LIMNOLOGY AND POLLUTION
IN LAKE VALENCIA, VENEZUELA
by
C. Kwei Lin
Great Lakes and Marine Waters Center
International Programs
Report Number 1
The University of Michigan
Ann Arbor, Michigan
1982
CONTENTS
Acknowledgments iv
List of Figures v
List of Tables ix
Introduction • 1
General Project Plan 3
Materials and Methods , 4
Morphology and Bath3niietry „ 12
Light 20
Water Current and Velocity 26
Temperature 29
Dissolved Oxygen 32
pH and Alkalinity 44
Specific Conductance 46
Total Dissolved Solids 46
Sulfur 49
Phosphorus 51
Nitrogen 62
Trace Metals 73
Chlorophyll a. , 77
Primary Productivity and Respiration 83
Phy toplankton 86
Zooplankton. 95
Coliform Bacteria 101
Pollutant Inputs from Three Major Tributaries 104
Macrophytes and Marsh System 116
Discussions and Conclusions 121
References 127
111
ACKNOWLEDGMENTS
This project was initiated and administered by the director of Direccion de
Investigacion del Ambiente (DISCA, now D.I. A.), Ing. Gustavo Parra Pardi, whose
great enthusiasm and full support were indispensable for completion of the
investigation throughout the project period in 1978. Ing. Luisa Damia deserves
my greatest appreciation for her patience in coordinating the cruises and
organizing raw data.
Marcos Mota Carpio, Jose Luis Franco, Luis Carrillo Nuiiez, and Zaida Duran
worked long hours during the cruises, and are gratefully acknowledged.
I am also thankful to the following laboratory personnel who participated in
supervising, processing, or analyzing samples for chemical and biological
parameters:
Chemists: Leopoldo Blumenkranz G.
Maria Moron de Ramirez
Vicenta Salazar
As is Alfonzo
Mercedes Garcia
Biologists: Elizabeth Bilbao de Marquez
Zaida Duran
I am also indebted to many others who assisted on the paper work or the
physical operations devoted to this project, particularly to T. Ladewski and^
P. Morissey for their assistance in data analysis, and to S. Schneider for
editorial service .
IV
LIST OF FIGURES
Figure Page
1. Monitoring stations for physical, chemical, and biological
parameters. Circled stations indicate the locations where
samples were taken for laboratory analysis of chemicals,
phytoplankton, and zooplankton. Distance between two
adjacent stations is approximately 3 km 5
2. Samipling scheme, volume, and treatment of water samples for
water quality analysis 11
3. Watershed boundary of Lake Valencia tributaries 14
4. Bathymetric map of water column in Lake Valencia
(contour in 5-m depth intervals) 15
5. Lake surface area and volumes corresponding to depth
intervals 17
6. Water levels recorded between 1962 and 1976 18
7. Water level fluctuation recorded in 1978 19
8. Incident solar radiation on lake surface recorded on
a clear day in February, 1978 21
9. Depths (m) of light penetration in water column at
stations along a transect on May 25j 1978 22
10. Depth contour (cm) of Secchi disc transparency during
four cruise periods in 1978. . 23
11. Depth contour (m) of 1% light penetration during six cruise
periods in 1978 24
12. Correlation between depths (m) of 1% light penetration and
Secchi disc transparency 25
13. Current direction and velocity in surface water (0.5 m)
during four cruise periods in 1978 27
14. Variations in direction and velocity of water current recorded
at various depths at station 20 on three dates in 1978 28
15. Annual isothermal variation (**C) of water temperature
recorded from station 20 during 1978 30
16. Vertical temperature profile recorded at station 31 during
six cruise periods 31
V
17. Depth-time isopleths of daily temperature variation at
station 20 on February 21, 1978 33
18. Depth-time isopleths of dissolved oxygen concentration
(rag/L) at station 20 during 1978 34
19. Vertical distribution of dissolved oxygen at station 31
recorded on six dates in 1978 36
20. Depth-time isopleths of dissolved oxygen concentration (mg/L)
during day hours on February 21, 1978 37
21. Depth-time isopleths of dissolved oxygen concentrations (mg/L)
on September 11, 1978 38
22. Changes of dissolved oxygen concentration throughout the water
column at station 20 during 12-hr of daytime on five dates
in 1978 39
23. Horizontal distributions of dissolved oxygen concentration
at five different depths during April 24-28, 1978 41
24. Vertical gradient of dissolved oxygen concentration (mg/L)
in north-south transect during six cruise periods in 1978 42
25. Vertical gradient of dissolved oxygen concentration (mg/L)
in east-west transect during six cruise periods in 1978 43
26. Seasonal variations in pH, conductivity, and concentration of
sulfate total dissolved solids in surface water at station 31.. 45
27. Distribution of specific conductance (ymho/cm @ 25**C)
recorded during three cruise periods in 1978 47
28. Distribution of sulfate concentration (ppm) recorded during
two cruise periods in 1978 50
29. Annual variation in total phosphorus, total dissolved phosphor-
us, and inorganic soluble orthophosphate at station 20, 1980.
Values are presented by surface area 53
30. Vertical distributions of total dissolved phosphorus,
total phosphorus, and orthophosphate at station 20
during six cruises in 1979-80 54
31. Horizontal distribution of total phosphorus
during six cruise periods 56
32. Time-depth distribution of total phosphorus concentrations,
in 20 pg/L gradient contour, during 1980 57
VI
33. Horizontal distribution of total dissolved phosphorus
during six cruise periods , . . . 59
34. Time-depth distribution of total dissolved phosphorus,
in 20 yg/L gradient contour, during 1980
35. Horizontal distribution of orthophosphate
during six cruise periods
37. Annual variation in total K nitrogen, NH3 - N, and NC3 + NO2
concentrations at station 20
38. Vertical distributions of total K nitrogen, NH3 - N,
and NO3 + NO2 at station 20 during six cruise period
39. Horizontal variation in total R nitrogen at selected stations
during six cruise periods
40. Time-depth distribution of total K nitrogen at
station 20 in 1980
46. Depth-time isopleths of chlorophyll a concentration at
station 31 in 1978 ""
48. Vertical variation in chlorophyll £ concentration at
station 20 on February 22, 1978
50. Vertical distribution of respiration in the euphotic zone at
station 20 on five dates in 1978
60
61
36. Time-depth distribution of orthophosphate concentration
during 1980 ^3
64
periods 66
67
68
41. Horizontal variation in NO3 - NO2 concentration
at selected stations during six cruise periods 70
42. Vertical variation in NO3 - NO2 concentration
at station 20 during 1980 71
43. Horizontal variation in NH3 - N concentration at
selected stations during six cruise periods 72
44. Vertical variation in NH3 - N concentration
at station 20 during 1980 74
45. Vertical distribution of iron, nickel, lead, zinc, copper,
mercury, calcium, and magnesium concentrations 75
79
47. Lakewide distribution of chlorophyll _a (mg/m^) during
six cruise periods in 1978 3q
82
49. Vertical distribution of primary productivity in the euphotic
zone at station 20 on five dates in 1978 34
85
Vll
51. Seasonal variation in total cell numbers of phytoplankton and
Microcystis at station 31 in 1978 87
52. Vertical distribution of total phytoplankton during six
cruise periods in 1978 88
53. Vertical distribution of Microcystis during six
cruise periods in 1978 90
54. Horizontal distribution of predominant genera of three m.ajor
phytoplankton classes during six cruise periods in 1978 91
55. Variation in zooplankton composition during October 1979 98
56. Horizontal variation in total zooplankton numibers during
October 1979 99
57. Vertical distribution of major zooplankton taxa at station 31
during October 1979. 100
58. Density of coliform bacteria at sample stations in May, July,
and October 1980. Blank circles and squares indicate total
coliform, and dotted S3niibols indicate fecal coliform 103
59. Distribution of major emerged macrophytes on the lake shore.... 119
Vlll
LIST OF TABLES
l£^. Page
1. Cruise schedules and activities 4
2. Morphometry ^ 13
3. Major tributaries and their lengths and areas.. 13
4. Areas (A) and volumes (V) in 5-m depth intervals 16
5. Mercury content in muscle and viscera
of Tilapia mossambica -79
6. Species composition and population density of the zooplankton
community in October, 1979 95
7. Density of total and fecal coliform (MPN/100 mL) sampled
at 12 stations on three dates in 1980 102
8. Typical contaminants discharged by agricultural industries
in the Lake Valencia basin 207
9. Chemical and physical parameters of discharges of three
major tributaries in the Lake Valencia basin , 108
10. Annual flows and discharges of contaminants from three
major tributaries in the Lake Valencia basin m
IX
INTRODUCTION
Lake Valencia is the largest natural freshwater lake in Venezuela.
The lake is located in one of the most populated areas in the country and its
watershed possesses approximately 8% of the total population, with a density of
350 persons/km^.
Increasing population in the watershed, coupled with rapid expansion of
industry and agriculture, has caused gross environmental contamination in Lake
Valencia. Once known for its great beauty and potential natural resources. Lake
Valencia water quality is currently undergoing a rapid deterioration.
The eutrophication of Lake Valencia has long been accelerated by the
pronounced natural desiccation and greatly enhanced by modern human activity in
the lake basin.
The desiccation phenomenon and its effect on lake hydrological balance was
first reported by von Humbolt (1856) and more recently by Bockh (1956), Cartaya
and Montano (1969), and MARNR (1980). Although several investigations have been
devoted to pollution problems in the lake (INOS 1971, Torrealba and Cardenas
1972, AVIS 1973, and Fuchs and Mosqueda 1975), little effort has been made to
obtain more basic and comprehensive limnological information.
As most available documents on eutrophication and pollution in the
freshwater environment were obtained from the northern temperate region, the
problems and consequences associated with those processes in tropical lakes are
relatively unfamiliar. Therefore, investigation on the fundamental limnological
features in Lake Valencia would generate valuable scientific information which,
in turn, may provide a practical guideline for improving water quality and
managing the resources of this aquatic ecosystem.
Several specific objectives were set in the present project:
1. To measure the velocity and direction of water movement. This infor-
mation is essential in order to understand the mixing process of the
water mass and dispersion patterns of contaminants from point sources.
2. To measure the light penetration and transparency in the lake water.
The light regime in the water column is of primary im.portance for
determining the depth of the euphotic zone which governs phytoplankton
production.
3. To analyze major nutrients, trace metals, and organic contaminants in
lake water.
4. To document the community composition and succession of planktonic
organisms and their environmental factors.
5. To survey the submerged and emerged macrophytes.
6. To survey the pollutant inputs of three major tributary inflows:
Rio los Guayos, Rio Guey, and Cano Central.
GENERAL PROJECT FLAN
To meet the project objectives, six cruises were conducted approximately
bimonthly from January to December 1978. The exact schedule and activities for
each cruise are listed in Table 1. All the cruises were carried out with a
24-ft Rotork boat, A normal cruise required five persons on shipboard to under-
take all research activities.
The general survey was conducted at 40 stations distributed evenly on a
grid of 3 km between neighbor stations (Fig. 1). Several field parameters were
measured at each of the 40 stations, and 17 of those stations were selected for
chemical and biological sampling. Those samples were delivered to either the
field laboratory in Valencia or the central laboratory in El Hatillo.
Each cruise included four types of investigations: (1) monitoring and
sampling for physical-chemical properties and plankton, (2) measuring primary
productivity of phytoplankton, (3) monitoring dissolved oxygen, current, and
diurnal cycle of temperature, (4) monitoring pollutant dispersion in Rio Los
Guayos and Rio Guey .
Measurement of primary productivity of phytoplankton was carried out in
situ at three fixed stations (17, 20, and 23), located in the western, central,
and eastern parts of the lake respectively. The diurnal changes of pH,
dissolved oxygen, temperature, and current were monitored throughout the water
column during 12-24 hours at the central location of the lake (station 20).
The last part of each cruise involved the monitoring of the dispersion of the
two major tributary inflows: Rio Los Guayos in the west part and Rio Guey in
the east part of the lake proper. Measurements of temperature, light, current,
dissolved oxygen, and chemical samplings were made at nine stations adjacent to
each river mouth covering an area of 9 km^ on a 3 km x 3 km grid.
TABLE 1. Cruise schedules and activities.
Cruise No. Date Activity
1 Jan. 11-14 Routine 40 stations
Jan. 25-27 Primary productivity, station 20
Pollution dispersion
2 Feb. 7-10 Routine 40 stations
Feb. 21-23 Primary productivity, station 20
Pollution dispersion
3 Apr. 23-28 Routine 40 stations
May 23-25 Primary productivity, station 20
4 Jul. 17-26 Routine 40 stations
Primary productivity, station 20
Pollution dispersion
5 Sep. 29-Oct. 6 Routine 40 stations
Primary Productivity, station 20
Pollution dispersion
6 Nov. 21-24 Routine 40 stations
Primary productivity, station 20
Pollution dispersion
MATERIALS AND METHODS
Physical Parameters
1. Water movement -
The velocity and direction of current flows were measured
with a Braystoke current flow meter with 5" diameter im-
peller. At each station the measurement was made at 5-m
intervals starting from the surface. At station 20 the
measurement was made at 3-hr intervals throughout the en-
tire depth over 12 hours. Current velocity is expressed in
cm/sec and direction in degrees of compass.
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2. Light penetration and transparency -
Photosynthetic light regime was measured using LAMBDA
underwater quantum meter (Lincoln, Nebraska) at 0, 0.5, 2,
and 10 meters at each station. Light level is expressed in
y Ein m"^ sec""-^, and penetration was calculated as percent
of surface light intensity. Transparency was determined at
all stations with a standard black and white Secchi disc.
3. Hydrolab parameters -
Other limnological measurements made in situ were depth,
temperature, pH, specific conductance, and dissolved
oxygen, using a Hydrolab Surveyor Model 6D Water Quality
Analyzer (Hydrolab®, Austin, Texas). Measurements were
made at a series of depth intervals throughout the water
column at all stations. Diurnal variations of those
parameters were recorded at station 20.
B. Biological Methods
1. Aquatic macrophytes -
Aquatic macrophytes growing along the coastal region of the
lake and islands were sampled for species identification
and their distributions were recorded on a map. Submerged
plants were collected by hand or hooks. The distribution
of emerged vegetation was surveyed from the lake by a small
boat and from land on foot. Several collections were made
to determine the vegetative and reproductive growth cycle
throughout the year. Specimens collected were pressed dry
for species identification.
2. Phytoplankton -
Phytoplankton samples were collected at selected stations
(0, 1, 2, 5, 8, 12A, 17, 18, 21, 23, 24, 25, 31, 33, 35,
39, and 40). At each station, a composed sample was taken
from surface (0.5 m) , mid, and maximum depths of the water
column using a centrifugal pump. At station 31, a series
of profile samples was taken from 0.5, 5, 10, 20, and 30 m.
