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Interspecific Comparisons and the Potential Importance
of Nutrient Excretion by Benthic Fishes in a Large
Reservoir
Keith B. Gido
a
a
University of Oklahoma, Biological Station, Department of Zoology, Norman, Oklahoma,
73019, USA
Version of record first published: 09 Jan 2011.
To cite this article: Keith B. Gido (2002): Interspecific Comparisons and the Potential Importance of Nutrient Excretion by
Benthic Fishes in a Large Reservoir, Transactions of the American Fisheries Society, 131:2, 260-270
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Transactions of the American Fisheries Society 131:260–270, 2002
q Copyright by the American Fisheries Society 2002
Interspecific Comparisons and the Potential Importance of
Nutrient Excretion by Benthic Fishes in a Large Reservoir
KEITH B. GIDO*1
Downloaded by [Kansas State University Libraries] at 10:14 09 October 2012
University of Oklahoma, Biological Station and Department of Zoology,
Norman, Oklahoma 73019, USA
Abstract.—Fishes can provide an important link between benthic and pelagic habitats by removing
nutrients from sediments and excreting them into the water column. The relative importance of
nutrients excreted by fishes to ecosystem productivity may vary among species and with abiotic
conditions. I measured excretion rates of three benthic feeding fishes, gizzard shad Dorosoma
cepedianum, smallmouth buffalo Ictiobus bubalus, and river carpsucker Carpiodes carpio, to examine the potential contribution of these species to the nutrient budget of a large southern reservoir.
All species showed a significant relationship between log excretion rate and log body mass but
no differences among species in the slopes or intercepts. For all species combined, the slope of
the relationship was less than 1, suggesting a less than proportional increase in excretion rate with
size. Using mean standing crop estimates from published cove rotenone surveys in Oklahoma, the
nutrient loading to Lake Texoma by these fishes was estimated to be 0.0096 kg · ha21 · d21 for
phosphorus and 0.3580 kg · ha21 · d21 for nitrogen. These values exceeded external nutrient-loading
rates from the watershed 12% and 43% of the time for phosphorus and nitrogen, respectively.
Numerous studies in aquatic systems have
shown that fishes can have important effects on
ecosystem processes. Fishes can directly affect primary productivity through control of grazer populations (e.g., Carpenter et al. 1985; Power 1990)
or through transport and recycling of nutrients
(Hurlbert et al. 1972; Lamarra 1975; Andersson et
al. 1978; Persson 1997). In particular, benthicfeeding fishes can provide a link between benthic
and pelagic processes by transporting materials between the two regions. For example, they can enhance primary productivity in the water column
by resuspending phytoplankton from sediments to
surface waters (Breukelaar et al. 1994) or by increasing nutrient regeneration through resuspending sediments and associated microbes (Shormann
and Conter 1997). Benthic fishes also have the
ability to release bioavailable nutrients derived
from sediments into the water column through excretion (Lamarra 1975; Brabrand et al. 1990;
Schaus and Vanni 2000). Estimating the relative
importance of fishes in regulating water column
productivity can help managers interested in the
use of these organisms to improve water quality
or to enhance sport fish production (e.g., through
biomanipulation; Carpenter et al. 1987; Drenner
et al. 1998). Moreover, estimating the degree of
* E-mail: kgido@ksu.edu
1 Present address: Kansas State University, Division
of Biology, Ackert Hall, Manhattan, Kansas 66506,
USA.
Received August 21, 2000; accepted September 13, 2001
interspecific differences in species effects can aid
in modeling the effects of individual species or of
a suite of species on ecosystem processes.
The ability of a species to regulate ecosystem
processes depends on its abundance (Power et al.
1996; Power 1997), population size structure
(Schaus et al. 1997; Mehner et al. 1998), trophic
level (Carpenter et al. 1992; Schindler et al. 1993),
and mode of feeding (Matthews 1998; Gido 2001).
In addition, abiotic conditions often mediate the
magnitude of a species’ effect in an ecosystem (Brabrand et al. 1990; Power et al. 1996). For example,
the effect of omnivorous fish on chlorophyll biomass in experimental tanks was shown to be greatest when nutrients were added (Drenner et al. 1998).
In natural lakes and reservoirs, the effects of nutrient enrichment by fishes (e.g., Schindler et al.
