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Eisenhower Consortium Bulletin 1 0
rS0(
Effects of Road Salting
on Aquatic Invertebrate Communities
Manuel C. Molles, Jr.
Molles, Manuel C. , Jr. 1980. Effects of road salting on stream in¬
vertebrate communities. Eisenhower Consortium Bulletin 10, 9 p.
U.S. Department of Agriculture, Forest Service, Rocky Mountain
Forest and Range Experiment Station, Fort Collins, Colo.
Salting and sanding roads favored production of oligochaetes
over other invertebrates but did not have a pronounced effect on
Ephemeroptera , Plecoptera, Trichoptera, or Coleoptera in the study
streams. Salting and sanding of roads affects stream invertebrates
mainly through increased input of fine sediments and can impair trout
condition and reproduction.
Keywords: stream invertebrates, road salting and sanding, invertebrate
production, invertebrate diversity, sediment, salinity, trout, Sangre
de Cristo Mountains, New Mexico
Cover photos courtesy of New Mexico State Highway Department
Eisenhower Consortium Bulletin lO
July 1980
Effects of Road Salting
on Aquatic Invertebrate Communities1
Manuel C. Molles, Jr.
Assistant Professor of Biology
Biology Department
University of New Mexico
1 This research was supported by the Rocky Mountain Forest and Range Experiment Sta¬
tion, Forest Service, U.S. Department of Agriculture, through the Eisenhower Consortium for
Western Environmental Forestry Research under Research Agreement 16-589-GR. Supervision
was provided by David R. Patton, Project Leader, Wildlife Habitat Research, Tempe, Ariz.
Contents
Page
Management Implications . 1
Introduction . 1
Study Area . 1
Materials and Methods . 2
Results and Discussion . 2
Biomass . 2
Numbers . 4
Species Diversity . 5
Effects of Dissolved Salts Versus Sediment . 5
Implications to Trout Production . 8
Summary . 8
Literature Cited . 9
Effects of Road Salting
on Aquatic Invertebrate Communities1
Manuel C. Molles, Jr.
Management Implications
Salting and sanding roads to ease passage of traffic
in winter has been shown to increase salinity and suspended
sediments in nearby streams. The present study examined the
effects of these impacts on stream invertebrates. Reaches
receiving drainage from such roads supported reduced biomass
and numbers of stream invertebrates and/or were dominated by
oligochaetes during certain seasons. It appears these ef¬
fects were the result of increased input of fine sediments.
Reduced invertebrate production and changes in invertebrate
composition have the potential of impairing trout condition
(length-weight ratio) and, perhaps, total trout production.
Increased sedimentation also has the potential of harming
trout reproduction. Examination of this possibility could
receive a high priority in future studies of the impact of
road salting and sanding. Management could take steps now
to reduce inputs of fine sediments to streams. Procedures
to be considered are (1) application of no more road salt
and sand than necessary to insure safe passage of traffic,
and (2) revegetation of roadsides with salt-tolerant species
to act as a barrier to sand movement.
Introduction
The application of salt and sand to roads, a
widespread practice used to ease access to winter
recreation areas, has been shown to increase
concentrations of dissolved salts (Gosz 1977a)
and suspended sediments (Gosz 1977b) in nearby
streams. Elevated salt concentrations in these
streams result mainly from inputs of chloride
(Gosz 1977a). The main sources of increased sed¬
iment load are (1) sand applied during sanding
and salting, and (2) increased erosion of soils
resulting from the killing of roadside vegetation
by accumulated salts. The purpose of this study
was to determine the effects of these impacts on
stream invertebrates.
Study Area
The study area is in the Sangre de Cristo
Mountains approximately 15 km northeast of Santa
Fe, N. Mex. The streams studied, Tesuque Creek
and the Rio en Medio, are crossed by a road to
the Santa Fe Ski Basin (fig. 1). This road
receives applications of a mixture of 25% salt
(99% NaCl) and 75% sand by the State Highway
Department2 * following snow storms with total
applications of this mixture ranging up to 4.35 x
104 kg per km of road per year.
