecosystems.  Energy 
units  are  interconvertible,  and,  there- 
fore, the  energy  "cost"  of  totally  dif- 
ferent processes  is  the  common  denominator 
by  which  these  processes  can  be  compared 
objectively  and  ranked  in  terms  of  their 
ecological  importance.  The  first  ecologist 
formally  to  apply  this  principle  to  the 
study  of  ecosystems  was  Raymond  Lindemann, 
who  in  1942  published  a  landmark  treatise 
on  the  partitioning  of  energy  flow  through 
an  ecosystem  (Lindeman  1942).  Since  then, 
it  has  become  common  practice  to  include 
energetics  in  ecological  research.  Good 
review  sources  on  bioenergetics  include 
Phillipson   (1966)  and  Wiegert  (1968). 

The  extant  oyster  literature  includes 
several  compilations  of  energy  budgets  for 
various  species  of  oysters  in  different 
areas.  Extrapolations  from  some  of  these 
studies  are  necessary  to  fill  in  energy 
budget  data  gaps  for  intertidal  oysters  in 
the  study  area. 

The  calculation  of  an  energy  budget 
for  a  population  of  organisms  involves  the 
use  of  one  or  another  equation  of  the 
general  form: 


P(net)  =  I-E-R-W 


(1) 


''(net)  ~  "^^   secondary  production 
rate,  or  growth  of  the  pop- 
ulation in  a  given  time 
(including  somatic  growth, 
gamete  production,  and  mor- 
tality losses) 
I  =  ingestion  rate 
E  =  egestion  and  excretion  rates 
R  =  respiration  or  metabolic  rate 
W  =  the  rate  at  which  external  work 
is  performed  by  the  organisms 

The  term  W  is  usually  ignored  (Wie- 
gert 1968),  but  for  some  animals  (such  as 
mound-building  termites  and  reef  oysters), 
work  may  be  substantial  because  these  or- 
ganisms build  vertical  structures  against 
gravity. 

In  mature  populations,  the  production 
equation  may  attain  a  steady  state,  in 
which  no  net  growth  can  be  measured  and 
annual  energy  inputs  equal  losses.  Oyster 
reefs  appear  to  attain  this  steady-state 
maturity  when  they  achieve  a  critical  ver- 
tical elevation  relative  to  tidal  stage  or 
when  oyster  growth  is  equal  to  maintenance 
costs. 

Before  a  rough  energy  budget  for  in- 
tertidal oysters  is  presented,  the  prob- 
lems involved  in  compiling  such  a  budget 
must  be  discussed.  The  terms  in  the 
energy  budget  Equation  (1)  are  measured 
for  an  oyster  population  in  the  following 
ways.  Net  production  P(net)  ""s  sometimes 
calculated  by  measuring  the  increase  in 
size  of  experimental  animals  over  a  unit 
of  time.  This  technique  requires  measur- 
ing the  individual  oysters.  Another  tech- 
nique calculates  time  elapsed  between  age 
classes  in  the  size-frequency  distribution 
of  a  natural  population.  The  latter  tech- 
nique is  tedious  since  age  classes  quickly 
become  indistinct  because  of  continuous 
waves  of  spawning  over  the  warm  season. 

Total  production  P(nross)  includes 
gamete  production  (and  release)  as  well  as 
mortality  and  predation  (and  harvesting) 
between  sampling  periods.  The  growth  rate 


93 


ts 


of  an  oyster  slows  as  its  gamete  produc 
tion  gradually  begins  to  dominate  its 
energy  budget  and  as  its  respiratory  rate 
"catches  up"  to  its  ingestion  rate  (Rod- 
house  1978),  as  illustrated  in  Figure  A-1. 

Ingestion  by  oysters  (I)  is  usually 
estimated  by  measuring  the  rate  of  clear- 
ance of  particles  in  a  suspension  to  which 
test  animals  are  exposed  for  a  unit  of 
time.  Walne's  (1972)  experiments  using 
Crassostrea  qigas  and  Ostrea  edulis  are 
exemplary  in  that  realistic  food  concen- 
trations and  a  wide  range  of  sizes  of  oys- 
ters were  used.  In  addition,  Walne  used 
flowing  water  conditions  rather  than  the 
usual  standing  water  experiments.  Haven 
and  Morales-Alamo  (1970)  also  measured 
oyster  ingestion  in  a  flowing  water  system 
but  did  not  use  a  wide  size  range  of  oys- 
ters. 