The 250-mL phytoplankton sample taken from each station was
immediately preserved with 5 mL of Lugol solution and
subsequently concentrated to 25 mL by settling. The
phytoplankton taxa were identified at the generic level and
all numbers of each taxonomic entity were counted using an
inverted microscope.
3. Chlorophyll a. -
A 250 to 500 mL phytoplankton sample, taken from each
station and from various depths at station 31, was filtered
through a GFC filter. The chlorophyll a. was extracted with
10 mL of 90% acetone in an opaque vial in ambient cold for
a minimum of 24 hours. The chlorophyll a concentration was
determined by spectrophotometric method (Strickland and
Parsons 1969).
4. Zooplankton ~
Twenty liters of sample composed equally of surface, mid, and
bottom lake water were filtered through a #25 plankton net
(mesh size 65 ym) . The sample was concentrated to 300 mL and
preserved with 5% formalin. Species of major genera were
identified and numbers of each taxon were enumerated.
7
5. Primary productivity of phytoplankton -
Phytoplankton productivity was measured several timies at
four locations in 1978, using the light-dark bottle
dissolved oxygen exchange method. Water samples taken from
surface, 0.5, 2, 3, 5, and 10-m depths were enclosed in
300-mL Winkler bottles (two light and two dark bottles for
each depth). Those bottles were suspended for 4 hours
(1000-1400 hours) in the water column in respect to the
original sampling depths. The dissolved oxygen
concentrations in those incubation bottles were measured at
the beginning and the end of the incubation period. The
productivity was calculated by the following formulae:
net productivity =03-0]^
gross productivity =03-02
where cj = initial concentration of dissolved oxygen
C2 = final concentration of dissolved oxygen in
dark bottle.
C3 = final concentration of dissolved oxygen in
light bottle.
To determine the specific productivity, phytoplankton
standing crop in each incubation bottle was filtered to
determine chlorophyll a^ concentration.
6. Coliform bacteria -
Sampling locations. To determine the sources and
distribution of total and fecal coliform in the lake,
8
twelve sample stations were chosen. Six were near the
outfalls of major tributaries and the remaining six
stations represent open lake water. Those locations are
designed as A, B, F, H, I, P, 5, 6, 12A, 31, 33, and 39.
Stations A and B are located at the mouths of Rio Los
Guayos and Cano Central, respectively. Stations 5 and 6
are adjacent to B and A on the open lake side. Another
heavily polluted area is the western end of the lake in the
vicinity of Rio Guey outfall where stations H and 33 are
located. Stations F, I, P, and 39 are situated near the
shore of plantations representing non-point source
discharges, and occasional discharges from cattle and pig
farms. Station P is located in the vicinity of Puenta
Palmita, which is the most popular bathing area for local
residents. Station 12 represents the least contaminated
area, remote from any point and non-point sources of
domestic and industrial waste.. Station 31 represents the
major mass of lake water because it is the deepest and most
turbulent area of the lake.
Sample logistics. Bacterial samples were taken from the
12 stations in the morning and completed within 2 hours.
At each station, triplicate samples were taken from surface
water and put in pre-sterilized bacterial sample bottles.
Those samples were transported to the field laboratory in
Valencia immediately after collection.
Laboratory procedures for determination of total and
fecal coliform density in the samples were according to the
multiple-tub fermentation procedures to obtain the Most
Probable Number (MPN) index. The details of the method are
in Standard Methods (APHA 1975).
Chemical Procedures
1, Water samples were collected at 17 selected stations
(Fig. 1) using a centrifugal pump. The water sample taken
from each station was composed of an equal mixture of
surface, mid, and bottom water, except for station 31 where
water samples were taken separately from five depths at
0.5, 5, 10, 20, and 30 m.
The sampling scheme, volume, and preservation for water
chemistry are shown in Figure 2. Chemical parameters
analyzed in the laboratory were chloride, heavy metals (As,
Cd, Cu, Fe, Pb, Mn, Zn, Hg, and Cr), nitrogen (ammonium,
nitrite, nitrate, and total Kjeldahl), pH, phosphorus
(soluble reactive othophosphate, total dissolved phosphate,
and total phosphate), solids (total, total dissolved), and
sulfate. Analytical procedures used for quantitative
determination of the chemical parameters were based on
Standard Methods (APHA 1975). Trace metals were analyzed
with a Beckman atomic absorption spectrophotometer, phos-
phorus and nitrogen with a Technicon autoanalyzer . Sulfate
was determined by the turbidimetric method and solids by
the gravimetric method.
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MORPHOLOGY AND BATHYMETRY
Lake Valencia occupies a drainage basin of 3,000 km^ in northern central
Venezuela. Geologically, the lake basin is situated on metamorphic basal rocks
and was formed by a relatively narrow depression (a graben) between two fault
scarps running WSW - ENE (Peeters 1968). The depression has been gradually
filled with Pleistocene and Holocene sediments. Accumulation of the sediments
blocked the drainage outlet of the Rio Paito to the Orinoco basin when the lake
elevation dropped to 429 m above sea level around the year 1727. Since then the
lake has become a closed system.
The modern lake morphometry is listed in Table 2. The surface area of the
lake is 356 km^, approximately 12% of the water shed area (Fig. 3). There are
16 tributaries distributed throughout the watershed (Table 3), but the lengths
of most tributaries are less than 30 km and originated from steep Andean
mountains in the north. Most of those rivers carry a considerable amount of
silts from surface runoff during the rainy season and have little flowing water
during the dry season. Only those rivers such as Los Guayos, Caiio Central, and
Rio Guey, which drain domestic and industrial waste water, have continuous flow
throughout the year.
The most recent bathymetric map (Fig. 4) was made in 1963 and the depth
contour and shoreline have since altered considerably. The maximum depth has
been reduced from 40 m to 37 m, and Champego Island is now connected to adjacent
land. The lake was relatively shallow at the western portion and the drop in
water level has created an extensive soft, muddy shore. The deeper part (35 m)
of the lake extends between Punta Palmita and Macapo-Yuma shore. Table 4 shows
12
TABLE 2. Morphometry.
Altitude 405 m (asl)
Maximum length (L) 30 km
Maximum width (b) 17.8 km
Mean width (b) 12.7 km
Area (A) 356 km^
Volume (V) 6,740 X 10^ m^
Maximum depth (Zm) 37 m
Mean depth (z) 18 m
TABLE 3. Major tributaries and their lengths and areas,
Tributaries
Aragua
Turmero
Guigue
Paya
Guayabita
Limon
Maracay
Guey
Tocoron
Noguera
Guacara
Guaica
Los Guayos
San Joaquin
Mariara
Cura
Length
(km)
58
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TABLE 4. Areas (A) and volumes (V) in 5-m depth intervals,
Depth (m)
A (km-^
356
5
286
10
243
15
215
20
177
25
132
30
99
35
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Vz (lO^m^)
Va (lO^m^)
100
80
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1,602
68
1,321
2,923
60
1,144
4,067
49
978
5,045
37
769
5,814
27
575
6,389
13
351
6,740
the water volumes at 5-m depth intervals, and the decrease of surface area may
be estimated by the loss of corresponding water volume as indicated in Figure 5.
Decrease of water level has been a historical and most serious problem in
Lake Valencia. As a rough estimate, the average annual water loss has been
60 X 10^ m^ in the past 170 years (Apmann 1973). The recent accelerated
desiccation has most probably been due to the destruction of forests, the slash
and burn land cleaning, and agricultural and industrial consumptions of both
surface and underground water in the lake basin. Figure 6 shows the decreasing
trend of lake elevation from 406 m in 1962 to 403 m in 1976. The water loss was
particularly pronounced between 1971 and 1976 during which the lake level
dropped 2 m. The annual fluctuation between 1978 and 1979 is shown in Figure 7.
In this yearly cycle the water level dropped 50 cm from the end of rainy season
in October 1978 to February 1979, but a large rise occurred starting in May
1979. The rise of water level in 1979 was accelerated by the diversion of Rio
Cabrales which added 4-5 m-^/sec of water from sources outside the watershed.
16
AREA(Km2)
Q,^ 50 10 150 200 250 300 350
1000 2000 3000 4000 5000 6000 7000
volume: (10® m^)
FIG. 5. Lake surface area and volumes corresponding to depth intervali
17
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LIGHT
The solar radiation on Lake Valencia's surface on an average day is 12
hours with a maximum intensity of approximately 2,000 +^ 500 VEm m ^ sec ^ near
mid-day. Figure 8 shows the distribution of light intensity throughout a
typical clear day in February. Day length and light intensity in this low
latitude (10'' N) are relatively constant throughout the year. Typical light
attenuation in the water column at several transect stations is shown in
Figure 9.
The depth of light penetration in Lake Valencia is extremely shallow. As
shown by Secchi disc transparency (Fig. 10), the visibility ranged from 23 cm to
284 cm, with the majority of values between 100 and 150 cm. The greatest
transparency occurred in January and the lowest in July. Lakewide , the central
area between Punta Palmita and Isla Otama had greater transparency than other
areas of the lake. There were two areas where water visibility was persistently
poor, with values less than 50 cm during most cruises. The poor transparency in
those areas was mainly caused by the heavy loading of suspended matter from Rio
Los Guayos and Cano Central in the southwest and Rio Guey in the northeast. The
turbidity, resulting from sediment resuspension, also contributed to low trans-
parency in the shallow waters (<2 m) of the western basin. The variation of
transparency in open water near the center of the lake was largely due to the
phytoplankton blooms.
Attenuation of photosynthetic light intensity as measured by quantum is
highly correlated to Secchi disc transparency values (Fig. 11), and the
calculated depth of 1% light penetration (Fig. 12) reached 5 m in the central
basin. The major portion of the lake received this light at 3 m. In situ
20
0655 0855 HOO 1305 1510 1715 1830
DAY TIME (Hour)
FIG. 8. Incident solar radiation on lake sjurface recorded on a clear day
in February, 1978.
21
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JANUARY 1978
JULY 1978
FIG. 11.
in 1978,
Depth contour (m) of 1% light penetration during six cruise periods
24
1.0
I
J L
JAN. 12-14, 1978
J L
0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
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MAR. 25 -28, 1978
0.8 0.9
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SECCHI DISC DEPTH (m)
FIG. 12. Correlation between depths (id) of 1% light penetration and Secchi
disc transparency.
25
measurements of oxygen production and consumption indicate that 1% light level
is a critical compensation zone where photosynthetic and respiratory activities
are nearly equal.
WATER CURRENT AND VELOCITY
Current direction and velocity of water mass in Lake Valencia are
illustrated for four cruise periods by Figure 13. The maximum velocity observed
during those cruise periods frequently reached 50 cm/sec or greater;
occasionally up to 100 cm/sec was recorded. The current directions were
extremely variable and durations of any given direction were normally no more
than a few hours. Large numbers of islands located in the southern half of the
lake complicated the current flow pattern as the velocity and direction of
surface current exhibited great regional differences.
The greatest surface currents often occurred in the central, eastern, and
western basins when the wind blew directly from the opposite end of the lake.
Several observations were made for the duration of current throughout the
water column, over a period of 12 hours or longer, at station 20 near the
geographical center of the lake (Fig. 14). Those observations indicate that the
movement of water mass was extremely responsive to wind stresses over the lake
surface. On February 21, a gust of wind (20 kph) occurred during the night.
The velocity of the water column was measured at 0650 hr on the following day.
At most depths, the current velocity reached 40 cm/sec. At depths greater than
20 m, the velocity was reduced to 25 cm/sec. The surface current direction was
east-north-east and shifted to east-south-east in the lower half of the water
mass. The current velocity normally dampened within a few hours, from 40 cm/sec
26
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to a standstill in 4 hours on Feb. 21, and from 60 cm/sec to flow in 5 hours
on May 1 .
Tlie wind data, collected from a northwestern lake shore weather station
(Base Sucre) by the Venezuelan Air Force, show most of the maximum wind velocity
was recorded in the afternoon and evening. Frequent changes in wind direction
throughout the day caused the velocity and direction of water current to be
extremely irregular and unpredictable.
Langmuir circulation appeared to be a common water movement pattern in
surface water as indicated by frequent occurrence of streaks. These streaks,
ranging from 0.5 to 2 m wide, contained mostly phytoplankton, aquatic
macrophytes, and invertebrate residues.
TEMPERATURE
The annual thermal variation in Lake Valencia is shown in Figure 15.
The maximum temperature (30**C) occurred in surface water during June and the
minimum temperature occurred in subsurface water during January and April. In
general, the low temperature (26°C) of the water mass prevailed in the early
part of the annual cycle and was relatively uniform throughout most of the water
column. The water temperature increased progressively from April to July and it
reached 27*'C in the deep water toward the end of the year.
Although the vertical thermal differential in the lake was small, approx-
imately 5C°, the formation of transient thermal stratifications occasionally
occurred during June and October (Fig. 16). Thorough mixing of the water col-
umn, as indicated by the isothermal conditions of the water mass, occurred in
December. Pronounced diurnal variations in water temperature occurred mostly at
the surface. For instance, on February 21, 1978, the surface water temperature
29
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TEMPERATURE (°C)
25 26 27 28 29 30
31
STATION 31
Jan. 12/1/78
Feb. 9/2/78
Apr. 28/4/78
July. 18-21/7/78
Sept. 26/9/78--2-6/I0/78
Nov. 21-24/11/78
FIG. 16. Vertical temperature profile recorded at station 31 during six
cruise periods.
31
in the morning was less than IS'^C and it rose to 28°C in late afternoon (Fig.
17), The heat that accumulated in the surface water was quickly dissipated
during the night. The temperature of the water mass below 3 m remained stable
at 25.5**C. The cool water mass in the subsurface upwelled during the night,
which may possibly have resulted from surface cooling at night.
DISSOLVED OXYGEN
Annual Variation
The annual variation of DO in the water column near the geographical center
of the lake is extremely dynamic (Fig. 18). Oxygen stratification, from
supersaturated to anoxic conditions, prevailed during most of the year.
Supersaturation frequently occurred near the surface (<2 m) , and severe oxygen
depletion deficit (<50% saturation) constantly occurred below 10 m depth.
During December-March the lake water was well mixed as indicated by the
isothermal condition. The oxygenation reached the maximum depth of the lake.
Although the mixing process effectively replenished the DO in the anoxic deep
water, the oxygen deficit in the entire water column became pronounced. On
January 22, 1978, the DO level in the surface water was depleted as low as
3 ppm. This event of extreme DO deficit, coupled with extensive H2S released
from premixing in the anoxic zone, caused a massive fish mortality. The strong
oxygen stratification began in early April, and a rapid DO depletion followed
during May and June. The extensive anoxic depth occurred below 20 m in mid-
April and surfaced to 8 m in October. The largest DO gradient in the water
column was observed between 5 to 15 m, particularly in May and October during
which the DO decreased from 9 ppm at the surface to 6 ppm at 5 m and to ppm at
10 m. Such a large change in DO gradient appeared to be related to a large
32
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34
phytoplankton biomass buildup in the euphotic zone as a result of calm weather.