Study sites, each encompassing a 60-m length
of stream, were established above and below the
road on both study streams (fig. 1). The above¬
road site on Tesuque Creek extended upstream from
a point approximately 35 m above the road. The
below-road site on Tesuque Creek, which receives
drainage from 0.5 km of road, extended down¬
stream from a point approximately 50 m below
the road. The above-road site on the Rio en
Medio, a reach of stream immediately below the
U.S. Geological Survey weir on that stream, was
approximately 1 km upstream from the below-road
site. The positioning of the control site this
distance above the below-road site was necessary
to preclude influences from the Santa Fe Ski
Basin. The below-road site on the Rio en Medio
extended downstream from a point 100 m below the
road. This reach of stream receives drainage
from 0.35 km of road and a dirt parking lot.
2James R^ Gosz, personal communication.
Professor of Biology, University of New Mexico,
Albuquerque.
1
Santa Fe
Figure l.--Map of the study streams showing loca¬
tion of above-road (A) and below-road (B)
study sites on Tesuque Creek and on the Rio
en Medio.
basis of relative number of individuals. The
assumption is that all species are approximately
the same size. However, in the present study,
there were great differences in size between
species and between instars of the same species.
In such instances, biomass seems a more meaning¬
ful index of relative importance of species.
Therefore, dry weights were used to calculate the
relative proportions of species in a collection
(^e., p. = w./W, where w. = dry weight of the
i species ancl W = total clry weight of the col¬
lection). Species evenness (Pielou 1969, p. 233)
was calculated as
T = H' H*
H' In # species
max
All Surber samples for a given date and
study site were pooled for calculations of spe¬
cies diversity. This procedure yields the same
value of H' as would be obtained if an average
H' per sample were calculated. In these calcula¬
tions, only insects of the orders Ephemeroptera ,
Plecoptera, Trichoptera, and Coleoptera were
included. Other groups were excluded because of
taxonomic difficulties.
Information on trout diets was taken from
the literature since densities of trout in the
study streams were too low (two individuals per
300 m of stream) to do a meaningful dietary
analysis .
Concentrations of dissolved salts were
determined using a conductivity meter. Bimonth¬
ly samples were analyzed from each study site.
Statistical comparisons between study sites
were made using Student's t-test or, when stand¬
ard transformations (Steele and Torrie 1960)
failed to normalize distributions, the Mann-
Whitney test (Zar 1974).
Materials and Methods
Benthic invertebrates were monitored by
sampling with a Surber Sampler (0.093 m2). Ten
to twelve Surber samples were taken at each study
site in summer, autumn, and spring. A thick
cover of ice prevented winter sampling. Samples
were taken in midstream in intervals of 5 m be¬
ginning at the downstream end of each study site.
The sections of streams studied consisted chiefly
of swift runs with little pool development. The
major difference between study sites was an ob¬
vious increase in sand bottom below the road on
both streams.
All invertebrates collected were sorted
from coarse materials in the field, fixed in 95%
ethanol, and later transferred to 70% ethanol.
Final sorting, identification, counting, and
weighing were done in the laboratory. Dry weights
were taken by drying specimens with an infrared
drying unit, cooling them in a desiccator, and
weighing on an electric semimicro balance (preci¬
sion = ±0.01 mg) .
Species diversity was calculated using
the Shannon-Wiener diversity index (Shannon and
Weaver 1949), H' = - I p . lnp . . In this index,
t^ proportion of a sample represented by the
i species, p^, is generally calculated on the
Results and Discussion
Over 30,000 stream invertebrates were col¬
lected, identified, counted, and weighed during
the study. Most of these (99%) were insects of
the orders Ephemeroptera, Plecoptera, Coleoptera,
Trichoptera, and Diptera. The remainder were
oligochaetes and turbellarians , which, though
few in numbers, comprised a substantial propor¬
tion of the biomass of some collections.
Biomass
The biomass of invertebrates inhabiting
Tesuque Creek during July and October was not
affected by the presence of the road. In these
collections, significant differences between
above-road and below-road samples were found only
for the Ephemeroptera, which were represented
by greater biomass below the road in both July
(P <0.01) and October (P <0.01) (table 1). How¬
ever, in April the biomasses of all groups but
oligochaetes were significantly higher above the
road .
In contrast to Tesuque Creek, total biomass
in the Rio en Medio in July was higher below the
road (P <0.05) (table 2) as the result of higher
2
Table 1 . --Comparisons of biomass and numbers of invertebrates collected
with a Surber sampler in Tesuque Creek above and below the road
in summer, autumn, and spring. The values listed are means. Sta¬
tistical comparisons are within dates only.