Egestion  (E)  is  measured  by  holding 
test  oysters  in  trays  in  which  feces  and 
pseudofeces  are  collected  and  measured 
during  a  known  time  interval  (Haven  and 
Morales-Alamo  1967;  Bernard  1974). 

The  respiration  rate  of  oysters  (R) 
is  usually  measured  by  documenting  the 
rate  of  decline  in  dissolved  oxygen  in 
water  in  which  oysters  are  immersed  or  by 
measuring  the  change  of  dissolved  oxygen 
in  water  as  it  flows  over  a  population  of 
oysters.  The  rate  of  change  of  CO2  is  not 
as  convenient  to  measure  with  oysters, 
partly  because  an  infrared  CO2  analyzer  is 
required,  partly  because  oysters  can  fix 
CO2  (Hammen  1969),  and  partly  because  they 
can  respire  anaerobically  and  release  COj 
from  the  dissolution  of  shell  carbonate 
(Hochachka  and  Mustafa  1972). 

One  major  problem  in  quantifying  in- 
dividual terms  in  the  oyster  energy  budget 
equation  is  that  most  terms  change  in  a 
nonlinear  fashion  as  an  oyster  (or  size 
class)  grows.  Small  animals  operate  at 
higher  metabolic  rates  than  large  animals. 
Another  problem  is  that  at  least  five  en- 
vironmental variables  affect  each  term: 
(1)  intertidal  elevation,  (2)  water  tem- 
perature, (3)  levels  of  food  and  other 
suspended  matter  in  the  water  column,  (4) 
dissolved  oxygen  levels,  and  (5)  current 
velocity.  To  further  complicate  the  pic- 
ture, the  size  of  the  animals  and  these 
other  variables  are  interrelated  in  com- 
plex (nonlinear)  ways. 


Energy  budgets  are  invariably  simpli- 
fied models  because  of  these  problems,  and 
the  present  budget  is  no  exception.  Some 
comments  about  the  variables  used  and 
assumptions  made  follow. 


VARIABLES 

Tide  Stage 

Oysters  obviously  cannot  pump  water 
to  respire  and  feed  unless  they  are  im- 
mersed. Intertidal  reef  oysters  are 
assumed  to  be  inundated  on  the  average  of 
only  50X  of  any  24-hr  day.  Other  workers 
have  made  similar  assumptions  on  feeding 
duration,  even  for  subtidal  oyster  popula- 
tions. Bernard  (1974)  assumed  50%;  Rod- 
house  (1978)  assumed  70%  feeding  time. 

Water  Temperature 

Temperature  affects  all  biochemical 
reactions,  including  oyster  energy  con- 
sumption. Intertidal  oysters  are  exposed 
to  water  temperatures  that  vary  by  a 
factor  of  about  three,  from  9°  to  31°C 
(Dame  1970;  Bahr  1974).  The  annual  pat- 
tern of  water  temperature  variation  in 
coastal  South  Carolina  is  illustrated  in 
Figure  A-2  (Dame  1970).  Over  this  temper- 
ature range  oyster  metabolism  is  estimated 
to  vary  by  a  factor  of  about  eight  (Bahr 
1976). 

Food  and  Other  Suspended  Matter 

Loosanoff  (1962)  showed  that  food  and 
other  suspended  matter  significantly 
altered  oyster  ingestion  rate.  Excess 
turbidity,  caused  either  by  suspended 
organic  or  inorganic  matter,  reduces  "oys- 
ter pumping."  It  can  be  assumed  that  sus- 
pended matter  in  the  study  area  is  close 
to  optimum  for  intertidal  oysters  and  that 
they  are  exposed  to  about  0.01  gC/liter  or 
0.04  kcal/  liter  when  inundated  (Odum  and 
de  la  Cruz  1967). 