The DO profiles measured during six cruise periods are shown in Figure 19.
The greatest fluctuation of DO content in the water column occurred between
January and February, increasing from 5 to 12 ppm at the surface 5m. A similar
distribution pattern, typical clinograde, was observed during April, July,
September, and November.
Daily Variation
The DO concentration exhibited in diurnal variation was often episodic and
the amplitude of variation depends primarily on rates of photosynthetic produc-
tion, respiratory consumption, and mixing depth. Figures 20 and 21 illustrate
two specific dates showing two different DO variation patterns in the water
column during the day. On February 21 a relatively uniform DO distribution was
recorded from the surface (6.0 ppm) to the bottom (5.5 ppm) in the morning, and
became highly stratified in the late afternoon. The greatest DO reduction
occurred between 3 and 5 m, with DO reducing from 9 to 6 ppm at 7 pm. The
photosynthetic oxygen production was 3 g 02/tn^/day in the surface and the
compensation depth was at 3 m; at that depth no apparent increase in net DO
production occurred. A rapid destratif ication of DO followed during the
evening. On September 11, when the anoxic layer was extensively developed
(below 15 m depth), the DO fluctuated between 8 and 10 ppm in the top 5 m and
drastically reduced to between 10 and 15 m. The mixing event had rarely
extended to the pyncnocline (anoxic zone).
The budget of DO in the water column was measured in several cruises
(Fig. 22). On January 25 and March 23, there were net gains of DO throughout
the water column, but during the remaining dates various degrees of DO deficit
35
DISSOLVED OXYGEN (mg/L)
4 5 6 7 8 9 . 10 II
12 13 14
STATION 31
Jan. 12/1/78
Feb. 9/2/78
Apr. 28/4/78
Jul. 18-21/7/78
Sept. 26/9-2-6/10/78
Nov. 21-24/11/78
FIG. 19. Vertical distribution of dissolved oxygen at station 31 recorded on
six dates in 1978.
36
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6.0 6.5
TIME (Hr)
100 1500
7.0 8.0
1900 2300
9.0 8.5 7.5
FIG. 21. Depth-time isopleths of dissolved oxygen concentrations (mg/L) on
September 11, 1978.
38
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39
prevailed in the subsurface water. As indicated in Figure 18, the lake remained
totally oxygenated during the early part of the year and the pyncnocline devel-
oped from May throughout the rest of the year. The oxygen depletion in subsur-
face water during the anoxic period was most probably enhanced by the greater
organic loading from surface runoff during the rainy season (from May to
November), and the greater phytoplankton standing crop (shown by chlorophyll
level in Fig. 33) reduced the light penetration and contributed dead biomass in
the aphotic zone.
Lakewide DO Distribution
The horizontal distribution of DO in the lake water varies a great deal
depending on the depth of the water column, the land use of the shoreline, the
influence of tributary inflow, the photosynthetic production, the respiratory
consumption of oxygen, and the mixing effectiveness by wind action. Figure 23
shows the lakewide DO distribution at depths of 0.5, 5, 10, 20, and 30 m during
the April cruise. The maximum DO value in the surface water (0.5 m) was 10 ppm
near the west and south portions, and gradually decreased to 7.5 ppm near the
center of the lake. The DO concentration was above saturation level in most of
the surface water except nearshore of the northeastern region where DO sagged to
4 ppm (50%). It is expected that the DO content progressively decreased in the
deeper water, and a major portion of the zone below 20 m was free of oxygen
during this period.
The horizontal and vertical DO distributions along the north-south transect
during six cruises in 1978 are shown in Figure 24, and the east-west transect in
Figure 25* During the January-February period the entire lake was oxygenated,
with the maximum DO of 13 ppm and minimum at 1 ppm. The low values were
40
DO (ppm)
0.5 m
APR.25-28J978
DO (ppm)
5m
APR. 25-28 J978
DO (ppm)
10 m
APR.24-28J978
DO (ppm)
20 m
iiil ppm
APR.24-28J978
0.2\ ^ K
DO (^ppm)
30 m'
ppm
APR. 24-28, 1978
FIG. 23. Horizontal distributions of dissolved oxygen concentration at five
different depths during April 24-28, 1978.
41
5
10
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t 20
UJ
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25
30
35
5
10
5 15
& 20
o
STATION
40 37 30 20 II
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—
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STATION
40 37 30 20 II
25
30
35
DO TRANSECT
APR. 25-28,1978
STATION
40 37 30 20 II
5
10
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30
35
5
10
i 15
X
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o
25
30
35
STATION
40 37 30 20 II I
DO TRANSECT
FEB. 7-I0J978
STATION
40 37 30 20 II
DO TRANSECT
JULY 18-21.1978
STATION
40 37 30 20 11
DO TRANSECT
NOV. 21 -24,1978
FIG. 24. Vertical gradient of dissolved oxygen concentration (mg/L) in
north-south transect during six cruise periods in 1978.
42
5
10
1 15
X
fc 20
25
30
35
STATION
25 24 23 22 21 20 19 18 17 16
• V
"^7
— \ • • • •
• •
• • 7
• • /
— \ * *
• •
• •
""X /
DO TRANSECT \^
, /
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- JAN.II-I3,l978 Xr-^
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STATION
25 24 23 22 21 20 19 18
17 16
35 L-
5
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X
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25
30
35 L-
STATION
25 24 23 22 21 20 19 18
17 16
DO TRANSECT
JULYI8-2IJ978
STATION
25 24 23 22 21 20 19
FIG. 25. Vertical gradient of dissolved oxygen concentration (mg/L) in east-
west transect during six cruise periods in 1978.
43
recorded from the water mass near the bottom (20-25 m) at stations 22 and 23 in
January and stations 16 and 17 in February. Those stations were adjacent to
either end of the east-west transect receiving the Outfalls of the Rio Los
Guayos and Rio Guey, respectively. The continuous deoxygenation trend which
developed on the eastern shore encroached into the entire lower strata of the
lake basin.
The persistent long-term oxygen depletion in subsurface Lake Valencia
presents a very serious water quality problem. Under unaerobic conditions, the
production of H2S and methane became prevalent, and regeneration of nutrients
was enhanced. Several major factors responsible for the oxygen depletion in
Lake Valencia are the external input of organic matter through tributaries, the
respiratory consumption of planktonic organisms (phytoplankton, zooplankton, and
bacteria), and sediment oxygen demand. In order to accurately assess the oxygen
budget in the lake, these major processes involved in oxygen loss must be
investigated,
PH AND ALKALINITY
Several measurements of alkalinity made in Lake Valencia during 1978 ranged
between 300 and 350 mg/L CaC03. The seasonal and spacial variations in
alkalinity were relatively small (Fig. 26).
The pH value in lake water ranged from 7.4 to 9.2, with high values in the
surface and low ones near the bottom. Seasonal pH variation in surface water
was minimal (Fig. 26). However, the pH in the bottom water was lowered to 7.6
and 7.4 in April and November, respectively.
44
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SPECIFIC CONDUCTANCE
Specific conductance in Lake Valencia ranged from 1,200 to 2,300 ymho/cm,
with mean value around 1,900 ymho/cm. The seasonal fluctuation in the surface
water of the main water mass remained relatively constant, with high values
(>2,000) recorded during February and September (Fig. 26). Vertical variation
at most stations also showed a great uniformity, with differences less than
200 ymho/cm, or 10%, between surface and bottom at the central deep station.
There were no persistent seasonal and horizontal variations in the surface water
(Fig. 27). During April 24-29, the maximum value (2,000 ymho/cm) was observed
in the northwest region of the lake, and gradually lowered to 1,200 ymho/cm in
the southern and eastern portions. During the September-October cruise period,
the value in most parts of the lake was 2,000 ymho/cm or greater.
The specific conductance of the common bicarbonate type freshwater lake is
closely proportional to concentrations of total dissolved solids, approximated
by the equation:
Sp. Conductance (ymhos/cm 25°C) x A = total dissolved solids.
A is the coefficient obtained experimentally. The factor A is generally
between 0.55 and 0.75. However, the analyses of Lake Valencia data on specific
conductance and total dissolved solids show little correlation.
TOTAL DISSOLVED SOLIDS
The lakewide distribution of total dissolved solids (TDS) in 1978 ranged
46
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mostly between 1,500 and 3,000 mg/L, with an average value around 2,000.
Concentration in the surface water at the central station rose from 1,500 mg/L
in January to 2,000 mg/L in July, and reached a maximum of 3,000 mg/L in
September (Fig. 26). Dissolved solids in Lake Valencia consist mainly of
sulfate, magnesium, calcium, chloride, sodium, potassium, carbonate, and
bicarbonate. In the 1978 investigation, sulfate and chloride were analyzed in
all six cruise periods, and other constituents were less frequently analyzed.
Their relative quantitative relationship which prevailed in Lake Valencia was:
Cations: Mg > Ca > Na
Anions: SO4 > CO3 > Cl
The measurements made on November 22 show that Mg:Ca ranged from 2-10. The most
uncommon feature of the chemical constituents in Lake Valencia is the unusually
high sulfate concentration, comprising approximately 30 to 50% of dissolved
solids .
From a health standpoint the TDS concentration in Lake Valencia has already
reached a harmful level of 2,000 mg/L (National Academy of Science 1973). It
was reported that when the sum of the Mg and SO4 content exceeds 1,000 mg/L, or
TDS content is above 2,000 mg/L, the water has a laxative effect on most
persons. A similar effect also occurs in domestic animals if the drinking water
contains TDS above 1,000 mg/L. It is important to note that water which
contains a high concentration of TDS may conceivably have toxic effects on many
individual organisms. Water containing a high TDS concentration is
objectionable in industrial uses because it may cause formation of boiler scale,
accelerate corrosion, and interfere with the clearness and color of finished
48
products (McKee and Wolf 1963).
The rapid increase in TDS over time is one of the most indicative features
of deterioration of water quality in Lake Valencia. From a simple input-output
model, Apmann (1973) estimated that the TDS concentration was 348 mg/L in 1727
and rose to 1,332 in 1970. The most rapid increase occurred after 1960 and the
value observed in 1978 rose above 2,000 mg/L, which was greater than that
predicted by Apmann' s model (1,700 mg/L in 1980). The difference may result
from the desiccation of the lake, which was not included in Apmann* s model.
The major external sources of TDS inputs to Lake Valencia are tributary
inflows, weathering of soil and rock in the lake basin, and atmospheric
precipitation and fallout. Examination of the existing chemical data on Lake
Valencia shows that the rapid increase in SO4 concentration contributed the
single largest component in TDS's loading budget during the past few years.
Desiccation in this closed lake was undoubtedly an important causal factor in
the rapid increase in TDS concentration.
SULFUR
Two major forms of sulfur present in Lake Valencia are sulfate and sulfide.
The sulfate concentration reported by INOS (1971) was less than 400 ppm. In
this study in 1978, the value ranged from 400 to 1,500 ppm. Figure 26 shows the
concentration measured for surface water in the center of the lake, with high
concentrations (>1,000 ppm) occurring during the January-February period and
decreasing gradually throughout the remaining periods. However, the lakewide
horizontal variation was rather large (Fig. 28). The major factors causing
large seasonal and regional changes may be the degree of sulfur reduction and
49
SULFATE (mg/L)
JULY 17-21, 1978
> 900 ppm
800-900 ppm
n 700-800ppm
600-700ppm
SULFATE (mg/L)
OCT 2-6, 1978
700-800ppm
600-700ppm
FIG, 28. Distribution of sulfate concentration (ppm) recorded during two
cruise periods in 1978.
50
stagnation in the subsurface water where anoxic situations prevailed frequently
during the year. Planktonic organisms may also influence the sulfate
concentration in the lake due to the mineralization of organic cellular sulfur
content. There are many sources of sulfate input to Lake Valencia. In addition
to natural weathering and erosion of sulfur-containing soil and rocks in the
watershed, human activities (through sewage and industrial discharges) probably
contribute a major input of sulfate to the lake. The preliminary data on
sulfate input from tributaries show an extremely high value. For example, the
sulfate concentration in Rio Guey during March through May ranged from 60 to 100
ppm, with an estimated total daily input of 6,000-10,000 Kg.
Sulfide (H2S) is expected to be a dominant form of sulfur in the anoxic
zone. Although we did not measure the actual H2S concentration in the lake
water sample, we did notice the powerful rotten egg odor in water devoid of
dissolved oxygen. In the February cruise, the obnoxious sulfide odor was
pronounced even in the surface water. Presumably the complete mixing during
that period brought up the sulfide from deep water. A massive fish kill was
also observed at that time of the year. Sulfides are known to be extremely
toxic to aquatic organisms. A concentration of total sulfides greater than
0.002 mg/L is unsafe for all aquatic organisms, including fish (National Academy
of Science 1973). The extent to which sulfides cause the fish mortality in Lake
Valencia remains an important feature for investigation.
PHOSPHORUS
The phosphorus was measured in three forms: total phosphorus (TP) that
includes dissolved and particulate, organic and inorganic; total dissolved
51
phosphorus (TDP) that includes organic and inorganic forms filterable with
0.45 ym pores; and inorganic orthophosphate (PO4-P) that is the simplest and
most available form in the lake water.
The annual fluctuation of phosphorus contents in those three forms, as
integrated throughout the depth at station 20, is presented in Figure 29.
During the early part of the year, the concentrations were lowest at
3,000 mg/m^, 200 mg/m^, and 1,500 mg/m^ for TD, TDP, and PO4-P, respectively.
Those concentrations increased steadily until they reached their maxima in
October at 5,500 mg/m^, 4,500 mg/m^, and 4,000 mg/m^, respectively. Comparisons
between the TP and TDP show that the TP content contained 60-90% TDP. In other
words, the particulate P, including organic and inorganic forms, constituted
less than 50% of the TP.
The vertical variation of TP, TDP, and PO4-P concentrations in the deepest
part of the lake during the annual six cruises are shown in Figure 30. In gen-
eral, the three forms of phosphorus followed close patterns in their vertical
distribution, and in most cases their concentrations were lower toward the
surface and greater in the deeper water column. Such a pattern was particularly
pronounced during the July-October period. For example, during September-
October concentrations of TP, TDP, and PO4-P were 129, 29, and 13 yg/L in the
surface water, compared to 289, 285, and 283 yg/L at the 30-m depth,
respectively. During this period, while the TP concentration was relatively
comparable with the rest of the year (100-150 yg/D , the concentrations of TDP
and PO4-P (<30 yg/L) were much lower than other periods (>50 yg/L). The most
homogeneous concentrations of those phosphorus forms in the water column
occurred in the period between November and March.