1996; Vanni 1996) are thought to be important only
during times when nutrient loading from the watershed (Brabrand et al. 1990) or when nutrient regeneration by plankton (Hudson et al. 1999) is relatively low. Therefore, species effects in ecosystems must be examined in the context of variable
biotic and abiotic factors (Power et al. 1996; Power
1997).
That fishes have the potential to affect ecosystem
properties by excreting limiting nutrients has been
well documented (e.g., Lamarra 1975; Brabrand et
al. 1990; Schaus et al. 1997). However, few studies
have compared excretion rates across taxa to examine the capacity of different species to regulate
ecosystem processes. In this study, I examined interspecific differences in nutrient excretion rates
260
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NUTRIENT EXCRETION BY BENTHIC FISHES
among three benthic omnivorous fishes (smallmouth buffalo Ictiobus bubalus; gizzard shad Dorosoma cepedianum; and river carpsucker Carpiodes carpio). All species are facultative detritivores
that forage benthically but differ in the relative proportions of detritus, algae, and invertebrates in their
diets. Gizzard shad primarily consumes detritus and
algae along with large amounts of inorganic sediments, whereas smallmouth buffalo and river carpsucker consume cyclopoid copepods and detritus
with little inorganic sediments (Dalquest and Peters
1966; Gido 2001). These species are very common
in reservoirs in the Mississippi River Basin and
contributed approximately 50% of the offshore fish
biomass captured during gill-net surveys of Lake
Texoma (Gido and Matthews 2000; Gido et al.
2000), where this study occurred. In addition to
interspecific comparisons of excretion rates, I used
the measured values to estimate nutrient loading by
this guild of fishes and compared this outcome with
nutrient loading from tributary rivers to assess the
relative importance of these fishes in the overall
nutrient budget of the reservoir.
Study Area
Lake Texoma is a 36,000-ha impoundment
(mean depth 5 8 m) of the Washita and Red rivers
on the Oklahoma–Texas border. Secchi depth
transparency typically ranges from 100 to 125 cm
but can decrease to 15 cm during turbid inflow
episodes (Matthews 1984). The gross primary productivity at my study site, determined by light–
dark bottle reactions (Wetzel and Likens 1991),
was measured biweekly between June and August
1998 at 0.5 m below the surface and ranged from
98.7 to 191.8 mg (mean 5 149.8 mg) of carbon
m23 · h21. Chlorophyll a concentrations ranged
from 3.1 to 43.7 mg/L and total phosphorus in the
reservoir ranged from 20 to 110 mg/L (Gibbs
1998). Amounts of both phosphorus and light appear to limit primary production in the reservoir
as suggested by a positive association of total
phosphorus and a negative association of turbidity
with chlorophyll a concentration (Gibbs 1998).
Reservoir surface temperature rarely exceeded
328C during summer. Fish were collected approximately 35 km uplake from Denison Dam, within
the Red River arm of Lake Texoma.
Methods
Measurement of excretion rates.—Excretion
rates for ammonia (NH3-N) and total reactive
phosphorus (TRP) were measured in the field for
each species by using methods similar to those of
261
Mather et al. (1995) and Schaus et al. (1997). Most
fish were collected by seine; however, because of
difficulty capturing smallmouth buffalo and river
carpsucker this way, several individuals were captured with gill nets, which were checked at 15–
30-min intervals. Gill nets sampled this frequently
did not appear to cause undo stress to the fish,
probably because of their large size. All fish collections were made between 1400 and 1600 hours,
when nutrient excretion rates were likely to be
greatest (e.g., Pierce et al. 1981; Schaus et al.
1997). Fish were immediately placed in Styrofoam
coolers lined with polyethylene bags and filled
with prefiltered (Whatman GF/F filters) water collected from Lake Texoma. Water samples (250 mL
each) were taken immediately after a fish was
placed in the cooler and again after 1 h. These
samples were placed on ice immediately after collection and analyzed within 24 h. Extreme care
was taken to minimize stress to fishes by minimal
handling, placement in dark chambers (coolers),
and provision of sufficient volumes of water to
avoid oxygen depletion. Although stress caused by
capture may affect nutrient excretion rates, Mather
et al. (1995) showed no difference in excretion
rates between field fish and laboratory-raised individuals accustomed to handling. Although the
potential bias of increased excretion rates caused
by handling (in the laboratory and field) must be
considered, direct measurement of excretion from
naturally feeding fishes in the field is likely to give
more realistic results than simulating field conditions in the laboratory (Schaus et al. 1997).