Dissolved  0-, 


Oyster  respiration  rates  are  unaf- 
fected by  dissolved  oxygen  concentrations 
unless  the  concentration  decreases  below 
one-half  saturation  level  (Ghiretti  1966). 
In  other  words,  dissolved  oxygen  in  estu- 
aries in  the  study  area  should  normally 
not  affect  respiration  or  feeding  rates 


94 


20- 


C/3 
< 


Figure  A-1.  Age-dependent  annual  production  of  soft  tissue,  shell  organics, 
gonad  output,  and  respiration  in  an  oyster  (adapted  from  Rodhouse  1978). 


95 


AMJJ  ASON 

D  (TIME  IN  ANGULAR  DEGREES) 


Figure  A-2.  Seasonal  variation  in  water  temperature  affecting  oyster  reefs  in 
South  Carolina  (adapted  from  Dame  1970). 


96 


but  it  could  become  a  factor  in  dredged 
areas  (Frankenberg  and  Westerfield  1968). 

Current  Velocity 

A  positive  effect  of  current  velocity 
on  oyster  feeding  could  be  surmised  from 
the  fact  that  oyster  reefs  tend  to  grow 
outward  toward  the  middle  and  more  rapidly 
flowing  portion  of  a  tidal  stream.  En- 
hancement of  oyster  feeding  as  a  function 
of  increase  in  current  velocity  was  demon- 
strated by  Walne  (1972). 

Given  the  above  assumptions,  the  addi- 
tional information  most  important  for  the 
calculation  of  an  energy  budget  for  inter- 
tidal  oysters  is  the  size-frequency  (or 
weight-frequency)  distribution  of  reef 
populations  and  the  effect  of  weight  on 
the  energy  budget  terms. 

All  of  the  terms  in  the  energy  budget 
equation  for  oysters  are  presumably  af- 
fected by  the  size  (or  weight)  of  individ- 
uals by  the  following  general  equation: 


F  =  a  W 


(2) 


where  F  = 

W  = 

a  and  b  = 


in  energy 


the  process  rate 

or  matter  units 

the  biomass  of  the  oyster 

(g  or  kcal ) 

constants   (represent  the 

effects  of  temperature  and 

the  surface  area-to-volume 

ratio,  respectively) 


It  is  generally  known  that  small  oys- 
ters ingest,  egest,  respire,  grow  (and 
die)  at  higher  rates  than  do  large  oys- 
ters, and  that  these  rates  increase  in  all 
oysters  with  increased  temperature.  Un- 
fortunately, no  general  agreement  exists 
in  the  bioenergetics  literature  concerning 
units  of  biomass.  Table  A-1  lists  some 
conversion  factors  for  oyster  biomass  that 
were  compiled  from  various  sources.  The 
numbers  are  only  approximate  because  the 
allometric  relationships  can  change  with 
gonadal  state  or  with  tidal  elevation  of 
the  population.  Dame  (1972a)  found  that 
intertidal  oysters  in  North  Inlet,  South 
Carolina,  had  a  significantly  higher  ratio 
of  shell  weight  to  dry  meat  weight  than 
subtidal  oysters  had. 

Because  small  oysters  process  energy 
at  relatively  higher  rates  than  large 


ones,  it  is  important  to  document  the  size 
(biomass)  frequency  of  intertidal  oysters 
in  the  study  area.  Bahr  (1974)  separated 
reef  oysters  at  Doboy  Sound,  Georgia,  into 
32  size  classes  at  5-mm  intervals  (2  to 
157  mm).  He  found  that  the  oyster  popula- 
tion in  the  central  (higher)  portion  of 
several  old  reefs  typically  showed  a  log 
normal  distribution,  especially  during  the 
late  fall.  Oysters  in  the  smallest  five 
size  classes  (up  to  19  mm)  dominated  the 
population,  and  oysters  above  100  mm  were 
rare.  Dame  (1976)  reported  a  similar  size- 
frequency  distribution  of  reef  oysters  in 
South  Carolina,  but  with  generally  lower 
overall  populations  and  reduced  dominance 
of  small  size  classes.  Figures  A-3  and 
A-4  illustrate  the  temporal  changes  in 
size-frequency  distributions  of  reef  oys- 
ters in  these  two  respective  studies. 