In general, the TP content in non-polluted natural waters in the world
52
CO
JC
=3
Q_
o
s^
X>
o
(I)
SI
>
o
w
o
o
w
n
C/J
b
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V-^
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h-
t—
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II
1
B
H 1 1 1 1 1-
o
CO
<
0009 OOOS OOOt
OOOe^ 0003 OOOL
^LU/DUU
D
t— 1
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•H
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CO
t>0
^
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c
•H
TD
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CO
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•
a
CO
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42
CO
CX ^
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TD
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rH
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HD
•H
<u
TD
4J
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rH
(D
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CO
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a
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(D
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u
D
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u
o
CO
^
OJ
a
D
CO
t-H
o
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x:
>
a
f— 4
•
CO
o
4J
00
o
<J\
4J
f-H
C
•«
•H
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CM
c
o
C
•H
o
•Ui
•H
CO
u
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M
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c
U
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42
Pu
CO
•
o
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£
CN
CL
O
• ^
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U
^
o
53
mg/m*
300
Oct. 23-31, 1979
mg/m'
300
Mar. 3-9, 1980
300
ipg/'^J
300
July 7-9, 1980
200
300
Sep.25 - Oct.2, 1980
mg/m*
100
T
300
May 19-20, 1980 Nov.25 - Dec.5, 1980
D = Total Dissolved Phosphorous
o = Total Phosphorous
- = PO4-P
FIG. 30. Vertical distributions of total dissolved phosphorus, total phos-
phorus, and orthophosphate at station 20 during six cruises in 1979-80.
54
extends over a wide range from <1 yg/L to MOO mg/L, but the majority of them
are between 10 and 50 vg/L. Separation of TP into inorganic and organic
fractions in a large number of lakes in the temperate region indicate that a
great proportion of the TP is in the organic phase (>85%) (Wetzel 1975). Of the
total organic P, about 70% or more is in the particulate organic fraction, and
the remainder is present as dissolved and colloidal forms.
In a detailed treatment on phosphorus and productivity of lakes, Vollen-
weider (1968) demonstrated that the amount of TP generally increases with the
algal productivity. Lakes with TP ranging from 30-100 yg/L are considered
eutrophic, and those greater than 100 yg/L are hypereutrophic .
Total Phosphorus . Figure 31 shows the lakewide distribution of TP con-
centration in Lake Valencia. With few exceptions, the total P concentration is
well above 100 yg/L in most areas during most times of the year. Persistently
higher concentrations were found near the western and eastern ends of the lake.
During the annual cycle, the lower TP concentrations occurred in October 1979
with lakewide concentrations ranging from 28 to 228 yg/L; the majority of values
during the rest of the year were between 100 and 200 yg/L. At the western end
of the lake, total P content ranged from 250 to 1,500 yg/L.
The annual cycle of TP concentration varied considerably with time and
depth (Fig. 32). It clearly shows that the TP concentration was relatively
homogeneously distributed throughout the water column during the period from
January through March, with the majority of concentrations ranging from 100-200
yg/L. It also varied very slightly around 120 yg/L in the surface 5 m during
the annual course. However, a marked gradient, increased from 80 yg/L at 10 m
to a concentration greater than 200 yg/L at 30 m, prevailed from June throughout
the end of the year.
55
Oct. 23-31, 1979
July 7-9. 1980
Mar. 3-9. 1980
Sep.25 - Oct.2, 1980
May 19-20. 1980
Nov.25 - Dec.5. 1980
o = 100mg/m'' Total Phosphorous
FIG. 31,
periods .
Horizontal distribution of total phosphorus during six cruise
56
u
o
o
c
o
u
c
•H
CO
U
DO
DO
;3L
o
o
CO
U
4J
c
<u
u
c
o
o
CO
o
u
o
CO
O
CD
o
c
o
Xi
(lu) mdaa
T3
I
OJ
6
•H •
H O
00
CN
CO OO
c
i-J D
{XI ^3
57
Total Dissolved Phosphorus . Distribution of total dissolved P (TDP),
as shown in Figure 33, exhibits a pattern similar to that of TP. The low
values, ranging from 13 to 90 yg/L, occurred in October 1979; high values
ranging from 52 to 117 yg/L occurred in May 1980. Concentrations ranging from
50 to 150 yg/L prevailed in the lake water during most of the year. On a few
occasions the TDP content was greater than 300 yg/L.
The pattern of annual depth variation of TDP concentration in the central
part of the lake was somewhat similar to that of TP (Fig. 34). The TDP
concentration in the surface 10 m was much smaller than TP, but the difference
narrowed with increasing depth. In the surface water, large portions of
phosphorus were present in particulate form, especially in phytoplankton cells,
which are capable of accumulating dissolved phosphorus from the surrounding
water. As phytoplankton cells sink to the aphotic depth, the phosphorus is
released back to the water medium upon decomposition of algal cells.
Orthophosphate . Inorganic orthophosphate (PO4-P) distribution in lake
water is presented in Figure 35. In October 1979, most areas in the lake
contained PO4-P less than 10 yg/L, but the concentration increased considerably
throughout the 1980 sampling periods, with the majority of the values exceeding
30 yg/L. Particularly high PO4-P content OlOO yg/L) was widespread from July
through October, 1980. The annual variation of vertical distribution of PO4-P
concentration determined for station 20 is shown in Figure 30. The PO4-P
concentration in the upper 10 m of lake water was 20 yg/L or less during the
beginning and second half of 1980 and reached 40 yg/L from February to June. A
considerably higher concentration of PO4-P persisted in the deeper water column,
i.e., the concentration was greater than 60 yg/L in the water below 20 m
throughout the year. Pronounced accumulation of PO4-P in deep water occurred
58
Oct. 23-31, 1979
July 7-9, 1980
Mar. 3-9, 1980
Sep.25 - Oct.2, 1980
May 19-20, 1980
Nov.25 - Dec.5, 1980
O =
100mg/m' Total Dissolved Phosphorous
FIG. 33. Horizontal distribution of total dissolved phosphorus during si
cruise periods.
IX
59
K>
zOi
E
D
O
O
_>
O
(/)
(A)
D
■♦-
O
o
CM
O
(lu) M+daa
U
D
O
c
o
o
c
(U
•H
CO
60
;3.
o
CO
u
o
CO
o
x:
a.
T3
>
r— I
O
CO
CO
CO
o
O
•H
CO
•H
CU
'V
I
0)
a
H O
00
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CO 60
C
o ^
M D
{*4 13
60
Oct. 23-31, 1979
July 7-9. 1980
Mar. 3-9, 1980
Sep.25 - Oct.2, 1980
May 19-20, 1980
Nov.25 - Dec.5, 1980
° = 100mg/m' PO^-P
FIG. 35. Horizontal distribution of orthophosphate during six cruise periods,
61
during the period from April throughout November, and it reached the maximum
concentration of 280 yg/L at 30 m in October (Fig. 36). This pronounced buildup
of PO4-P superimposed on the anoxic event in the lake water presents a classic
example demonstration of the mobilization of PO4-P in an anoxic hypolimnion. As
the oxygen in the hypolimnion is removed by active decomposition of organic
matter, which in turn releases soluble phosphate, the reductions of iron and
manganese are expected to increase markedly. With the reduction of ferric
hydroxides and complexes, ferrous ions and absorbed phosphates are mobilized and
appear in the water. The existence of hydrogen sulfide in the anoxic water
column may further enhance the PO4-P accumulation in deep water. It has been
shown that in very productive lakes, where the decomposition of organic matter
produces anoxic conditions and hydrogen sulfide in the hypolimnion, ferrous
sulfide (FeS) precipitation occurs. If FeS precipitation is sufficiently large
to remove large proportions of the iron, phosphorus may be kept and accumulated
in solution.
NITROGEN
Four major chemical groups of nitrogen compounds were analyzed for Lake
Valencia water. They are total ammonia (NH3), nitrite (NO2), nitrate (NO3), and
total Kjeldahl nitrogen (TKN). Nitrogen compounds may vary their chemical forms
throughout the water column depending on the degree of prevailing oxidation and
reduction process at a given specific depth. To accurately estimate the total
quantity of the various nitrogen compounds from surface to the bottom of the
lake, values integrated over the depth for TKN, NH3, and NO3 + NO2 are shown in
Figure 37. Content of TKN was relatively stable in comparison with either total
(lu) mdea
O
00
c
•H
u
c
o
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c
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63
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M 4-»
Ii< CO
64
NH3 or NO3 + NO2. While the concentration of NH3 increased from around 800
mg/m in March to near 2 x 10^ ing/m^ in October, the concentration of NO3 + NO2
declined steadily from 4,000 mg/m^ in March to 50 mg/m^ in December. The
concentrations of NH3 and NO3 + NO2 clearly fluctuated in a mirror image
pattern. During the early part of the annual cycle, the NH3 content was lower
and the NO3 + NO2 was higher as the lake water was mixed and oxygenated
throughout the water column. However, as the prevailing deoxygenation increased
in depth during the second half of the year, the nitrogen transformed to the
reduced form, NH3.
Vertical variations of TKN, NH3, and NO3 + NO2 concentration are shown in
Figure 38. There were two sampling periods, March and November-December, during
which all three nitrogen forms were distributed more or less homogeneously in
the water column. TKN concentration ranged between 1,000 and 3,000 yg/L.
Except for the March period, the NO3 + NO2 concentration was relatively low,
often below our detectable level at 4 yg/L. Such a low value most likely
resulted from phytoplankton uptake.
T otal Kjeldahl Nitrogen (TKN) . The lakewide horizontal TKN distribution
ranged from 200 to 4,500 yg/L, with the majority of the values exceeding
1,000 yg/L (Fig. 39). In general, the higher concentrations were persistently
observed at the western basin of the lake where values ranged between 2,000 and
3,000 yg/L.
Annual vertical variation of TKN content in the water column is shown in
Figure 40. The concentrations were between 1,000 and 2,000 yg/L in the upper
10 m throughout the year, and in the entire water column from March to
September. During the period from September to November, a marked vertical
concentration gradient prevailed, increasing from 1,000 yg/L at 30
m.
65
mg/m'
ixio' ixio2 uM^ um*
mg/m*
txK)' 1xl02 1xl03 1x10*
Oct. 23-31, 1979
mg/m*
,102 1x,K)3 ijclO*
July 7-9, 1980
mg/m*
IxW' 1xl02 IxW' 1x,10*
Mar. 3-9, 1980
mg/m'
IxW' lx,K)2 1)(,»3 ix,10*
Sep.25 - Oct.2, 1980
mg/m*
IxW' 1x,l02 IxK)' lx,10*
May 19-20, 1980
o = Total K Nitrogen
o = NH,-N
* = NO,+NO,
FIG. 38. Vertical distributions of total K nitrogen, NH3 - N, and NO3 + NO2
at station 20 during six cruise periods.
66
Oct. 23-31, 1979
July 7-9, 1980
Mar. 3-9. 1980
Sep.25 - Oct.2. 1980
May 19-20, 1980
Nov.25 - Dec.5. 1980
° = 1 mg/L Total K Nitrogen
FIG. 39. Horizontal variation in total K nitrogen at selected stations during
six cruise periods.
67
-J
E
c
O
D
O
o
(lu) qjdaa
o
00
o
CM
o
CO
4J
CO
u
CO
c
o
u
c
1^
CO
4-)
O
o
•H
4-»
CO
•H
a
I
e
H
o
d
68
Destratification of the lake water normally occurred between December and March,
during which the TKN content showed a great degree of homogeneity.
Nitrite (NQ9) and Nitrate (NQ-^) . The lakewide horizontal distribution of
the oxidized nitrogen, NO3 + NO2, as shown in Figure 41, is highly variable
although the concentrations are generally very low. With few exceptions,
concentrations were below 100 yg/L and the contents of many samples, in fact,
were less than 4 yg/L. Among the six sampling periods, the overall concentra-
tion was highest in March, with values ranging from 6 to 172 yg/L. Unlike other
nutrients, the NO3 + NO2 concentrations were generally lower in the vicinity of
point contamination sources, i.e., tributary discharges, as the effluents and
lake water in those regions often contained little oxygen. As nitrate is the
major nitrogen form assimilated by phytoplankton, the content is expected to be
lower in surface water where there is a large algal standing crop. Figure 42
shows the vertical distribution of NO3 + NO2 in the lake water. The peak
concentration was built up in March (lOO-KSO yg/D, and was followed by a sharp
decline to the lowest level at 4 yg/L from June throughout the rest of the year.
As the rapid deoxygenation occurred in the water column after March, the NO3 was
denitrified to NH3 in the anoxic zone. The combination of prevailing
denitrification in the subsurface water and phytoplankton uptake in the surface
euphotic zone resulted in the disappearance of NO3 in the lake water.
Ammonium (NH-^-N) . Ammonium is an important, prevailing form of nitrogen in
Lake Valencia. Its seasonal and partial distribution in the lake water was
extremely variable. Figure 43 shows the lakewide horizontal distribution of
NH3. Concentrations were lowest at the beginning of the year (March), ranging
from 6 to 51 yg/L, and were highest during the September-November period, with
most values greater than 100 yg/L. The vertical distribution of NH3 in an
69
Oct. 23-31. 1979
July 7-9, 1980
Mar. 3-9, 1980
Sep.25 - Oct.2, 1980
May 19-20, 1980
Nov.25 - Dec.5, 1980
O = 100mg/m' NO3-NO
FIG. 41. Horizontal variation in NO3 - NO2 concentration at selected stations
during six cruise periods.
70
-Q
o
00
c
D
O
CN
C
o
CO
CO
C
o
•H
4-»
CO
M
•u
C
0)
a
C
o
o
CN
o
2
CO
O
2
(lu) mdea
C
O
u
CO
>
CO
>
CN
fX4
71
Oct. 23-31. 1979
July 7-9, 1980
Mar. 3-9. 1980
Sep.25 - Oct.2, 1980
May 19-20. 1980 Nov.25 - Dec.5. 1980
o = 100mg/m' NH3-N
FIG. 43, Horizontal variation in NH3 - N concentration at selected stations
during six cruise periods.
72
annual cycle is shown in Figure 44. Very low NH3 (<100 ]ig/L) appeared in the
surface 10 m from January to October. A pronounced vertical concentration
gradient began to establish below the 10 m depth after June and reached the
greatest level at 1,700 yg/L near 20 m in October. The development of the NH3
gradient was closely related to the oxygen depletion pattern in the hypolimnion.
TRACE METALS
The concentrations of several heavy metals were analyzed at several depths
at station 20 (Fig. 45). During January, when the lake water mass was well
mixed, a greater degree of homogeneity in vertical concentrations occurred.