Ammonia nitrogen concentration was determined by the phenate method and TRP was determined by the ascorbic acid method (APHA et
al. 1992). In a preliminary study, measurements
of dissolved reactive phosphorus (i.e., filtered
through 0.45-mm pore-size membrane filters) gave
values about the same (,2% difference) as for
TRP, suggesting that little particulate phosphorus
was suspended in the water. Thus, values of TRP
are close approximations of the soluble reactive
phosphorus that has been measured in previous
studies (e.g., Schaus et al. 1997).
Because nutrient concentrations in holding
chambers could potentially be taken up by microorganisms egested by the fishes (e.g., Meyer and
Schultz 1985), occasionally water samples (n 5 9)
were taken 1 h after fish were removed from coolers to test for nutrient uptake. Although mean nutrient concentrations were lower 1 h after fish were
removed from coolers than at the time of removal
(294.2 [SE 5 201.0] and 22.2 [SE 5 4.76] mg/
262
GIDO
TABLE 1.—Estimates of standing crop (kg/ha) for all fishes and three omnivorous fishes based on cove rotenone
surveys in reservoirs.
Smallmouth
buffalo
Gizzard
shad
River
carpsucker
276
227
187
351–934
52.7a
27
21.3
20–1,200
24.2 (37.6)
117.6
195
137
103–798
417
118.5 (177.0)
Reservoir
All fishes
Oklahoma reservoirs
Clear Lake
Lake Catherine
West Point Reservoir
Acton Lake
NRRP database b
9.2 (22.4)
Source
Jenkins 1976
Lambou and Stern 1959
Mathis and Hulsey 1959
Timmons et al. 1979
Schaus et al. 1997
NRRP, unpublished data
a
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All buffalofishes.
b Mean (SD) standing crop estimates (cove rotenone surveys) from 360 reservoirs across the United States compiled
by the National Reservoir Research Program (NRRP).
L for NH3-N and TRP, respectively), the differences were not significant (t-test, P . 0.05). Thus
I assumed uptake by microbes was negligible and
made no adjustments to the data.
Estimates of whole lake effects.—Nutrient excretion rates were extrapolated to estimate areal
nutrient loading by fishes (L) to Lake Texoma.
Nutrient loading (L; kg · ha21 · d21) was defined
as:
LS 5 ES · SCS · DS
where E is the per capita NH3-N or TRP excretion
rate (kg of N or P kg21 · h21) described above, SC
is standing crop (kg/ha wet mass), and D is a coefficient between 0 and 1 to account for diel variation in excretion rates (see below). The subscript
S denotes species. Because the amount of standing
crop probably varies across space and time within
Lake Texoma, nutrient loading was estimates
across a range of standing crop estimates. I calculated the combined nutrient-loading rate for
these three species, assuming relative densities of
61.1, 27.5, and 11.1% for gizzard shad, smallmouth bass, and river carpsucker, respectively
(values that were based on cove rotenone surveys
of 20 Oklahoma reservoirs, including Lake Texoma; Jenkins 1976). As reference points, L was
calculated for standing crop estimates of 192 kg/
ha, the mean for Oklahoma reservoirs (Jenkins
1976), and 1,200 kg/ha, which represents a maximum loading rate to the system based on values
reported from cove rotenone surveys of 360 North
American reservoirs (see Table 1 for examples).
Because standing crop values can vary within and
among reservoirs, I felt that using an average estimate across reservoirs in the region (192 kg/ha)
would be more reliable than an estimate from a
single period of time, which might be subject to
a high degree of seasonal and annual variation.
Because nutrient excretion rates are likely to
vary over a 24-h period, loading rates were adjusted to account for a decline in excretion associated with diel changes in feeding activity (D). D
was estimated by examination of gut fullness
across daylight hours. I assumed that a significant
decline in gut fullness (a 5 0.05) indicated cessation of feeding. To measure this, fishes were collected with monofilament gill nets with mesh sizes
from 25 to 102 mm bar measure at 4-h intervals:
from 0800 to 1200 hours, from 1200 to 1600 hours,
and from 1600 to 2000 hours. This sampling was
conducted monthly at nine locations from May
through August in 1997 and 1998. The alimentary
canals from a maximum of 6 individuals (mean 5
3.8 [SD 5 6 1.7] individuals for gizzard shad, 4.5
[SD 5 6 1.3] individuals for smallmouth buffalo,
and 2.4 [SD 5 6 2.3] individuals for river carpsucker) of each species were taken from each time
period and immediately placed on ice. In the laboratory, the gut contents were cleared from the
entire length of the intestine and dried to a constant
weight at 608C (for approximately 48 h). The mean
weight of the dried gut contents was used to determine differences in feeding activity across time
periods. Because nutrient excretion continues into
the night (between 2000 and 0800 hours) but possibly at a lower rate, I estimated nighttime excretion to be 20% less than that measured during midday. Schaus et al. (1997) reported nighttime excretion rates for gizzard shad were 82% of maximum daily rates.