The  equation  that  describes  the  size- 
frequency  distribution  of  reef  oysters  in 
Doboy  Sound,  Georgia  (Bahr  1974)  is  as 
follows: 


log^Q  Y  =  -0.02  X^.  +  2.32 


(3) 


where  Y  =  the  number  of  oysters  per 


0.1 


1^  in  size  class  X,- 


X.  (i  =  2,  7,  12. ..157)  =  5-mm 
^  size  class 

The  relation  between  individual  oys- 
ter size  and  biomass  from  Bahr  (unpub- 
lished data)  is  described  by  another 
regression  equation  as  follows: 


0.02  X  -1.8 


(4) 


where  Y  =  logjg  afdw  (g)  of  total 

oysters   including  shell, 

X  =  height  of  each  oyster  in  mm 

The  r  of  this  relationship  is  0.84 
with  78  degrees  of  freedom.  The  experi- 
mental animals  were  collected  at  eight 
different  times,  including  all  seasons. 

To  simplify  the  computation  of  the 
energy  budget  of  the  reef  oyster  popula- 
tion. Equations  3  and  4  were  used  to 
describe  a  typical  reef  oyster  population, 
intermediate  in  both  numbers  and  biomass. 
Thus,  the  numerical  dominance  of  small 
oysters  is  offset  by  the  higher  biomass  of 
(rare)  large  oysters,  and  oysters  from  40 
to  80  mm  in  height  (mean  60  mm,  or  0.25  g 
afdw)  are  functionally  typical  (See  Figure 


97 


Table  A-1.  Conversion  factors  for  oyster 
biomass  units  (intertidal  oysters). 


Whole  oyster 


Total  wet  wt 


100% 

Total 

dry  wt 

100% 

Total 

afdw^ 

Wet 

shell  wt 

72% 

Dry 

shell  wt 

97.1% 

Shell  afdw*^ 

42% 

Meal 

:  only 

Dry 

wt 

Wet 

meat  wt 

28% 

Dry 

meat  wt 

2.8% 

Meal 

;  afdw*^ 

100%  42%  58% 


Wet  wt  Dry  wt  afdw 

100%  14.9%^  12.0^ 


^Gametes  may  comprise  up  to  50%  of  this  proportion, 
afdw  =  ash-free  dry  weight. 


98 


HEIGHT  IN  MM 


Figure  A-3.     Seasonal   changes  in  size-frequency  distribution  of  reef  oysters  in 
Georgia  (Bahr  1976) . 


99 


-I    2 


Figure  A-4.  Seasonal  changes  in  intertidal  oyster  size-frequency  distribution 
in  South  Carolina  (adapted  from  Dame  1976). 


100 


A-5).  The  entire  oyster  biomass  of  the 
reef  population  is  therefore  considered 
here  as  divided  among  0.25-g  oysters. 
Bahr  (1974)  reported  that  the  average  bio- 
mass of  the  reef  oyster  population  was  970 
g/m2  afdw  (total  wt);  thus  one  can  postu- 
late a  hypothetical  reef  populated  by 
60-rnni  oysters  at  a  density  of  about  4,000 
oysters/m^.  The  dry  meat  weight  of  an 
oyster  of  0.25  g  total  afdw  would  equal 
approximately  0.18  g  (from  Table  A-1). 

Before  one  estimates  the  value  of  the 
terms  of  Equation  1  for  the  "average"  reef 
oyster  population,  it  is  appropriate  to 
consider  two  independent  studies  that  were 
conducted  at  approximately  the  same  time 
and  that  attempted  to  measure  certain 
aspects  of  the  energy  budget  of  oyster 
reefs.  Bahr  (1974,  1976)  and  Dame  (1970, 
1972a-,  1972b,  1976,  1979)  studied  oyster 
reefs  in  Georgia  and  South  Carolina, 
respectively.  Significant  differences 
between  the  studies  are  compared  in  Table 
A-2. 