Iiron. Iron concentration was 50 yg/L from the surface to 20 m and 200 yg/L
at 30 m. During the lake turnover period (December to March) the lake water was
thoroughly oxidized. It is expected that the ferric ion would be the predomi-
nant form that in turn precipitates to the sediment as a hydoxide, Fe(0H)3. In
the anoxic water during the remainder of the year the presence of H2S probably
removed iron in the form of ferrous sulfide (FeS), which resulted in the
production of black mineral muds.
Ni£kel. Nickel concentration was 100 yg/L throughout the water column.
High concentrations of nickel were reported to be toxic to plant life
O500 yg/L) and to fish reproduction (>700 yg/L). However, concentrations at or
below 100 yg/L have no apparent harmful effects on aquatic organisms.
Le_ad. Lead concentration ranged between 40 and 50 yg/L, which is the
permissible level for domestic water supply in the USA. It was shown that
30 yg/L concentration caused reproductive impairment in Daphnia magna (Biesinger
and Christensen 1972).
73
•o
E
CM
O
(tu) mdaa
o
00
ON
oo
o
O
CM
CO
4J
CO
CO
C
O
CO
U
u
C
cu
o
c
o
u
I
en
PC
c
o
CO
CO
O
U
(1)
>
74
CONCENTRATION (ppb)
100 200 50 150 40 80 10 30 50
5
10
J 15
S] 20
25
30
— (
MM
1 '
• 1
>
>
i 1 i
1 " 1 1 1 1
\
(»
\
— (
\
•
»
)
\
:..\
Ni
[pb
Zn
•
JAN 10-14,1978
5
10
I 15
X
O
25
30
CONCENTRATION (ppb)
10 30 50 10 30 50 70
M/M
7
M M «k l~T
10 30
I I I
Zn
JULY 18-21,1978
CONCENTRATION (ppm)
30 50 70 400 4 40 480 520 560 600
I Ivf I
FIG. 45. Vertical distribution of iron, nickel, lead, zinc, copper, mercury,
calcium, and magnesium concentrations.
75
zinc . Zinc concentration ranged from 20 to 40 yg/L and from 10 to 35 Pg/L
in July 1979, Those concentrations were relatively low compared to the mean
concentration (64 pg/L) in U.S. waterways. Numerous bioassays have been
conducted to test the toxic effect on all levels of aquatic organisms. Zinc is
also an essential trace metal required by all kinds or organisms (Bowen 1966,
Underwood 1971). Minimum concentration required for optimal algal growth was
reported at 50 yg Zn/L (Eyster £t jd. 1958). Zinc concentrations toxic to human
are quite high (>5 mg/L). The test for chronic effects of zinc on fathead
minnow reproduction showed that 30 yg/L had no effect, whereas 180 g/L caused a
93% reduction in fecundity.
Copper . Copper concentration was between 20 and 50 yg/L at station 20 in
Lake Valencia. The vertical profile shows that the highest concentration was at
30 m. All organisms require minute quantities of copper for growth. Free
copper ions at concentrations greater than 50 yg/L have an adverse effect on
phytoplankton species. The copper toxicity to algae depends on the temperature
and alkalinity of the water. Deleterious effects of free copper concentrations
on fish and zooplankton were greater than 18 yg/L in soft water (<30 mg/L CaC03)
and 30 yg/L in hard water (>200 mg/L CaCOs) (Mount 1968).
Mercury . Mercury concentration from the surface to 30 m ranged between 15
and 70 yg/L at station 20 in July 1978. The toxicity of mercury has been known
to man for a long time. Although it may exist in inorganic (Hg^ and Hg'**^) and
organic forms in the environment, the organic methyl mercury (HgCH^) is 50 times
more toxic to organisms than inorganic forms (Schroeder 1974). Algae and
aquatic plants accumulate mercury by surface adsorption, and fish take it up
both directly from the water and from food (Hannerz 1968). The accumulation
rate was shown to be fast while the elimination rate was slow, leading to
76
concentration factors of 3,000 fold and higher. Concentration factors by fish
in excess of 10,000 times that in the surrounding water have been reported
(McKim 1974), and 1 yg/L of mercury showed a distinct effect on marine
organisms. Two chronic toxicity tests on Daphnia magna showed that mercury as
mercuric chloride and methylmercuric chloride caused significant reproductive
impairment at concentration of 2.7 and 0.04 yg/L mercury, respectively
(Biesinger 1974).
To detect the mercury level in fish in Lake Valencia, samples of Tilapia
mossambica , the most abundant species in the lake, were taken for analysis.
Table 5 shows the mercury content in fish muscle and viscera. Mercury content
in fish muscle and viscera ranged from 20 to 70 yg/kg of fish in wet weight.
The levels of mercury in those fish samples were relatively low, compared to the
safety levels (500 yg/kg) in livestock recommended for human consumption. The
accumulation factor from lake water appears to be extremely small. One
explanation would be that the mercury in Lake Valencia water reacted with the
prevalent hydrogen sulfide, forming extremely insoluble precipitates which make
it unavailable to organisms for uptake.
CHLOROPHYLL a
The chlorophyll a^ concentration fluctuated widely over depth, time, and
location. Figure 46 shows that at the beginning of 1978, at station 20, the
chlorophyll concentration was low (10-20 mg/m"-^) and relatively homogeneous
throughout the water column. As indicated by the depth profiles of temperature
and dissolved oxygen, the water mass was well mixed during the January-February
period. Chlorophyll a concentration in the upper 10 m increased progressively
77
TABLE 5. Mercury content in muscle and viscera of Tilapia mossambica ,
sampled at Turmero rivermouth and Macapo in Lake Valencia.
Sample No. Muscle Viscera
Turmero 2 0.02 0.04
Turmero 4 0.03 0.03
Turmero 6 0.03 0.07
Turmero 8 0.03 0.04
Turmero 10 0.04 0.05
Turmero 12 0.03 0.07
Macapo 5 0.02 0.05
Macapo 6 0.06 —
Macapo 7 0.02 0.07
Macapo 9 0.07
Macapo 11 0.02 0.02
Macapo 13 0.02 0.02
Macapo 15 0.04
and the vertical gradient became pronounced toward the latter part of the year
(Fig. 46). The maximum concentration in the surface water (5 m) occurred in
October with value greater than 50 mg/m^ and less than 10 mg/m^ at 25 m.
Chlorophyll level remained relatively low and constant at approximate 10 mg/m^
in the deep water below 20 m. As this depth was far below the euphotic zone
(<5 m) , chlorophyll was not expected to carry out photosynthetic activity. But
the chlorophyll appeared to be healthy and functional as indicated by a high
chlorophyll/phaeophytin ratio.
The lakewide chlorophyll distribution during six cruise periods showed
large regional variation throughout the year (Fig. 47). In general, high
concentrations occurred in the southwestern portion of the lake, and this
pattern was particularly pronounced from July to November. During the January-
April period, a major portion of the lake contained chlorophyll at 10-20 mg/m^
78
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with considerable shifting, and maximum concentration (>30 mg/m^) occurred only
in a small area. During the July-October period the chlorophyll concentration
increased considerably, with values in the major portion of the lake greater
than 20 mg/m^. Annual maximum chlorophyll production occurred in November,
during which approximately half of the lake contained >50 mg/m^ chlorophyll.
The most productive region was located in the extreme southwestern sector where
the maximum chlorophyll content was 176 mg/m^ (station 5).
The most obvious factors that cause the spatial heterogeneity of
chlorophyll distribution in lakes are variation in nutrient concentrations,
herbivore grazing pressure, water movement pattern, and sinking rate. However,
the nutrient levels (phosphorus and nitrogen) in Lake Valencia were sufficiently
high to support optimal chlorophyll production within the limited euphotic zone.
As the predominant phytoplankton species in Lake Valencia was Microcystis
aeruginosa > the heterogeneity caused by zooplankton grazing and the sinking
process is expected to be minimal. Wind action appeared to be the most
significant factor in mixing and redistributing the surface-inhabiting
Microcystis aeruginosa .
The chlorophyll heterogeneity also occurred in the euphotic zone at all
locations over a short time. Figure 48 shows the changing chlorophyll concen-
tration in surface water (5 m) at station 20 on a clear, calm February 22, 1978.
At 0700 hr, the depth profile of chlorophyll in the euphotic zone was 10-11
mg/m. As the day progressed, the chlorophyll content in the immediate surface
water decreased and the maximum value (16 mg/m^ at 1900 hr) occurred at 3 m.
81
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82
PRIMARY PRODUCTIVITY AND RESPIRATION
The gross productivity measured by oxygen production at station 20 on five
dates in 1978 is shown in Figure 49. Maximum productivity mostly occurred in
the surface 1 m, ranging from 2-5 g 02/ni3/day. On July 27 and November 29, the
productivity at the immediate surface (10 cm) was less than that at 0.5 m,
presumably inhibited by excessive solar radiation that caused chlorophyll
damage. Comparison of the productivity integrated throughout upper 10 m per m^
per day (Z P in Fig. 49) shows that the highest production (16 g C2/m2/day)
occurred on November 29, and the lowest (1.42 g 02/ni2/day) on February 21.
The specific productivity based on unit weight of chlorophyll £ shows that
the maximum value was 184 mg 02/nig Chi £/day at station 17 at 10 cm on July 27
and the value decreased to 25 mg 02/mg Chi £/day at 5 m; and the gross
productivity in unit surface area at stations 17, 20, and 23 were 13.07, 13.30,
and 13«55 g 02/m2/day, respectively. The vertical variation in specific
productivity was strikingly different among those stations.
The respiratory oxygen consumption measured at station 20 on five discrete
dates is shown in Figure 50. Respiration rate at the surface was less than
0.5 g 02/m3/day and the consumption increased toward the aphotic zone (>5 m) in
most cases. Total DO consumption rates integrated over depth at station 20 were
0.94, 4.47, 16.11, 4.55, and 2.77 g 02/ni2/day on February 21, May 25, July 27,
October 5, and November 29, respectively. Net oxygen productivity in the top
50 m water from photosynthesis and respiration at station 20 during those five
dates were 0.77, 5.56, -2.81, 5.25, and 13,23 g 02/m3/day, respectively.
Oxygen contributed by phytoplankton production is the largest source of
dissolved oxygen in Lake Valencia. A rough estimate based on the average
83
GROSS PRODUCTIVITY (gOa/mVdy)
-2-1 I 2 3 4
IP=I.42
STATION 20 FEB 21,1978
GROSS PRODUCTIVITY (g02/mVdy)
12 3 4 5
X
o
4
T
T
1 — r
STA'
10
Y.P= 10.03
ION 20 MAY 25,1978
GROSS PRODUCTIVITY (gOz/m^/dy) GROSS PRODUCTIVITY (gOa/mVdy)
01 2345601 2345
IP= 13.30
STATION 20 JULY 27, 1978
GROSS PRODUCTIVITY (gOs/m^/dy)
1 2 3 4 5 6
e 9 _
3 2
X
o
4 -
5 -
—
1 1 •^L 1
—
y/ IP=I6
V
STATION 20 NOV 29,1978
3 -
10
FIG. 49. Vertical distribution of primary productivity in the euphotic zone
at station 20 on five dates in 1978.
84
J 2
I
I-
uj ^
o
4
5
10
RESPIRATION (gOz/mVdy)
-2 -I 12 3
1
IR = 0.95
STATION 20
U FEB. 21, 1978
X
RESPIRATION (gOa/m^/dy)
1
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1
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STATION 20
JULY 27,1978
I
i ^
X
t 3
tu
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5 -
RESPIRATION (gOa/mVdy)
I 2 3 4 5
T
IR= 2.77
STATION 20
NOV. 29, 1978
10
RESPIRATION (gOz/mVdy)
12 3 4 5
- 2
i[L
ILlI
'=^4
T
I R = 4.47
STATION 20
MAY 25, 1978
RESPIRATION (gOg/rnVdy)
12 3 4 5
T
T
T
IR= 4.55
STATION 20
OCT. 5,1978
FIG. 50. Vertical distribution of respiration in the euphotic zone at station
20 on five dates in 1978.
85
observed photosynthetic rates at stations 17, 20, and 23 in five discrete
measurements showed that the gross oxygen production in the top 10 m over the
whole lake ranged from 1.5 x 10^ to 5.7 x 10^ g O^/day, with an annual average
of 2 X 10^ g 02/day. Oxygen generated in the eutrophic zone diffuses downward
by a mixing process to replenish DO in the deeper water. In comparison, the
oxygen influx from external aeration into Lake Valencia is relatively small.
Apmann (1973) estimated that the transfer rates from air to water over Lake
Valencia were 10-30 x 10^ g/yr assuming a reaeration coefficient of 0.1 g/m^/hr
under various hydrodynamic conditions.
PHYTOPLANKTON
Phytoplankton taxa and their quantitative distribution in Lake Valencia
during six cruise periods in 1978 were identified to a total of 37 genera
belonging to Chlorophyta, Cyanophyta, Chrysophyta, and Pyrrhophyta. Population
density of the phytoplankton community among lake-wide stations during the study
period ranged from 28,210 to 572,880 cells/mL. As those samples were composed
of surface, mid, and lower depth at each station, the actual density is expected
to be larger in the surface and smaller in the bottom water. The seasonal
variation of total phytoplankton and Microcystis cell numbers, shown in
Figure 51, indicates that the maximum standing crop occurred in January and the
minimum in October. During most of the year. Microcystis occupied more than 90%
of the total cell counts, and the minority species were in significant
proportion only in January. Figure 52 illustrates the vertical distribution of
total phytoplankton at station 31 on six discrete sampling dates. In January
and February large populations (28-45 x 10^ cells/mL) occurred in the surface
86
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TOTAL PHYTOPLANKTON ( CELLS x 10^/ m^ )
100 200 300 400 100 200 300
5 h
10
15 -
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30 -
^ 10
e
- 15
X
P4 20
25
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5
10
15
20
25
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APR.
OCT.
JULY
NOV.
FIG. 52. Vertical distribution of total phytoplankton during six cruise
periods in 1978.
88
(1 m) and decreased drastically at/below 5 m. But the variation was relatively
small in other sampling periods. As Microcystis bloomed persistently, its
vertical distribution dominated the community vertical distribution pattern
(Fig. 53).
In January, Ankistrodesmus and Nitzschia populations reached 36,022 and
28,210 cells/mL in the surface water, respectively, and constituted the second
and third largest phytoplankton populations.
Lake-wide quantitative distribution of predominant phytoplankton genera
during six cruises in 1978 is shown in Figure 54. The major species of
Cyanophyta were Microcystis , Anabaena, Oscillatoria , and Spirulina . Among those
entities. Microcystis and Oscillatoria were particularly abundant.