July 31, 1976
October 16,
1976
April 23, 1977
Above
N=10
Below
N=10
Signifi¬
cance
Above
N=10
Below
N=10
Signifi¬
cance
Above
N=10
Below
N=10
Signifi¬
cance
Biomass
Coleoptera
1.45
1.26
1.03
0.97
2.45
0.95
P<0 . 05
Diptera
2.29
4.33
0.77
1.21
9.86
4.49
P<0.01
Ephemeroptera
20.47
39.52
P<0.01
3.87
10.73
^O.OOl
13.59
4.60
P<0.001
Oligochaeta
7.77
0
27.73
6.05
8.05
5.59
P<0.01
Plecoptera
10.81
9.31
16.76
17.09
12.73
6.93
P<0 . 05
Trichoptera
51.86
14.72
18.67
13.81
41.33
9.36
P<0.001
Turbellaria
0.52
1.14
C1)
0.14
1.40
C1)
0.37
1.91
PC0.001
Total
95.17
70.27
68.97
51.30
88.45
33.82
P<0.001
Numbers
Coleoptera
1.90
2.90
5.40
3.80
14.80
5.30
Diptera
15.40
22.90
6.50
8.90
176.00
67.20
P<0.001
Ephemeroptera
45.10
47.10
25.40
44.90
P<0 . 05
213.10
77.40
P<0.01
Oligochaeta
0.90
0
P<0 . 05
2.30
0.50
P<0 . 05
1.20
1.20
Plecoptera
15.50
17.60
48.70
75.70
62.00
88.50
Trichoptera
12.30
4.20
34.30
27.80
66.00
35.10
P<0.01
Turbellaria
0.30
0.90
C1)
0.20
1.00
1.20
6.00
P<0.05
Total
91.40
95.60
122.80
162.60
534.30
280.70
P<0.01
1
Mann-Whitney Test,
Student's t-Test used
in other
analyses
Table 2. --Comparisons of biomass and numbers of invertebrates collected
with a Surber sampler in Rio en Medio above and below the road
in summer, autumn, and spring. The values listed are means. Sta¬
tistical comparisons are within dates only.
July 30,
1976
October 16,
1976
May 17,
1977
Above Below
N=10 N=10
Signifi¬
cance
Above Below
N=10 N=10
Signifi¬
cance
Above
N=10
Below
N=10
Signifi¬
cance
Biomass
Coleoptera
1.
.00
0.
.54
4.
,17
1.
,81
4.
.04
1 ,
.30
P<0.
001
Diptera
4.
.81
4.
.34
4.
.53
1.
.40
P<0.01
39.
.31
6.
. 18
P<0.
001
Ephemeroptera
11 .
.86
9.
.19
15.
.08
2.
.91
1P<0.001
71.
.83
15,
.32
P<0.01
Oligochaeta
0.
.79
53,
.23
P<0
.01
0.
.11
35.
.76
1P<0.001
8.
.39
5.
.71
Plecoptera
4.
.38
2.
.67
44.
.07
24.
.77
(:)
4.
.05
3.
.66
Trichoptera
2.
.58
0.
.02
Tl
A
O
.05
4.
.80
4.
.48
2.
.68
4.
.11
Turbellaria
0.
.17
0
(x)
1 .
.67
0
2.
.32
0.
.21
Total
25.
.59
70.
.00
P<0
.05
74.
.44
71.
,06
132.
.60
36.
.49
'-d
A
O
001
Numbers
Coleoptera
5.
.70
1.
.80
13.
.90
3.
.90
P<0.01
24.
.83
10.
.42
P<0.
001
Diptera
40.
.30
19.
.10
27.
, 10
6.
.50
P<0.001
452.
.42
185.
.17
P<0.
001
Ephemeroptera
15.
,40
14.
.40
93.
.50
6.
.70
P<0.001
251.
.08
118.
.08
P<0.
001
Oligochaeta
0.
.30
3.
.50
1P<0.(
JO 1
0.
.60
4.
.60
^<0.01
1 .
.08
2.
.83
Plecoptera
8.
.50
18.
.50
70,
.70
38.
.90
16.
.67
54.
.58
Trichoptera
1.