Some  differences  between  the  two  sets 
of  conclusions  are  explainable  on  the 
basis  that  test  reefs  in  Dame's  studies 
were  significantly  lower  in  the  intertidal 
zone  than  were  the  reefs  in  Bahr's  work, 
although  the  absolute  elevation  of  Dame's 
reefs  with  respect  to  mean  low  water  (MLW) 
was  not  reported.  This  elevation  dif- 
ference perhaps  indicates  a  significant 
difference  in  inundation  time,  which  could 
explain  the  higher  production  reported  by 
Dame.  In  Dame's  studies,  oyster  produc- 
tion estimates  for  large  oysters  were 
based  on  holding  oysters  in  trays  beneath 
a  pier  (presumably  shaded)  and  therefore 
not  in  as  stressful  a  setting  as  on  a 
natural  reef.  A  real  difference  probably 
existed  in  intertidal  oyster  reef  produc- 
tion (higher  in  South  Carolina).  The  ac- 
tive commercial  harvest  of  South  Carolina 
reef  oysters  is  proof  that  net  production 
of  large  oysters  occurs  there.  Lunz  (1943) 
reported  that  oysters  can  grow  to  3  inches 
in  2  years  in  South  Carolina  reefs.  Using 
a  calorific  coefficient  of  3.3  kcal/g  Oj, 
one  can  estimate  that  reef  oysters  respire 
the  equivalent  of  13,000  kcal/m^/yr.  The 
implication  of  this  high  metabolic  rate  is 
that  the  total  biomass  turns  over  on  the 
average  about  once  every  0.38  yr,  or  2.6 
times  per  year  (13,000  kcal/m2  /yr  t  5,000 
kcal/m2). 


Energy  expended  for  gamete  production 
increases  with  the  age  of  a  particular 
oyster  but  remains  about  half  the  respira- 
tion rate  (Figure  A-1).  Bernard  (1974) 
estimated  that  a  subtidal  population  of  C^. 
gigas  expended  as  much  energy  on  gamete 
production  as  on  respiration  (Figure  A-6). 
Thus,  between  7,500  and  13,000  kcal/m^/yr 
of  the  energy  assimilated  by  reef  oysters 
would  be  converted  to  gametes  and  released 
into  the  water  column.  At  least  99%  of 
this  energy  "investment"  would  never  reach 
"maturity"  but  would  be  consumed  by  other 
members  of  the  salt  marsh  ecosystem. 

The  rate  of  external  work  (W)  per- 
formed by  oysters  is  the  rate  at  which  a 
unit  weight  of  shell  material  is  elevated 
above  the  mud  surface,  multiplied  by  its 
elevated  distance.  In  energy  terms  this 
translates  into  the  cost  to  oysters  of 
producing  the  shell  protein  that  comprises 
1.3%  of  the  total  shell  dry  weight  or 
about  400  g  protein/m^  (2,000  kcal/m^). 
The  maximum  elevated  distance  is  1.5  m 
(see  Section  3.1),  but  unfortunately  we 
have  no  reliable  estimate  of  reef  growth 
rates.  Bernard  (1974)  estimated  that  sub- 
tidal  oysters  (C^.  gigas)  in  British  Colum- 
bia only  expend  about  10  kcal  /m^/yr  on 
shell  production.  This  is  equivalent  to 
(30  kcal/m2/yr)  for  oysters  in  the  study 
area,  calculated  by  using  Bernard's  data 
but  correcting  for  biomass  differences 
between  the  two  different  populations. 
We  suspect  that  this  estimate  is  much  too 
low.  The  rate  of  predation  on  oyster 
reefs  is  discussed  in  Section  3.4. 

Energy  Budget  Summary 

An  energy  budget  for  reef  oysters  is 
presented  in  the  following  paragraphs,  and 
the  rationale  and  values  for  the  terms  of 
the  equations  are  discussed.  Because  of 
the  method  used  in  estimating  net  produc- 
tion [P  (net)^  '■'^  ^^^  studies  discussed 
above,  we  are  inclined  to  agree  with  the 
conclusions  of  the  Georgia  study.  Charac- 
teristically, net  secondary  production  of 
reef  oysters  is  low  in  the  upper  portion 
of  high  reefs  and  large  oysters  are  quite 
old,  perhaps  even  5  to  10  years  or  more. 
In  these  reefs,  somatic  growth  is  balanced 
by  mortality.  In  lower  "immature"  reefs, 
f'(netj  ''^  undoubtedly  significant.  Because 
the  South  Atlantic  Bight  includes  large 
areas  of  low  "immature"  reefs,  especially 


101 


CO 


(A 

>- 

o 


o 


450 


400- 


350- 


300 


■MEASURED  OYSTER  FREQUENCY  (tt/O.lm^) 

APPROXIMATE  FREQUENCY  (ft/O.lm^) 

OYSTER  BIOMASS  CALCULATED 

FROM  SIZE  CLASSES 

(gafdw/O.lm^i 


APPROXIMATE 

BIOMASS 

(gafdw/O.lm^) 


u.        250- 


200- 


150 


100- 


50- 


-11.0 


10.0 


-8.0 


I     I     I     I    I     ^     I     I     I    I    I     I     r 

0     10    20   30   40    50 


-6.0    2 

m 
O) 


-4.0 


75  100 

SIZE  (mm) 


125 


150 


Figure  A-5.  Reef  oyster  height-frequency  relationship  and  cumulative  biomass 
curves. 