As Microcystis is well adapted to flotation the size, shape, and patchiness
of its colonies are dependent on water movement and wind action. This blue-
green alga bloomed during most of the year, with population density ranging from
20,000 to 500,000 cells/mL. Oscillatoria was most abundant during February,
with a maximum population density of 10^ cells/mL, and had a relatively low
profile (<10^ cells/mL) during the rest of the year. Like Oscillatoria , the two
genera of nitrogen fixer, Anabaena and Anabaenopsis , never developed a
population competitive with that of Microcystis . Chlorophyta, although
numerically much smaller than Cyanophyta, consisted of diverse taxa. In January
and February there were 10-12 genera present, and the composition became more
complex during the remainder of the year. There were 20 genera in September.
Among those entities, Ankistrodesmus and Cosmarium were most abundant in
January, but became sporadic during the rest of the year. No other genera of
green algae developed any significant population size.
89
-10
£ 20
25
30
5
10
15
20
25
30
FIG. 53.
in 1978.
MICROCYSTIS (CELLS x l06/m3)
100 200 300 400 100 200 300 400
JAN.
APR.
OCT.
FEB.
JULY
NOV.
Vertical distribution of Microcystis during six cruise periods
90
373,000 1 '69,000
461,000
467,000 198,000 1 260,000
JAN.I0-I4,I978
CYANOPHYTA
Microcystis
b Anabaena
c Oscillatoria
d Spirulina
252,000
1 50.000 cells
ml
FEB.7-9.1978
a Microcystis
b Oscillatoria
c Anabaena
FEB. 7-9,1978
CHLOROPHYTA
a Ankistrodesnnus
b Oocyst is
c Closteriopsis
d Closterium
e Closteridium
20,000 cells
ml
JAN. 10-14, 1978
CHRYSOPHYTA
a Nitzschio
b Cyclotella
c Diatomella
FEB. 7-9,1978
FIG. 54. Horizontal distribution of predominant genera of three major
phytoplankton classes during six cruise periods in 1978.
91
75,000 cells
APR 25-28,1978
CYANOPHYTA
Q Microcystis
b Oscillatoria
c Anabaena
d Lyngbya
e Anabaenopsis
JULY 18-21,1978
CYANOPHYTA
a Microcystis
b Oscillatoria
c Anobaeno
<j Anabaenopsis
e Lyngbya
f Spirulino
APR. 25- 28, 1978
CHLOROPHYTA
Tetraedron
b Chlorococcum
c Glenodinium
d Coelastrum
e Ankistrodesmus
20,000 cells
nnl
JULY 18-21,1978
CHLOROPHYTA
Ankistrodesmus
b Tetroedron
c Elakatothrix
d Chroococcus
20,000 cells
ml
APR. 25-28,1978
CHYSOPHYTA
Nitzschia
b Cyclotella
20,000 cells
ml
JULY 18-21, 1978
CHRYSOPHYTA
Nitzschia
b Cyclotella
FIG. 5A. (continued),
92
20,000 cells
ml
SEP 26 -OCT. 2, 1978
CYANOPHYTA
a Microcystis
b Oscillatoria
c Anabaena
<i Spirulina
e Anabaenopsis
NOV. 21 -24,1978
CYANOPHYTA
a Microcystis
b Oscillatoria
c Anabaena
d Spirulina
e Anabaenopsis
20,000 ceNs
ml
SEP 26 -OCT 2, 1978
CHLOROPHYTA
Tetraedon
b Chiorello
c Treubaria
d Oocystis
CHRYSOPHYTA
a Nitzschia
b Cyclotella
NOV. 21 -24, 1978
FIG. 54. (continued)
93
Diatoms consisted of 5-10 genera in Lake Valencia, Only one entity,
Nitzschia sp. , occurred persistently throughout the year with population size
ranging from 10^ to 10^ cells/mL. This species was particularly abundant during
January, February, and July. Other genera occurred sporadically in relatively
low numbers.
Three genera of dinof lagellates recorded were Glenodinium , Gymnodinium , and
Peridinium . None of those entities developed to a significant percentage of
phytoplankton populations.
Assessment of standing crop and succession pattern of phytoplankton
communities in Lake Valencia is difficult because of the great heterogeneity in
horizontal distribution. Any attempt based on a few selected stations is
inadequate to draw lake-wide information on phytoplankton.
In an attempt to use the species diversity of phytoplankton communities to
indicate trophic state (Margalef 1958), the Shannon-Weaver index (Shannon and
Weaver 1963) was calculated based on the following equation:
n. sp
H = - z Pi log Pi
i = 1
Where Pi is the probability of occurrence of the i th species. In lakes, H
values normally range from slightly greater than to as high as 4.5. Margalef
(1958) used the diversity index to define the trophic state as follows:
oligotrophic, >3.5; mesotrophic, 2.5-3.5; and eutrophic, <2.5. The values
calculated for each station during all cruise periods ranged from 0.4 to 1.7.
Those extremely low diversity indexes undoubtedly indicate that Lake Valencia is
in a highly eutrophic state.
94
ZOOPLANKTON
Zooplankton samples taken during 1978 cruises were mostly used for
preliminary taxonomical identification. The zooplankton results presented in
this report are samples taken during October 1979 cruises.
A total of 18 taxa were recorded for the zooplankton community (Table 6),
including three species of copepods, two species of cladocerans, six species of
rotifers, and seven species of ostracods. Among them, the rotifers were most
abundant, particularly Branchionus calyciflorus and B^. havanaensis whose
populations consistently constituted 50% or greater of the zooplankton community
(Fig. 55). The total zooplankton individuals ranged from 100 to 1,926
organisms/L among 23 sample locations, and a population density greater than
1,000 org/L occurred in the shallow water along the southern and northern shores
(Fig. 56). It is noteworthy that the greatest densities were found at stations
5 and 33 with 1,837 and 1,926 org/L, respectively.
Those two stations are respectively situated near the mouths of Cano
Central and Rio Guey, which are major point sources of contamination in Lake
Valencia. The zooplankton community in those regions was dominated by rotifer
populations, which comprised more than 80% of the community.
Another interesting aspect is that many taxa of ostracoda exist, although
in low number. It is speculated that those ostracods found in plankton samples
were actually benthic dwellers stirred up along with sediment resuspension at
those shallow stations.
Figure 57 shows the vertical distribution of predominant zooplankton
entities at station 31 in October 1979. The largest population density
(313 org/L) occurred at the surface and the lowest (24) near the bottom.
95
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= 1000 Organisms /Liter
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FIG, 56. Horizontal variation in total zooplankton numbers during
October 1979.
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Branchionus havanaensis , the most abundant species, showed the most distinct
density gradient in its vertical distribution.
COLIFCRM BACTERIA
Table 7 shows the densities of total and fecal coliform at 12 sampling
stations during three different periods in 1980. Highest densities (MPN/100 m)
were found persistently at stations A and B adjacent to the outfalls of Rio
Los Guayos and Cano Central. At station A, the total coliform bacteria ranged
from 1.4 X 10^ to 8.4 x 10^ MPN/100 mL, and fecal coliform from 9.3 x 10^
to 3.7 x 10". Those high bacteria densities declined drastically in a short
distance toward the open lake. For example, in May the density of total
coliform at station 5 was reduced to 1.2 x 10^ MPN/100 mL, less than 20% of that
of station B at a distance of 2 km. Similar reductions also occurred during
other sampling periods and other locations (Fig. 58).
The second most contaminated area in the lake was at the western end near
the outfall of Rio Guey, represented by stations H and 33. The total coliform
index at stations H and 33 ranged from 5.3 x 10^ to 9.3 x 10^ and from 4.6 x 10^
to 7.6 X 10 . On most occasions, the fecal coliform densities were the major
component of the total coliform in this area.
In the areas where there was no apparent adjacent contamination source, the
bacteria density was relatively low. At stations F, P, 12, and 39, located on
the fringe of farm land, the total coliform index was often less than 500
MPN/100 mL. The density at open lake station 31, representing the major water
mass of the lake, was less than 1,300 MPN/100 mL for total coliform, and fecal
colifoim was persistently less than 300 MPN/100 mL.
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102
MAY 1980
(^3 10®MPN/100ML
O < 500
JULY 1980
FIG. 58. Density of coliform bacteria at sample stations in May, July, and
October 1980. Blank circles and squares indicate total coliform, and dotted
symbols indicate fecal coliform.
103
The coliform group reported here was unusually high in the vicinities of
tributary discharges, where the MPN index is in the range of municipal sewage
bacterial density (5-50 x 10^). The extreme degree of contamination in Lake
Valencia undoubtedly indicates that the river water entering the lake consisted
of concentrated waste water from domestic and industrial sources.
The concentration of waste water in the tributaries fluctuated considerably
throughout the year depending on the dilution of the storm runoffs. Accurate
estimates of the bacteria density should consider the pattern of flow volumes of
the tributaries. The drastic decrease in bacterial density in the open lake
water probably resulted from dilution of the river discharges as they disperse
in the open lake.
Although the coliform bacteria data are mainly used to indicate the degree
of contamination from human and animal wastes, coliform group bacteria include a
pathogenic genus, Klebsiella . Large concentration of Klebsiella often occur in
certain industrial wastes such as pulp and paper, sugar, and food processing
effluents. The presence and implications of this group of bacteria are yet to
be determined.
POLLUTANT INPUTS FROM THREE MAJOR TRIBUTARIES
In general, the external loadings of contaminants to Lake Valencia
originated from three major sources: point, non-point, and atmospheric inputs.
The point source input from tributary discharges contributes the largest
quantity of pollutants into the lake.
104
Lake Valencia receives discharges from 16 tributaries (Fig. 3), which carry
heavy silt loads from surface runoff during the rainy season. Most of those
tributaries have little flow during the dry season, and only three maintain
measurable flow throughout the year. The source of those flows during the dry
season are domestic and industrial effluents, which become the most serious
contamination inputs to Lake Valencia. Those three tributaries are Rio Guey,
Rio Los Guayos, and Cano Central.
To estimate the quantity of pollutant discharge received by the lake and
its effects on water quality, it is necessary to obtain information on the
variety and quantity of contaminants in those tributaries. During 1979-80,
water samples were taken from each of those three tributaries for analysis of a
large inventory of pollutants, including organic compounds, solids, inorganic
nutrients, heavy metals, and coliform bacteria. The analyses were mostly
carried out by DIA laboratory personnel in Valencia and at El Hatillo.
Description of Tributaries
As shown in Figure 3, the Rio Guey receives drainage from the area north-
east of the lake basin including the city of Maracay. The Rio Guey discharges
into the lake through a relatively short, steep, and deep channel with a
forceful plume. There was little marsh or shoal featuring the river mouth.
The Rio Los Guayos drains the west side of the lake basin, receiving run-
off from a large area of agriculture land and the waste water of the city Los
Guayos. Adjacent to the Rio Los Guayos, the Cano Central enters the lake at the
southwest corner. Cano Central drains a relatively large catchment area and has
a long flowing course. Since the beginning of 1979, the tributary has received
a large volume of flow from Rio Cabrales that was previously emptied into a
105
different watershed, Cachinche. Rio Cabrales collects drainage from the city of
Valencia and Central Tacarigua, and the present diversion of this river into
Lake Valencia has become a serious source of contamination to the lake.
Unlike the Rio Guey, the inflows of Rio Los Guayos and Cano Central enter
the lake through a large area of marshes and extensive shoal.
The chemical nature of the industrial effluents is diverse. Table 8 shows
the contaminants that are commonly found in the effluents of light industries in
the Lake Valencia basin.
Pollutant Loadings
The original results of tributary study were obtained by the Tributary
Group of the Lake Valencia Research Project and some of the data were presented
in Weber* s report (Weber 1981),
Table 9 shows the annual means of physical and chemical parameters of the
discharge from Rio Guey, Los Guayos, and Caiio Central. Based on the discharge
of each tributary and mean annual values of pollutant content, the total yearly
inputs of the major contaminants are presented in Table 10.
Discharge volume. The largest flow among the three tributaries came from
Cano Central with the total annual flow of 1.5 x 10° m-^ and a mean of
4.8 m-^/sec, ranging from 2.3 to 11.9 m^/sec. Rio Guey and Rio Los Guayos
discharged 6.28 x 10^ m^ and 5,48 x 10^ m^/year, respectively. The flows in
those tributaries fluctuate 5 to 10 fold between low and high water levels. In
fact, during the rainy season, the flow regime was in a plug and flow phenomenon
because the short course of the river bed and steep terrain of the surrounding
mountains and hills flush out runoff in short periods of time after rains.
106
TABLE 8. Typical contaminants discharged by agricultural industries
in Lake Valencia basin.
Type of Industry
Contaminants
Animal Feed
Soft Drink
Alcoholic Beverage
Rubber Industry
Soap Products
Fruit and Vegetables
Flour Mill
Milk Products
Wood and Paper Products
Agricultural Chemicals
Tannery
Textiles
Tobacco
Oil and grease j, detergents, sewage, solids,
acids, bases
Oil and grease,, detergents, sewage, solids,
acids, bases
Oil and grease, detergents, sewage, solids,
acids, bases
Oil and grease, detergents, sewage, solids,
acids, bases
Oil and grease, detergents, sewage, solids,
acids, bases
Detergents, solids, sewage
Solids, sewage
Solids, sewage, acids, bases
Solids, sewage, acids, oil and grease,
sulfates, and sulfites
Solids, sewage, dyes, detergents, pesticides,
fertilizers, trace metals
Acids, bases, solids, sewage
Acids, bases, oil and grease, solids, deter-
gents, dyes, trace metals, sewage, chloride
Solids, sewage
107
TABLE 9. Chemical and physical parameters of discharges of three major
tributaries in the Lake Valencia basin.
RIO GUEY
Cone.
Cone
. R£
Parameters
X
S.D.
mge
Flow budget (m^/sec)
1.99
1.14
0.71
8.00
Temperature Cc)
29.83
1.96
26
—
33
BOD (ppm)
241.67
121.92
68.8
—
930
COD (mg/L)
386.20
166.55
150
_
768
DO (ppm)
1.72
1.88
0.0
-
6.3
PH
7.88
1.37
2.1
-
9.5
Turbidity (J.U.)
240.83
60.77
130
—
330
Cyanide (mg/L)
0.10
0.07
0.01
_
0.27
Alkalinity (mg/L)
216.33
60.05
50
—
300
Total organic carbon (mg/L)
119.07
48.19
38
-
192
Oil and grease (mg/L)
89.92
110.89
2.4
-
534
Chloride (mg/L)
79.87
48.01
25
_
188
Sulfate (mg/L)
60.73
29.80
13
—
104
Detergents ABS (mg/L)
0.66
0.41
0.09
—
1.32
Color (C.U.)