To quantify the relative importance of nutrient
excretion by fishes, I compared the nutrient loading
by fishes to estimates of external loading from the
watershed. External loading was derived from daily
inflow data for Lake Texoma (U.S. Army Corps of
Engineers, Denison Dam, unpublished report) and
nutrient concentrations taken from monthly sampling at U.S. Geological Survey gauging stations
on the Washita River near Dickson, Oklahoma
NUTRIENT EXCRETION BY BENTHIC FISHES
263
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(Gage U07331000), and the Red River near Gainsville, Texas (Gage U07316000). Because the total
phosphorus concentration and the discharge of these
tributaries were not related (r2 5 0.03, P 5 0.25,
df 5 52), external loading was calculated by multiplying mean nutrient concentration by daily inflow
volume during summer (May–August) 1995–1998.
In contrast, the total nitrogen concentration in tributary rivers did show a significant positive relationship with discharge (r2 5 0.11, P 5 0.01, df 5
52); thus, daily total nitrogen concentration was
predicted from the equation:
total nitrogen (mg/L)
5 0.0046 inflow(m 3 /d) 1 1.079.
Frequency histograms for daily nutrient-loading
values for this period were generated and used for
comparison with estimates of nutrient loading by
fishes.
Data analysis.—Interspecific differences in
NH3-N, TRP excretion, and the N:P ratio were
determined by using analysis of covariance (ANCOVA), with body weight as the covariate. The
relationship between log body mass and log nutrient excretion rate was determined with leastsquares regression analysis for individual species
and for all species combined. To examine diel differences in gut fullness, a two-way analysis of variance (ANOVA) was used to detect differences
among the main effects of month, time of day, and
their interactions.
Results
Nutrient excretion was measured for fishes during
summer (May–August) 1998 when surface water
temperature ranged from 25.58C to 30.58C. All three
species showed a significant (P , 0.02) positive
association between log body mass and log excretion rate for NH3-N and TRP but not for N:P ratio
(Figure 1; Table 2). The slope coefficient of the
relationship between log body mass and log nutrient
excretion rate for both NH3-N and TRP did not
differ from 1 (i.e., within the 95% confidence interval) for smallmouth buffalo and gizzard shad
(Table 2). The same relationship for river carpsuckerhad a slope coefficient of less than 1. However, ANCOVA with body mass as a covariate revealed no significant difference among species for
rates of NH3-N excretion (F2,39 5 0.837, P 5 0.44),
TRP excretion (F2,39 5 1.604, P 5 0.21), or molar
N:P ratio (F2,39 5 2.038, P 5 0.14). Because I found
no interspecific difference in the slopes of the relationship between log body mass and log excretion,
FIGURE 1.—Relationship between body mass and nutrient excretion (NH3-N, TRP, and N:P ratio) for three
benthic fishes in Lake Texoma. Least-square regression
lines are drawn for each species.
I performed a separate regression with all species
combined. In contrast to individual regression analyses for gizzard shad and smallmouth buffalo, the
slope of this overall relationship was less than 1,
suggesting a less than proportional increase in excretion rate with body mass. For example, the mean
weight of smallmouth buffalo (1.88 kg) measured
in this study was 8.1 times that of gizzard shad (0.23
kg) but the smallmouth excretion rates for NH3-N
and TRP averaged only 2.8 and 3.1 times, respectively, that of the shad.
No significant differences in gut fullness were
found among times of day for any species (P .
0.09), which suggests continuous feeding through-
264
GIDO
TABLE 2.—Coefficients and P-values from regression analyses relating log body mass to log nutrient excretion rates
(ammonia nitrogen [NH 3-N] and total reactive phosphorus [TRP]) and molar N:P ratio for three benthic fishes examined
separately and for all three species combined.