102 


Table  A-2.  Comparison  of  two  sets  of  oyster  reef 
energy  parameters  collected  within  the  study  area. 


Sources 

Bahr  (1974,  1976)  Dame  (1976) 

Parameter  Georgia  South  Carolina 

P/n..ce^^  —  4,500/yr 


(gross) 
'(net) 


P,_.^  0-1,000/yr  '^  3,460/yr 


R  13,000/yr  6,000/yr 

b'*  5,000  (total   oysters)  2,050  (meat) 

10,000-20,000  1,000-4,400 


-6 


^All  figures  unless  otherwise  noted  represent  kcal/m  (rounded). 

Includes  growth,  mortality,  and  gonadal  products. 
''Minimum  P  from  "old"  high  reef,  maximum  P  from  "young"  low  reef. 

B  =  biomass. 


^F  =  oyster  frequency  (#  /m  ). 


103 


POTENTIAL 
22052  Kcal 


GAMETES 

2% 
502  Kcal 


NOT  RETAINED 
89% 


1545  Kcal 

7% 

DEPOSITED 


Figure  A-6.     Schematic  representation  of  percentage  distribution  of  potential 
food  expressed  in  kilocalories   for  1-year  period  in  1  m^  of  subtidal 
Crassostrea  gigas  population   (adapted  from  Bernard  1974). 


i04 


in  South  Carolina,  we  will  assume  the 
''(net)  °^  1,000  kcal/m^/yr  is  a  conserva- 
tive estimate. 

The  ingestion  rate  (I)  of  a  reef  pop- 
ulation, as  expressed  by  the  "functionally 
average"  60-mm  oyster,  approximates  the 
ability  of  oysters  in  the  population  to 
filter  about  100  ml  of  water  per  minute 
(extrapolated  from  values  reported  by 
Walne  [1972]  for  C^.  qiqas  of  the  same 
height).  The  feeding  experiments  by  Walne 
were  carried  out  at  temperatures  approxi- 
mating the  median  level  for  our  study  area 
(19°  C).  During  one  day  (12  hours  of  pump- 
ing time),  the  oysters  occupying  a  typical 
square  meter  of  reef  could  filter  288,000 
liters  of  water  (4,000  oysters  x  0.1  li- 
ter/mi n  X  12  hr  x  60  min).  With  an  aver- 
age POC  load  of  0.01  g/liter  assumed  (Odum 
and  de  la  Cruz  1967),  this  would  equal  a 
potential  maximum  ingestion  rate  of  300 
gC/m2/day,  or  1  x  106gC/m2/yr  (5  x  105 
kcal/m^/yr)  if  the  oysters  filtered  at 
100%  efficiency.  If  filtration  is  40% 
efficient  (Haven  and  Morales-Alamo  1970), 
ingestion  of  organic  carbon  would  occur  at 
the  rate  of  about  2  x  10^  kcal/m2/yr.  Only 
a  small  fraction  of  this  cartjon  would  be 
assimilable,  however.  The  remainder  would 
be  egested  and  biodeposited  as  feces  or 
pseudofeces,  or  excreted  as  organic  nitro- 
gen. Mathers  (1974)  reported  that  large 
oysters  of  the  species  £.  anqulata  could 
completely  filter  water  at  the-  rate  of 
54  ml/g  (wet  wt)/hr  or  about  0.45  liter/g 
(afdw)/hr.  This  translates  to  about 
2  X  lO^'gC/m^/yr  or  1.0  x  10^  kcal/m Vyr 
for  reef  oysters,  twice  the  estimate  of 
Walne  (1972).  These  two  estimates  illus- 
trate the  approximate  nature  of  this  meas- 
urement. 