135
99.49
20
—
300
Phenols (mg/L)
0.34
0.35
0.032
—
1.824
Total solids (mg/L)
695.43
150.55
388
-
964
Dissolved solids (mg/L)
533.17
169.13
272
—
848
Suspended solids (mg/L)
152.27
77.14
40
-
292
Sedimental solids (mg/L)
4.64
2.27
0.7
-
8.2
Total nitrogen (mg/L)
24.17
14.94
4.2
-
64.4
Organic nitrogen (mg/L)
24.00
15.16
2.8
—
64.4
NO2-N (mg/L)
0.037
0.052
0.002
-
0.302
NO3-N (mg/L)
0.31
0.15
0.13
-
0.66
Total phosphorus (mg/L)
6.72
3.89
2.03
-
15.39
Total dissol. P (mg/L)
5.09
3.21
0.43
-
12.45
PO4-P (mg/L)
3.34
1.95
0.23
-
6.90
Total hardness (mg/L)
132.23
65.54
35
-
345
Magnesium hardness (mg/L)
47.70
52.07
0.0
-
270
Calcium hardness (mg/L)
84.87
28.18
30
—
185
108
TABLE 9. (Continued),
RIO LOS GUAYOS
Cone .
Cone
. R
Parameters
X
S.D.
ange
Flow budget (m^/sec)
1.737
0.892
0.686
4.419
Temperature Cc)
28.22
1.95
25
-
31
BOD (ppm)
410.88
216.78
165.2
-
975
COD (mg/L)
615.31
436.04
150
—
2,704
DO (ppm)
1.13
1.31
0.0
-
4.9
PH
7.00
0.47
6.1
-
8.5
Turbidity (J.U.)
369.22
185.95
150
-
950
Cyanide (mg/L)
0.198
0.210
0.022
—
0.94
Alkalinity (mg/L)
182.87
69.18
30
—
330
Total organic carbon (mg/L)
164.09
100.86
2
-
387
Oil and grease (mg/L)
84.26
93.80
2
-
725
Chloride (mg/L)
132.93
82.67
32
—
382
Sulfate (mg/L)
46.37
30.29
3
-
174
Detergents ABS (mg/L)
0.32
0.26
0.01
-
0.99
Color (C.U.)
67.27
70.90
15
-
300
Phenols (mg/L)
1.024
1.147
0.032
-
4.420
Total solids (mg/L) 1
,286.25
992.89
520
-
6,480
Dissolved solids (mg/L)
900.34
910.95
232
-
6,240
Suspended solids (mg/L)
412.06
312.94
64
-
1328
Sedimental solids (mg/L)
12.26
14.97
0.1
-
98
Total nitrogen (mg/L)
19.03
11.73
6.3
-
56
Organic nitrogen (mg/L)
18.97
11.84
5.6
—
56
NO2-N (mg/L)
0.015
0.016
0.002
_
0.079
NO3-N (mg/L)
0.22
0.09
0.06
-
0.46
Total phosphorus (mg/L)
-
-
0.97
-
16.04
Total dissol. P (mg/L)
3.02
2.11
0.75
—
9.25
PO4-P (mg/L)
1.88
1.45
0.23
-
6.94
Total hardness (mg/L)
139.06
40.22
65
-
250
Magnesium hardness (mg/L)
47.81
36.63
5
-
140
Calcium hardness (mg/L)
i . . rr
91.25
26.98
45
—
145
wcontmuec
109
TABLE 9. (Continued).
CANO CENTRAL
Cone.
Cone
. R
Parameters
X
S.D.
ange
Flow budget (m^/sec)
4.808
2.17
2.334
T
11.90
Temperature (°C)
27.24
2.03
24
-
31
BOD (ppm)
70.70
74.47
3.8
-
250
COD (mg/L)
166.78
187.31
26
-
792
DO (ppm)
0.68
0.76
0.0
-
2.6
pH
6.90
0.42
6.0
-
7.5
Turbidity (J.U.)
59.62
40.52
1.4
-
163
Cyanide (mg/L)
1.30
1.79
0.011
-
7.1
Alkalinity (mg/L)
182.33
44.27
110
-
300
Total organic carbon (mg/L)
24.65
9.06
10
-
48
Oil and grease (mg/L)
Chloride (mg/L)
58.73
29.44
21
-
178
Sulfate (mg/L)
87.24
52.65
20
-
187
Detergents ABS (mg/L)
0.58
0.36
0.08
-
1.28
Color (C.U.)
82.73
57.69
10
-
200
Phenols (mg/L)
0.11
0.11
0.004
-
0.560
Total solids (mg/L)
618.87
262.26
400
-
2,144
Dissolved solids (mg/L)
516.67
230
332
-
2,076
Suspended solids (mg/L)
109.20
100.58
8
-
400
Sedimental solids (mg/L)
0.75
0.51
0.2
-
1.7
Total nitrogen (mg/L)
10.53
7.48
4.2
-
33.3
Organic nitrogen (mg/L)
8.90
6.36
2.8
-
28
NO2-N (mg/L)
7
?
?
-
?
NO3-N (mg/L)
0.114
0.107
0.01
-
0.71
Total phosphorus (mg/L)
2.21
0.92
0.97
-
3.88
Total dissol. P (mg/L)
1.53
0.78
0.68
-
2.90
PO4-P (mg/L)
1.617
1.079
0.52
-
4.19
Total hardness (mg/L)
195.13
63.64
125
-
445
Magnesium hardness (mg/L)
71.80
64.82
5
-
330
Calcium hardness (mg/L)
129.50
25.03
100
"~
195
110
TABLE 10. Annual flows and discharges of contaminants from three major
tributaries to the Lake Valencia basin.
Parameters
Rio
Guey
Los Gua
lyos
Cano Central
Total
Flow (m^/yi
•)
6.28
X
107
5.48
X
107
1.52
X
108
2.70
X
108
POLLUTANTS
(Kg/yr)
BOD
1.52
X
107
2.25
X
107
1.07
X
107
4.84
X
107
COD
2.43
X
107
3.37
X
107
2.53
X
107
8.33
X
107
TOG
7.48
X
106
8.99
X
106
3.7A
X
106
2.02
X
107
ci-1
5.02
X
106
7.28
X
106
8.91
X
106
2.12
X
107
SO4-2
3.81
X
106
2.54
X
106
1.32
X
107
1.95
X
107
SOLIDS
Total
4.37
X
107
7.05
X
107
9.38
X
107
2.08
X
108
Dissolved
3.35
X
io7
4.93
X
107
7.83
X
107
1.61
X
108
Suspended
9.56
X
106
2.25
X
10^
1.65
X
106
3.37
X
107
Sediment al
2.91
X
105
6.71
X
105
1.14
X
10^
9.73
X
105
Total N
1.52
X
10^
1.04
X
10^
1.60
X
106
4.16
X
10^
Organic N
1.51
X
106
1.04
X
106
1.35
X
10^
3.90
X
10^
NO2 + NO3
2.18
X
10^
1.29
X
10^
-
—
3.37
X
10^
Total P
4.22
X
105
2.00
X
105
3.35
X
"l05
9.57
X
105
Total dis.
P
3.20
X
105
1.65
X
105
2.32
X
105
7.17
X
105
P04~P
2.10
X
105
1.03
X
105
2.45
X
105
5.58
X
105
Hardness (Ca)
5.33
X
106
5.00
X
106
1.96
X
107
2.99
X
107
Hardness (Mg)
3.00
X
106
2.62
X
106
1.09
X
107
1.65
X
107
Cyanide
6.34
X
103
1.08
X
10*^
1.98
X
10^
3.40
X
10^
Oil and Gre
ase
5.64
X
106
4.62
X
106
3.74
X
105
1.06
X
107
Detergent
4.14
X
10^
5.48
X
103
8.81
X
103
1.05
X
105
Phenols
2.14
X
10^
5.61
X
10^
1.67
X
103
7.92
X
10^
Cr
6.28
X
103
1.19
X
10^
-
—
1.82
X
10^
Cu
6.28
X
103
1.51
X
10^
-
—
2.14
X
10^
Ni
3.14
X
103
2.51
X
103
-
—
5.65
X
103
Pb
1.19
X
10^
4.08
X
10'^
-
5.27
X
10^
Zn
8.16
X
103
9.92
X
10^
-
—
1.07
X
105
COLIFORM BACTERIA
(MPN/100 mL X 107)
Total 2.7
Fecal 0.7
3.0
0.7
0.9
0.5
6.6
1.9
111
Dissolved oxygen. Extremely low mean DO content was recorded for all three
tributaries throughout the year. The mean DO values were 1.7, 1.1, and 0.6 mg/L
for Rio Guey, Los Guayos, and Cano Central, respectively. The water in those
rivers was occasionally anoxic. Because of the short running course and large
volumes of untreated sewage, those rivers merely function as open sewers that
carry waste water to the lake.
Biochemical oxygen demand (BOD). The total BOD loading from those three
tributaries amounted to 4.8 x 10^ kg/yr, with an average of 179.25 mg/L.
The annual input from Caiio Central was 1.07 x 10^ kg/yr and the mean value
70.7 mg/L. Rio Guey and Los Guayos, despite the fact that the flow volume was
much smaller than that of Cano Central, contributed 1.52 x 10^ and 2.25 x 10-^
kg/yr of BOD into the lake.
Chemical oxygen demand (COD) and total organic carbon (TOC) . The total
COD loading from the three rivers was 8.33 x 10^ kg/yr, approximately twice the
BOD input. The total organic carbon input from the three tributaries was
2.02 X 107 kg/yr.
BOD, COD, and TOC are all important parameters for measuring the potential
requirement of dissolved oxygen from receiving water. BOD in a sample of raw
water is due almost entirely to microbial oxidation of carbonaceous organic mat-
ter, which is mostly contained in the TOC. However, BOD loadings resulted in
depressing dissolved oxygen concentrations to levels harmful to aquatic organ-
isms. The average content of BOD5 in effluents from secondary treatment of mu-
nicipal waste water in the United States is 25 mg/L (Weinberger ^ _al. 1966).
In sharp contrast, the mean content of BOD was 70.7 mg/L in Cano Central, and
410.8 mg/L in Rio Guey. The BOD determination has shortcomings, including the
112
oxygen demand of reducing inorganic compounds and inhibition of toxic substances
to microorganisms.
Total organic carbon (TOC) content is the sum of particulate and dissolved
organic carbon. Particulate organic carbon (POC) is composed of living and non-
living particles, which provide substrate for microorganisms and food for
zooplankton and fish. Dissolved organic carbon (DOC) includes large varieties
of compounds of both natural and anthropogenic sources. In the tributary
effluents of Lake Valencia, the major organic pollutants contain cyanide
(3.4 X 10^ kg/yr), phenols (7.9 x 10^ kg/yr), detergent (1.05 x 10^ kg/yr) , and
oil and grease (1.06 x 10^ kg/yr). Quantitatively, the content of oil and
grease account for one half of the TOC (2.02 x 10^ kg/yr) input. As shown in
Table 8, the oil and grease contamination in Lake Valencia's tributaries came
from a large variety of agricultural industries. Among those pollutants,
cyanide and phenols are highly toxic to aquatic organisms. For instance, fish
were killed by water containing phenols greater than 0.5 ppm (USEPA 1974). The
levels of phenols in Rio Guey, Los Guayos, and Cano Central were 0.34, 1.02, and
0.11 ppm, respectively. The safety limit of cyanide to freshwater organisms is
set at 5 ppb in U.S. waters, and the levels found in Rio Guey, Los Guayos, and
Cano Central were 0.1, 0.2, and 1.3 ppm, respectively. However, the cyanide ion
combines with many heavy metal ions to form metallocyanide complexes which are
often extremely stable in the aquatic environment.
Heavy metals. The five metals which were sampled and analyzed for Rio Guey
and Los Guayos are listed in Table 6. Relatively high concentrations of Zn and
Pb were found in Los Guayos with 150 and 650 yg/L, respectively. The total
annual inputs of Cr , Cu, Ni, Pb, and Zn from those two rivers were 1 . 82 x 10^,
2.14 X 10^, 5.65 X 10^, 5.27 x 10^^, and 1.07 x 10^ kg, respectively.
113
Phosphorus. The input of total phosphorus (TP) from the three major
tributaries was 9.57 x 10^ kg/yr, with annual mean concentrations of 6.72, 3.65,
and 2.21 mg/L in Rio Guey, Los Guayos, and Cano Central, respectively. Those
values are much higher than the permissible P concentration (2 mg/L) in the
effluents of many municipal sewage treatment plants in the United States. To
prevent the development and control of nuisance algal blooms, total P should not
exceed 50 yg/L in any stream at the point entering the lake or reservoir.
However, the amount of annual input of total P from those three major
tributaries alone would increase the mean P concentration of the lake water in
Lake Valencia by 0.106 mg/L/yr,
Total dissolved phosphorus (TDP). A high proportion of the TP content in
those tributary waters is in dissolved form, including both organic and
inorganic. The annual input of TDP was 7.17 x 10^ kg, which was approximately
75% of the TP. The largest input was from Rio Guey, with a mean concentration
of 5.09 mg/L and a total annual input of 3.02 x 10^ kg; Los Guayos had a mean
value of 3.02 mg/L and an annual total of 1.65 x 10^ kg; and Cano Central had a
mean of 1.53 mg/L and a total of 2.32 x 10^ kg.
Orthophosphate (PO4-P). The total input of PO4-P from those tributaries
was 5.58 X 10^ kg/yr, approximately 78% of the TDP and 58% of the TP. Cano
Central contributed the largest annual input at 2.45 x 10^ kg, followed by Rio
Guey at 2.10 x 10^ kg. However, the mean annual concentrations in these three
waters were 1.62, 3.34, and 1.88 mg/L, respectively.
Most of the phosphorus content in those tributaries undoubtedly originated
from domestic waste. The physiological level of phosphorus excretion for adult
humans is estimated 1.5 g P/day (Fair e_t ^al. 1968). In recent years, domestic
use of detergents containing polyphosphate, such as pyrophosphate and
114
tripolyphosphate, may have increased the P content in sewage as much as four
times over sewage containing human excreta only (Stumm and Morgan 1962).
Nitrogen. Three forms of nitrogen were determined in Lake Valencia
tributary water samples. The input of total nitrogen was 4.16 x 10^ kg/yr, with
1.52 X 10^, 1.04 X 10^, and 1 . 60 x 10^ kg/yr from Rio Guey, Los Guayos, and Cano
Central, respectively. Most nitrogen was in organic form, with an annual input
of 3.96 X 10^ kg, approximately 94% of total N. The highest organic nitrogen
concentration was found in Rio Guey at 24.17 mg/L and the lowest in Caiio Central
at 8.9 mg/L; Los Guayos was measured at 18., 97 mg/L. The input of inorganic
nitrogen (NO3 + NO2) was 3.37 x 10^ kg/yr, less than 1% of total N. Annual mean
concentration was highest in Rio Guey with 0.34 mg/L. The ammonia content,
although not measured, is expected to be high because the tributary waters were
often deprived of oxygen, host organic nitrogen is released by human and animal
wastes. Microbial decomposition of organic nitrogenous compounds is rapid and
releases ammonia.