.80
0.
.30
P<0
.05
4.
.30
4.
.00
9.
.08
9.
.75
Turbellaria
0.
.10
0
1.
.60
0
6.
.58
0.
.33
P<0.
001
Total
72.
. 10
57.
.60
211.
.70
64.
.60
P<0.001
760.
.92
381 .
.08
P
CO
Collection dates
Figure 3 . --Comparisons of mean biomass per Surber
sample (0.093 m2) taken from Tesuque Creek
and the Rio en Medio above (A) and below (B)
the road in summer, autumn, and spring. An
(*) indicates significant differences be¬
tween above-road and below-road study sites.
Statistical comparisons were made within
dates only.
Species Diversity
Despite the effects on numbers and biomass
noted above, salting and sanding of the road ap¬
pears to have had little effect on either composi¬
tion or diversity of insects. A total of 38
species of the orders Ephemeroptera , Plecoptera,
Trichoptera, and Coleoptera were identified
from Surber samples (table 3). Thirty-three of
these species were collected in the Rio en Medio.
Of these, three were confined to below-road
collections and another three species were con¬
fined to above-road collections. In Tesuque
Creek, 5 of 34 species were confined to above¬
road collections and 1 species was found below
the road only. The total biomass of species
confined to above-road or below-road sites com¬
prised less than 1% of the biomass of the four
orders of insects included in this analysis.
In all three collections from the Rio en
Medio more species were collected above the road.
In Tesuque Creek more species were collected
above the road on two of the three dates. How¬
ever, overall species diversity, H' , was higher
for all three below-road collections from Tesuque
Creek and for one of three below-road collections
from the Rio en Medio. In all four cases this
was the result of higher species evenness, J,
below the road.
Effects of Dissolved Salts Versus Sediment
Road salting and sanding had two principal
effects on the water quality of the Rio en Medio
and Tesuque Creek: (1) increased salinity (figs.
4 and 5), and (2) increased sediment load below
the road for the sampling period from October
1976 to September 1977 (Gosz 1977b) as shown
in the tabulaton below. Related to these changes
in water quality below the road were reductions
in invertebrate numbers and biomass. However,
no pronounced differences in species diversity
between study sites were observed. These results
suggest that invertebrates below the road were
affected mainly by sedimentation and not by ele¬
vated salinities.
Above road Below road
Rio en Medio 684 kg 1262 kg
Tesuque Creek 1466 kg 6544 kg
Species diversity is one of the most sensi¬
tive indicators of toxicity effects (Hawkes
1977). Where salt pollution has been severe,
reductions in species diversity have been re¬
corded (Harrel 1966, Crowther and Hynes 1977).
Failure to demonstrate any pronounced effect on
composition or diversity during the present study
indicates that salt concentrations below the
road were not toxic to stream insects.
The conclusion that salt concentrations be¬
low the road were not toxic to insects inhabiting
the study sites is supported by the dissolved
salt levels recorded. The maximum concentration
of dissolved salts recorded (in the Rio en Medio
below the road), ~ 66 mg/1 (fig- 5), was approx¬
imately one-half the world average for freshwater
of 120 mg/1 (Wetzel 1975) and less than the "to¬
tal dissolved solids" of 14 of 18 mid-elevation
trout streams in the Rocky Mountains (Pennak
1977). In addition, Crowther and Hynes (1977)
found that road salting affected the behavior of
stream insects in southern Ontario streams only
when Cl concentrations exceeded 1,000 mg/1, two
orders of magnitude greater than the maximum con¬
centration recorded in the present study.
The reduction in invertebrate numbers and
biomass observed in both study streams is con¬
sistent with the effects of sedimentation that
have been observed in a wide variety of streams
(Cordone and Kelley 1961). Gosz (1977b) docu¬
mented an increased sediment load below the road
on both study streams, and even casual observa¬
tion confirms a higher percentage of sand bottom
at below-road sites, especially on the Rio en
Medio. Retention of sand appears to be enhanced
5
Table 3 . --Comparisons of species diversity and species composition of
Coleoptera, Ephemeroptera , Plaecoptera, and Coleoptera collected
with a Surber sampler in the Rio en Medio and Tesuque Creek above
and below the road. Except where indicated otherwise, values
listed are average dry weights (mg) per 0.093 m2 .
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