Egestion,  excretion,  and  pseudofecal 
production  (E)  by  reef  oysters  can  be 
expressed  in  terms  of  a  reef  population  of 
60-mm  oysters.  Bernard  (1974)  reported 
that  large  specimens  of  C^.  gigas  (  '^10  g 
dry  wt  of  meat)  produced  about  5.9  x  IC* 
kcal  per  oyster  per  year  as  biodeposits. 
If  an  extrapolation  were  made  to  the  60-mm 
reef  oyster  (dry  meat  weight  =  0.18  g),  we 
could  conservatively  predict  that  it  would 


biodeposit  the  equivalent  of  1,000  kcal/ 
yr,  or  4  x  10^  kcal/m^/yr  for  the  entire 
oyster  population.  If  one  judges  by  the 
estimated  maximum  ingestion  rate,  however, 
(see  above)  this  estimate  is  equal  to  80% 
of  ingestion,  implying  a  20%  assimilation 
rate  (A  =  I-E).  This  estimate  may  be  high 
because  only  a  small  portion  of  the  total 
of  all  ingested  carbon  can  be  assimilated 
by  oysters. 

Of  the  terms  in  Equation  (1),  respi- 
ration rates  (R)  are  best  known  for  reef 
oysters.  Bahr  (1974,  1976)  calculated  that 
the  reef  oyster  population  accounted  for 
approximately  48%  of  the  mean  oxygen  up- 
take of  the  total  reef  community,  or  about 
3,900  g02/m2/yr.  This  estimate  was  derived 
by  combining  individual  oyster  respirome- 
try  experiments  (carried  out  seasonally  at 
ambient  temperatures  and  on  different 
sized  animals)  with  the  relative  propor- 
tion of  the  reef  oyster  biomass  repre- 
sented by  each  size  class. 

From  data  reported  by  Dame  (1970)  and 
Bahr  (1974),  the  following  equation  de- 
scribes the  relationship  between  oyster 
oxygen  uptake  and  biomass  at  the  approxi- 
mate median  water  temperature  in  the  study 
area  (20°  C). 


0.53X 


0.71 


(5] 


where  Y  =  mg  O2  used  per  hour  and 
X  =  total  afdw 

Solving  this  equation  for  a  function- 
ally typical  oyster  of  0.25  g  afdw,  one 
would  predict  that  a  single  oyster  would 
consume  0.20  mg  02/hr.  When  this  figure  is 
multiplied  by  12  hours  of  inundation  time/ 
day,  365  days/yr,  and  4,000  oysters/m^, 
the  resulting  estimate  of  oxygen  require- 
ments is  3,500  g  Oa/m^/yr,  very  close  to 
the  above  estimate  of  3,900  g  Oa/m Vyr 
(Bahr  1974). 

The  final  estimates  of  the  parameters 
in  the  energy  budget  Equation  (1)  are  pre- 
sented in  Section  2.5  and  illustrated  in 
Figure  12. 


105 


5027J  -101 


REPORT  DOCUMENTATION 
PAGE 


1,_  REPORT   NO. 

FWS/OBS-81/15 


3.    Recipient's  Accession   No 


4.  Title  and  Subtitle 


THE  ECOLOGY  OF  INTERTIDAL  OYSTER  REEFS  OF  THE  SOUTH  ATLANTIC 
COAST:  A  COMMUNITY  PROFILE 


5.   Report  Date 

May  1981 


7.   Author(s) 


8.    Performing  Organization   Rept.   No. 


Leonard  M.  Bahr  and  William  P.  Lanier, 


9.   Performing  Organization  Name  and  Address 

Louisiana  State  University 
Baton  Rouge,  Louisiana  70803 


10.  Project/Task/Work  Unit  No. 

11.  Contract(C)  or  Grant(G)  No. 
(C) 

(G) 


12.  Sponsoring  Organization  Name  and  Address 

U.S.  Fish  and  Wildlife  Service 
Office  of  Biological  Services 
Department  of  the  Interior 
W3':;hingtnn,  D.C. 20240 

15.  Supplementary  Notes 


13.  Type  of  Report  &  Period  Covered 


16.   Abstract  (Limit:  200  words) 

The  functional  role  of  the  intertidal  oyster  reef  community  in  the  southeastern  Atlantic 
coastal  zone  is  described.  This  description  is  based  on  a  compilation  of  published  data, 
as  well  as  some  unpublished  information  presented  as  hypotheses. 