Solids. The total solids, including dissolved, suspended, and sedimental
fractions, were measured. A large quantity, 2.08 x 10^ kg (2.08 x 10^ tons), of
total solids was discharged by the three tributaries into Lake Valencia
annually. Cano Central contributed the largest amount with 9.38 x 10^ kg/yr,
Los Guayos was second with 7.05 x 10'^ kg/yr, and Rio Guey was the smallest at
4.37 X 10^ kg/yr. Total dissolved solids were 1.61 x 10^ kg/yr, which
constitutes 78% of the total solids in the tributary discharges. The dissolved
solids mainly consisted of chloride, organic carbon, sulfate, calcium,
magnesium, carbonate, oil and grease, and detergent. Other substances, such as
phosphorus, nitrogen, and metals, were minor constituents.
115
Calcium was the largest item among the major dissolved solids, and annual
input from the three tributaries was 2.99 x 10^ kg, followed by chloride at
2.02 X 10^ kg/yr, sulfate at 1.95 x 10^ kg/yr, and magnesium at 1.65 x 10^
kg/yr. The total dissolved solids discharged by those tributaries would
increase the concentration in the lake water by approximately 24 mg/L/yr. This
amount appears to be much lower than the apparent increases during the last few
years. It is likely that the remaining large input of total dissolved solids
has come from other sources, such as the atmosphere, non-point runoff, and other
tributaries. The drastic increase in TDS during the past few decades has also
been attributed to concentration resulting from the chronic desiccation.
MACROPHYTES AND MARSH SYSTEM
Wetland is a transitional zone formed between terrestrial and aquatic
systems, which resulted from deposition of surface runoff from the land phase.
The substrates of wetlands are often rich in nutrients and support dense growth
of semi-aquatic vegetation that forms marshes. The accumulation of detrital
materials resulting from seasonal succession of marsh vegetations may in turn
form a thick layer of peat-like substrate.
Marshes are ecologically important for a number of reasons. The dense
growth of vegetation provides habitat for wildlife. Marshes are also important
in relation to the water quality of their adjacent aquatic environment, because
the thick substrates may function in several ways to improve water quality.
(1) They intercept siltation from terrestrial runoff and settle out particulate
matter. (2) They process decomposition of organic matter and absorb inorganic
nutrients. Recognizing those functions, marshes have been used in recent years
as biological treatment systems for sewage effluents (Tilton et al. 1976).
116
However, most of those practices are experimented with in temperate climates
where the vegetative growth is disrupted during the winter. In tropical cli-
mates, growth is continuous all year around and it may be even more practical to
use marshes for purification of sewage effluents.
Marsh System in Lake Valencia
The formation of m-arsh land around Lake Valencia is prominent due to two
major processes. First is the continuing long-term desiccation that causes the
level of lake water to drop more than 10 cm a year. Consequently, the surface
area of the lake is reduced and the emerged aquatic macrophytes invade the
shallow zone along the lake shore. The second process is the sediment
deposition caused by surface runoff in the watershed. This process is par-
ticularly significant near the mouths of major tributaries, which carry a heavy
load of organic and inorganic particulates during the rainy season.
The existence and distribution of marshes around the Lake Valencia basin is
extremely dynamic. It is expected that the continued drop in lake water level
will shift the marsh lakeward, and the outer zone will become part of the
terrestrial system. The distribution of marshes has been progressively
destroyed by channelization of the tributaries and development of farmland.
The seasonal succession of the marsh vegetation is primarily regulated by
the dry-rainy cycle. Marsh land expands during the rainy season and reduces
during the dry period. The vital biomass of the vegetative growth also responds
according to such a cycle.
Figure 59 shows the species distribution of emerged macrophytes around the
lake shore. The most predominant macrophytes were species of Typha and
Chemopodium . Those species are most densely developed on the east and
117
southwestern shores during the wet season from June to November. A smaller
stand remained toward the water's edge during the dry period.
At present there are substantial marsh lands along the eastern and western
shores of the lake (Fig. 59). Those marshes are particularly valuable as they
receive the outflow of several tributaries (Los Guayos, Cano Central, Rio Guey,
and Rio Aragua) that constitute major point sources of pollution in Lake
Valencia. Because the formation of marshes will be a prominent feature of the
lake ecosystem as the severe siltation and desiccation take place in the
lacustrine environment, protection of the wetland and creation of a green belt
around the lake will be a measure urgently needed to stall the accelerated
eutrophication in Lake Valencia. Full utilization of the marshes for
purification and removal of contaminants from sewage effluent will be a highly
appropriate technology for the social-economical-environmental system in the
Lake Valencia watershed.
Submerged Macrophytes
The distribution of submerged aquatic macrophytes is limited to shallow
water around islands and the southern shore where the substrates are sandy or
gravelly. In the soft, muddy portion of the lake very little submerged
vegetation was observed. In shallow muddy areas, vigorous sediment resuspension
caused great turbidity and siltation which provided poor conditions for
macrophyte growth. In addition to the turbidity problem, the large seasonal
fluctuation in water level made the immediate shore zone undergo an expose-flood
cycle, creating unfavorable growth conditions for submerged vegetation.
118
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The submerged macrophyte species composition is dominated by Chara sp. and
Potamogeton sp,
Chara sp.
One or more species of Chara (Characeae) occurs sparsely in the shallow
water, often found less than 1/2 m deep in the sandy or gravel bed. They
commonly occur around the south shore and around the islands, where the water
quality is relatively good.
The plant appears in vivid green and each plant grows in tufts of half a
dozen to a dozen shoots which converge at the base. Shoots are unbranched, 5-
10 cm high. No reproductive structure was observed during the survey.
Potamogeton latifolius (Robbins) Morong.
This is the most abundant submerged plant found in Lake Valencia. It grows
almost continuously around the lakeshore and islands, except where the water has
high turbidity and/or there are badly polluted river mouths. The most dense
growth is found along the south shore of the lake and around the islands, but
there is little growth at the east and west ends of the lake.
The plant is distributed in the littoral zone from 0.5 to 4 m deep, and the
length of the plant varies considerably with the depth of the water column in
which it grows. In shallow water, the plants average 20-40 cm in length, while
in deep zones they reach over a meter in length. The predominant flowering
season is not certain and only a small number of the plants bore flowers during
the period of this survey.
A filamentous alga (Chlorophyceae) , its species yet to be identified, is
commonly associated with the upper shoots of Potamogeton . This association is
120
extremely productive as indicated by the abundant tiny air bubbles evolving from
the vegetation beds.
In addition to submerged species, a floating species, Eichhornia crassipes
(Mart.) Solms., was observed to grow near the river mouth, and individual plants
were occasionally dispersed to the open lake. A dense growth occurred on the
north side of the Rio Guey mouth. The environmental conditions that prevent
this plant from becoming wide spread are not known.
DISCUSSIONS AND CONCLUSIONS
The physical features of Lake Valencia have been undergoing a rapid change
due to chronic desiccation which results in the rapid reduction of surface area,
volume, and depth.
The lake has dwindled to an enclosed stagnant system that accumulates and
condenses incoming soluble and insoluble substances. Results of the present
study indicate that the depth of the water column has decreased to a level
sufficiently shallow to allow frequent circulation to the maximum depth of the
water column. As the wind action in the Lake Valencia basin is extremely
variable in direction and velocity, the movement of water masses driven by wind
stress shows little persistent pattern. The results also indicate that the
water movement is so episodic and ephemeral that it does not appear to exert an
effective vertical mixing mechanism, as indicated by dissolved oxygen during
most of the year. It was difficult to obtain a simultaneous lakewide water
movement pattern as the measurements were made using one current meter over
several days during each cruise. More comprehensive features of the lakewide
water movement pattern in Lake Valencia may be obtained by simultaneous
measurements using a number of current meters at several stations in the lake,
121
or by continuous monitoring of water movement at several depths near the center
of the lake.
The light regime in Lake Valencia plays a very important role in governing
the phytoplankton ecology. Despite the high intensity of incident light in the
tropical climate, the attenuation of light penetration in Lake Valencia is
extremely great and the maximum depth of Secchi disc transparency is less than
2 m. Accordingly, the maximum depth of the euphotic zone extends to
approximately 5 m, and on a volume basis only 25% of the lake water is
autotrophic and 75% heterotrophic. In reality, the proportion of the auto-
trophic and heterotrophic zones is much smaller as the average transparency in
most areas is much less than 2 m during most of the year. Accumulation of
phytoplankton biomass on the surface water is apparently the major cause of the
shallow euphotic zone.
The dynamics of dissolved oxygen in Lake Valencia present the complex
limnological processes in a single parameter, and the severe oxygen depletion
imposes a most serious managerial problem. The major source of dissolved oxygen
in the lake is phytoplankton photosynthesis. Input through atmospheric
diffusion is expected to be insignificant because the surface lake water is
frequently supersaturated with dissolved oxygen that prevents efficient air
diffusion into the lake water.
On the other hand, the dissolved oxygen is consumed by several major
components in the lake system, i.e. respiration of phytoplankton, zooplankton,
and fish; sediment oxygen demands; chemical oxygen demands; and biochem^ical
oxygen demands of the large quantity of organic materials from tributary inputs.
Those processes result in a negative dissolved oxygen budget in Lake Valencia
and create a severe oxygen depletion in the aphotic zone where an anoxic state
122
prevails in a large proportion of the water column. Predictably, the continuous
external loading of organic substances from tributaries will demand a greater
quantity of the dissolved oxygen produced by phytoplankton and drive the anoxic
layer upward to the surface.
Further understanding of the dynamic role of dissolved oxygen in Lake
Valencia will require systems analysis and development of a numerical mass
balance model. This type of model is essential for any managerial attempt to
improve oxygen tension in the lake.
Water chemistry in Lake Valencia is probably far more comxplex than any
general survey can comprehend.
The exceedingly high concentrations of total dissolved solids
(-2,000' mg/L) with sulfate as a major constituent (500-1,000 mg/L) represent an
unusual case in freshwater lakes of this size. Unlike most natural lakes with
relatively long'-term stable chemical characteristics which are governed largely
by the geological features of the lake basin, the major chemical constituents in
Lake Valencia are anthropogenic. The composition and quantity of those chemical
constituents are constantly changing with increasing population and land use
patterns in the lake basin. The concentration of total dissolved solids has
risen progressively since the lake became a closed body of water: it has been
increasing exponentially during the last two decades. According to a m^ass
balance predictive equation (Apmann 1973), the total dissolved solids will rise
to 6,000 by the year 2020. During recent years, the increase of total dissolved
solids has paralleled a rise in sulfate concentration. Although sulfate is
generally considered a conservative ion in the aquatic environment, the effects
of exceedingly high concentrations on aquatic biota are not well known.
Certainly, sulfate is the most available form to be reduced to sulfide in the
123
prevailing anaerobic subsurface water in Lake Valencia. The formation of
sulfide in lake water may have a significant impact on the chemistry of heavy
metals as the sulfide of heavy m.etals forms notoriously insoluble precipitates.
Theoretical treatment indicates that the insolubility of ferrous sulfide in the
sediment prevents release of certain metals such as copper, silver, lead, and
cadmium to lake water (Hutchinson 1957). The oxidation of hydrogen sulfide
during the mixing period may also enhance the oxygen depletion in the lake
water.
The nutrient data presented in this report indicate that the levels of
phosphorus and nitrogen were in excessive supply for phytoplankton growth. A
large proportion of those nutrients was in the organic or particulate forms,
presumably retained mostly by the phytoplankton cells. The internal loading of
nutrient through organismal cycling and sediment regeneration is likely to be an
important source of nutrient supply in Lake Valencia. Under the prevailing
anoxic conditions in the sediment and deep water, a substantial quantity of
nutrients might be regenerated and released to the euphotic zone. Therefore,
the analysis of sediments is an important tool for the study of the chemical and
biological functions of the overlying water body. Transfer of dissolved matter
between sediments and overlying water is of considerable importance in the
control of the chemical processes taking place in the sediments and interstitial
and basin waters. Some of the primary concerns are those of the role of
sediments in the cycling of nutrients and as a sink of oxygen. These processes
are governed by complex biochemical mechanisms. One of these processes is
sediment oxygen demand (SOD). In Lake Valencia, incorporating SOD into a
quantitative model will be indispensable to predict mass balance of dissolved
oxygen for lake restoration.
124
Source of phosphorus (P) and nitrogen (n) released from the sediment to the
overlying waters is expected to be of critical importance for successful
prediction of the effects on lake trophic status resulting from the removal of
nutrients by treatment.
The external loadings through tributary discharges constitute the largest
pollutant input to Lake Valencia. As those loadings contain mostly untreated
domestic and industrial discharges and silts from surface runoffs, the lake
proper adjacent to the tributary outfalls acts as a treatment which may be
conveniently termed a self-purification system for the pollutant. Several
processes are involved in the system: sedimentation of solid material,
decomposition of organic substances, and release of inorganic nutrients. As a
result, those areas of the lake show a large sedimentation rate, severe oxygen
depletion, and high nutrient concentrations.
Thus, the chemical constituents of the pollutants, the physical features of
the treatment area, and the biological responses are primary factors involved in
the self-purification capacity. Our previous investigation indicates that the
river mouth of Cano Central exhibits a great capacity of self-purification.
Such a process is particularly important in this area because it receives the
largest quantity of domestic discharge to Lake Valencia. Physically, the
extensive shallow contour provides efficient aeration which results in the
active aerobic decomposition of organic materials. The nutrients released from
organic forms stimulate massive phytoplankton blooms. Population density of the
zooplankton community has been found greater here than in other parts of the
lake, particularly the density of rotifers which are believed to be bacteria
feeders. Ostracods are also commonly found in this area. The pollutant
discharge, dispersion pattern, and heterotrophic and autotrophic processes in
125
the Cano Central region should be another focus for future research.
Results of this investigation indicates that excessive nutrient loading and
severe oxygen depletion present the most urgent issues in Lake Valencia, To
estimate nutrient and oxygen budgets in the lake water, we roust consider both
external and internal loadings. The external sources of nutrients and
biochemical oxygen demand are primarily contributed by tributary discharges
which are being thoroughly investigated. The internal loading of nutrient and
oxygen involves uptake and release by various constituents in the water column
as well as in the sediment. However, the magnitude of uptake and release of
nutrients and oxygen through internal compartments of the lake is virtually
unknown in Lake Valencia. The dynamics of nutrient and oxygen concentrations
are governed by physical, chemical, and biological processes, and to understand
the complexity of those processes requires system analysis and numerical
modeling.
126
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130
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