Coefficient
Lower 95%
confidence
point
Upper 95%
confidence
point
P-value
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River carpsucker
NH 3-N
Intercept
Slope
TRP
Intercept
Slope
Molar N:P
Intercept
Slope
1.593
0.698
0.794
0.410
2.393
0.986
20.401
0.683
20.977
0.476
0.176
0.891
113.186
0.178
2145.661
293.015
372.034
93.371
0.9965
0.0005
,0.001
Smallmouth buffalo
NH 3-N
Intercept
Slope
TRP
Intercept
Slope
Molar N:P
Intercept
Slope
1.094
0.809
20.049
0.457
2.236
1.162
0.0002
21.554
1.017
24.151
0.216
1.043
1.818
0.0165
202.470
227.176
2497.523
2243.121
902.463
188.768
0.791
Gizzard shad
NH 3-N
Intercept
Slope
TRP
Intercept
Slope
Molar N:P
Intercept
Slope
0.762
1.060
20.465
0.534
1.988
1.585
0.0006
21.186
1.086
22.528
0.509
0.156
1.664
0.0004
91.291
24.404
2169.183
2116.015
351.765
107.208
0.934
All species
NH 3-N
Intercept
Slope
TRP
Intercept
Slope
Molar N:P
Intercept
Slope
1.858
0.585
1.594
0.490
2.123
0.679
,0.0001
0.101
0.515
20.322
0.364
0.524
0.666
,0.0001
24.671
27.608
278.479
29.191
127.821
64.407
out daylight hours (Figure 2). Thus, because I assumed excretion rates to be maximal during the
day (D 5 1.0 for 12 h) and to decline by 20%
during the night (D 5 0.8 for 12 h), for the full
24-h period, D 5 0.9.
Inflow from tributaries of Lake Texoma ranged
from 2.4 3 105 to 4.7 3 108 m3/d during summer
1995–1998. Median nutrient loading from the watershed was 0.435 kg · ha21 · d21 (range, ,0.01–
344.06 kg · ha21 · d21) for total nitrogen and 0.081
kg · ha21 · d21 (range, 0.002–3.895 kg · ha 21 · d21)
for total phosphorus (Figure 3). Assuming a stand-
0.137
ing crop of 192 kg/ha, nutrient loading by benthic
fishes was predicted to be 0.358 kg · ha21 · d21 for
nitrogen and 0.0096 kg · ha21 · d21 for phosphorus.
Nitrogen loading by fishes exceeded nutrient loading from tributary rivers on approximately 43% of
the days modeled in this analysis, whereas phosphorus excreted by fishes exceeded external loading 12% of the time (Figure 3). Given a maximum
standing crop estimate of 1,200 kg/ha, loading by
fishes was greater than that from the watershed for
78.2% and 39.2% of the days modeled for nitrogen
and phosphorus, respectively.
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NUTRIENT EXCRETION BY BENTHIC FISHES
265
F IGURE 2.—Mean gut fullness for three omnivorous fishes during three time periods. Gut fullness is represented
as dry weight of gut contents as a percentage of body weight (wet weight) of each fish. Vertical bars represent
1 SE.
Discussion
Interspecific Differences in Nutrient Excretion
Nitrogen and phosphorus excretion rates were
positively associated with body mass for all three
species. Although the slope of these relationships
did not differ among species, the large variance in
excretion rates combined with a limited range of
body sizes measured within species limited the
statistical power to detect interspecific differences
or give precise slope estimates for these relationships. For example, when species were examined
separately, the slope of the relationship between
log body mass and log excretion rate did not differ
from 1 for smallmouth buffalo and gizzard shad.
This contrasts with the predicted slope of less than
1, which is expected because of a typical per capita
decrease in metabolic rate with body size (Gerking
1955; Brett and Groves 1979). Although low statistical power may have led to the discrepancy between my results and predicted values, some evidence suggests the measured slopes for smallmouth buffalo and gizzard shad may be near 1 for
the size range of individuals measured in this
study. For example, in a study of the metabolic
rate of smallmouth buffalo (Adams and Parsons
1998) weighing between 0.6 and 2.3 kg, the slope
coefficient of log active metabolic rate and log
body mass was approximately 1.08 (S. R. Adams,
Southern Illinois University, personal communication), which is consistent with the measured
slope of log NH3-N excretion and log body mass
(1.09) found in this study. Whereas Schaus et al.