The  profile  is  organized  in  a  hierarchical  manner,  such  that  relevant  details  of  reef 
oyster  biology  (autecology)  are  presented,  followed  by  a  description  of  the  reef 
community  level  of  organization.  Then  the  reef  community  is  described  as  a  subsystem 
of  the  coastal  marsh-ecosystem  (synecology) .  This  information  is  also  synthesized  in  a 
series  of  nested  conceptual  models  of  oyster  reefs  at  the  regional  level,  the  drainage 
basin  level,  and  the  individual  reef  level.  The  final  chapter  includes  a  summary 
overview  and  a  section  on  management  implications  and  guidelines. 

Intertidal  oyster  reefs  are  relatively  persistent  features  of  the  salt  marsh  estuarine 
ecosystem  in  the  southeastern  Atlantic  coastal  zone.  The  average  areal  extent  of  the 
oyster  reef  subsystem  in  this  larger  ecosystem  is  relatively  small  (about  0.05%).  This 
proportion  does  not  reflect,  however,  the  functional  importance  of  the  reef  subsystem 
in  stablizing  the  marsh,  providing  food  for  estuarine  consumers,  mineralizing  organic 
matter,  and  providing  firm  substrates  in  this  otherwise  soft  environment. 


17.  Document  Analysis     a.  Descriptors 


Oysters,  Reefs,  Ecology,  Intertidal  zone,  Food  chain,  Estuaries 


b.   Identifiers/Open-Ended  Terms 


Salt  marsh,  conceptual  models 


c.  COSATI  Field/Group 


18.  Availability  Statement 

Unlimited 


19.  Security  Class  (This  Report) 

Unclassified 


20.  Security  Class  ahis  Page) 


21.  No.  of  Pages 

105 


(See  ANSI-Z39.18) 

■it     U.S.  GOVERNMENT  PRINTING  OFFICE:  1981  -  773-803 


Sea  Instructions  on  Reverse 


OPTIONAL  FORM  272  (4-77) 
(Formerly  NTIS-35) 
Department  of  Commerce 


LEGEND 

_A_  Headquarters  ■  Office  of  Biological 

V4  Services,  Washington,  D.C. 

-.  National  Coastal  Ecosystems  Teann, 

-_      ^-^  Slidell.  La. 

C])\T)  Regional  Offices 


U.S.  FISH  AND  WILDLIFE  SERVICE 
REGIONAL  OFFICES 


REGION   1 

Regional  Director 

U.S.  Fish  and  Wildlife  Service 

Lloyd  Five  Hundred  Building,  Suite  1692 

500  N.E.  Multnomah  Street 

Portland,  Oregon  97232 

REGION  2 

Regional  Director 

U.S.  Fish  and  Wildlife  Service 

P.O.Box  1306 

Albuquerque,  New  Mexico  87103 

REGION  3 

Regional  Director 
U.S.  Fish  and  Wildlife  Service 
Federal  Building,  Fort  Snelling 
Twin  Cities,  Minnesota  55111 


REGION  4 

Regional  Director 
U.S.  Fish  and  Wildhfe  Service 
Richard  B.  Russell  Building 
75  Spring  Street,  S.W. 
Atlanta,  Georgia  30303 

REGION  5 

Regional  Director 

U.S.  Fish  and  Wildlife  Service 

One  Gateway  Center 

Newton  Corner,  Massachusetts  02 1 58 

REGION  6 

Regional  Director 

U.S.  Fish  and  Wildlife  Sen/ice 

P.O.  Box  25486 

Denver  Federal  Center 

Denver,  Colorado  80225 


REGION  7 

Regional  Director 
U.S.  Fish  and  Wildlife  Service 
1011  E.Tudor  Road 
Anchorage,  Alaska  99503 


l.s.  1 

FISHaWll.UI.IFE 
SEHVK-K 


DEPARTMENT  OF  THE  INTERIOR  N^ 

II.S.  FISH  AND  WHDIIFE  SERVICE       v 


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sibility for  most  of  our  nationally  owned  public  lands  and  natural  resources.  This  includes 
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