(1997) found a slope of less than 1 for the relationship between the excretion rate of NH3-N and
soluble reactive phosphorus and the body mass
ofgizzard shad, the average size of individuals
measured was five times smaller than those I measured in this study (42.6 versus 222.7 g). Thus,
excretion rate possibly does scale proportionally
with body mass in larger individuals.
In contrast to the separate analyses of these species, pooling all data gave an overall relationship
between log body mass and log excretion rate for
both NH3-N and TRP of less than 1. Because there
GIDO
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266
FIGURE 3.—Frequency histogram of external nitrogen (top graph) and phosphorus (bottom graph) loading to
Lake Texoma based on estimated inflow and nutrient concentrations in tributary rivers during summer 1995–1998.
The dashed line represents the median nutrient loading from the watershed. Arrows represent predicted nutrient
loading rates by fishes given standing crop estimates of (A) 192 kg/ha (based on cove rotenone surveys of Oklahoma
reservoirs) and (B) 1,200 kg /ha (maximum standing crop estimate for this guild of fishes based on rotenone surveys
of 360 North American reservoirs). Loading rates by fishes were based on relative densities of gizzard shad,
smallmouth buffalo, and river carpsucker at 61.1%, 27.5%, and 11.1%, respectively.
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NUTRIENT EXCRETION BY BENTHIC FISHES
was little overlap in body size of individuals from
different species, this result was largely caused by
lower per capita excretion rates of the larger species (e.g., smallmouth buffalo) relative to the
smaller species (e.g., gizzard shad). Unfortunately,
because of the size differences among species measured in this study, I could not separate whether
differences in per capita excretion rates were a
function of body size (i.e., slope , 1 for all species) or reflected idiosyncratic differences in nutrient release among species (i.e., slopes differed
among species). Nevertheless, estimates of the
contribution of this guild of fishes to the nutrient
budget of the reservoir may depend on species
composition, simply because of the differences in
body size of the adults of the three species.
Differences in the N:P ratio of excreted materials can affect ecosystem properties, particularly
algal species composition (Drenner et al. 1996;
Vanni 1996). N:P ratios measured in this study
(mean 5 82, range 5 36–160) were greater than
those reported in previous studies (e.g., Mather et
al. 1995; Schaus et al. 1997). However, the majority of the fish I measured were much larger
(mean 5 741 g, range 5 81–2,748 g) than those
in the studies listed above and so would be predicted to have greater N:P excretion ratios. Because body N:P declines with increasing fish mass
(Davis and Boyd 1978), larger fish should sequester more, and subsequently excrete less, phosphorus relative to nitrogen than would smaller fish
(Vanni 1996; Schindler and Eby 1997). Nonetheless, the results of this and other studies suggest
that the relationship between body mass and the
N:P ratio of excretion within this guild of fishes
may differ from the predicted relationship. For example, the slope of the relationship between N:P
ratio and log body mass in this study did not differ
from zero when individuals from each species
were examined separately (but see above comments on statistical power), or when they were
combined, regardless of the wide range of body
sizes examined. Moreover, although Schaus et al.
(1997) reported much lower N:P ratios than in this
study, they also found a significant negative rather
than positive relationship between N:P and log
body mass (slope of24.43) for gizzard shad.
Whereas the overall high N:P ratios reported in
this study may be due to the large body size of
individuals measured, evidence within this guild
of fishes was insufficient to suggest a clear relationship with body mass.
Are there differences in how these three species
might affect ecosystem processes in reservoirs? If
267
nutrient excretion increased proportionally with
body mass (i.e., slope 5 1), nutrient loading by
fish could be predicted simply by knowledge of
the standing crop or biomass of fishes present.
However, because of large size difference among
species, and the observed slope of less than 1 with
all species combined, I chose to control for body
size (and species content) by estimating nutrient
loading for each species separately and then combining those estimates to get a total loading for
this guild of fishes. Under this model, I would
hypothesize that an assemblage dominated by the
larger-bodied smallmouth buffalo may contribute
substantially less, per unit biomass, to the nutrient
budget of a reservoir than would an assemblage
comprising primarily smaller-bodied gizzard shad.
Relative Importance of Nutrient Excretion
Results of this study indicate that large-bodied,
omnivorous fishes have the potential to contribute
to the nutrient budget of reservoirs through transport and recycling of nutrients. However, because
of the highly variable reservoir environment and
the presence of confounding variables that I could
not account for in this study, the role of fish in the
nutrient budget of Lake Texoma is somewhat speculative and depends on several factors. First, the
importance of fishes in the reservoir nutrient budget depends on other sources of nutrients, such as
those from the watershed. In Lake Texoma, phosphorus is positively associated with chlorophyll a
concentrations, suggesting that phosphorus is a
limiting nutrient (Gibbs 1998). Figure 3 suggests
that phosphorus excreted by fish, even at high densities (1,200 kg/ha), is of little importance during
periods when nutrient loading from tributaries exceeds 0.06 kg · ha21 · d21, an event that occurred
approximately 60% of the days on which nutrient
loading was modeled. Although external nitrogen
loading is probably more complex than phosphorus loading (e.g., because of atmospheric deposition), benthic fishes may contribute more to the
nitrogen budget of the reservoir than they contribute to the phosphorus budget. For approximately 43% of the days modeled, nitrogen loading
by fishes exceeded that from tributaries to the reservoir (assuming a standing crop of 192 kg/ha).
Apparently, internal loading of nitrogen by fishes
is potentially important during periods of low inflow, not only because the external loading is minimal, but because receding water levels will concentrate the fish, which will in turn cycle a greater
proportion of nutrients into the system.
Other internal sources of nutrient regeneration
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268
GIDO
also are important in regulating primary producers,
even during periods of low inflow from the watershed. Recently Hudson et al. (1999) reported a
strong linear relationship between total phosphorus concentration and phosphorus regeneration
rates by plankton. They further suggested that nutrient regeneration by fishes (i.e., through excretion) is relatively small compared with phosphorus
regeneration by plankton. Using their equation and
given a range of total phosphorus concentrations
between 20 and 110 mg/L in Lake Texoma (Gibbs
1998), I calculate the phosphorus regeneration rate
in Lake Texoma should range between 2,581 and
14,359 ng · L21 · d21. By converting my areal
estimates of phosphorus loading to their volumetric regeneration rates (using an estimated mean
depth of 8 m for Lake Texoma), phosphorus loading by fishes is about 120 ng · L21 · d21, assuming
a fish density of 192 kg/ha. Although this is a small
fraction of the predicted regeneration rate of phosphorus by plankton, nutrients provided by benthic
feeding fishes are more likely to increase productivity if they transport nutrients from benthic sediments to the water column than if they simply
recycle nutrients in the water column by foraging
on suspended phytoplankton or zooplankton (Brabrand et al. 1990; Kraft 1992; Mehner et al. 1998).
Thus, by moving nutrients from sediments, they
increase the total phosphorus content in the water
column (e.g., Shormann and Conter 1997; Schaus
and Vanni 2000).
In conclusion, this study provides a specific example of the potential for large-bodied benthic
fishes to contribute to the nutrient budget of a large
southern reservoir in a context-dependent manner.
Several studies have focused on the importance of
gizzard shad in these systems (e.g., Stein et al.
1995; Vanni 1996; Schaus et al. 1997), but other
large-bodied fishes such as smallmouth buffalo
and river carpsucker also are abundant in reservoirs and may have similar functional roles in the
ecosystem. However, because of differences in
body mass among species, their relative effects
may differ among species. Future research that accurately models the nutrient loading by fishes and
directly compares this to other sources of nutrients
(e.g., atmospheric deposition and nutrient regeneration by plankton) across a range of spatial and
temporal scales will clarify the importance of fishes in regulating primary production in reservoirs.
Acknowledgments
Field and laboratory assistance was provided by
D. Cobb, A. Marsh, D. Lutterschmit, and J. Schae-
fer. J. Pigg provided data on nutrient concentrations in the Red and Washita Rivers and S. Miranda
provided data on standing crop estimates from the
National Reservoir Research Program. This manuscript benefited by critical reviews by L. Canter,
C. Hargrave, M. Kaspari, W. Matthews, J. Schaefer, W. Shelton, C. Vaughn, M. Weiser, and three
anonymous reviewers. The University of Oklahoma Biological Station provided logistic and financial support. Thoughtful discussions concerning
design and analysis were provided by E. MarshMatthews, B. Narin, M. Schaus, G. Wellborn, T.
Wissing, and B. Ziebro. This manuscript was submitted as partial fulfillment of a Ph.D. to the faculty of the Department of Zoology, University of
Oklahoma.
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