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Effects of fish biomass and planktivore
type on plankton communities
JOSÉ LUIZ ATTAYDE* AND ROSEMBERG F. MENEZES
UNIVERSIDADE FEDERAL DO RIO GRANDE DO NORTE, CENTRO DE BIOCIÊNCIAS, DEPARTAMENTO DE BOTÂNICA, ECOLOGIA E ZOOLOGIA, NATAL, RIO GRANDE
DO NORTE
59072-970,
BRAZIL
*CORRESPONDING AUTHOR: [email protected]
Received February 20, 2008; accepted in principle April 17, 2008; accepted for publication May 1, 2008; published online May 6, 2008
Corresponding editor: Mark Gibbons
We performed a field experiment in 15 fish ponds manipulating the size and biomass of the Nile
tilapia, Oreochromis niloticus, to test the hypothesis that the effects of planktivorous fish on plankton dynamics and community structure depend on the planktivore type and fish-stocking biomass.
Juveniles and adult tilapia were either stocked alone or combined in the same proportion at 20 and
40 g/m3. Because tilapias show an ontogenetic niche shift from visual predation on zooplankton
to filter (suction) feeding on both zooplankton and phytoplankton, we expected differences in the
biomass-related effects of juveniles and adults on plankton communities. Our results show that
rotifer abundances were higher in treatments stocked with juvenile tilapia whereas cladoceran abundances were higher in treatments stocked with adults. However, total zooplankton and copepod
abundances, chlorophyll a concentrations and Secchi depth were not affected by tilapia size,
suggesting that variation in the feeding mode of tilapias affects the structure of zooplankton communities but that this does not cascade down to affect phytoplankton biomass and water transparency. Competitive interactions among the fish constrained their potential to depress zooplankton
abundances and increase algal biomass as fish-stocking biomass increased. Competition among
size classes of tilapias was highly asymmetric and juveniles were better competitors than adults for
plankton resources in our experimental ponds.
I N T RO D U C T I O N
Since the effects of planktivorous fishes on zooplankton
and phytoplankton communities were first recognized
(Hrbacek et al., 1961; Brooks and Dodson, 1965), there
have been many studies attempting to investigate the
impacts of size-selective predation by planktivorous fish
on zooplankton and their indirect effects on phytoplankton and the water transparency of lakes, reservoirs
and ponds (Carpenter et al., 1985; Carpenter and
Kitchell, 1993). The direct response of zooplankton
communities to visual fish predation (i.e. size-selective
predation on zooplankton) has been the focus of much
limnological research, whereas the multilevel effects of
omnivorous filter feeding fish ( predation on zooplankton plus grazing on phytoplankton) have been less investigated (Drenner et al., 1986, 1987, 1996; Lazzaro et al.
1992; Stein et al., 1995).
Previous studies suggest that the two contrasting
planktivore types (visual feeders and filter feeders) have
different effects on plankton communities (Drenner
et al., 1986; Lazzaro 1987; Lazzaro et al., 1992; Stein
et al., 1995). Visual feeders locate and attack single zooplankton prey, whereas filter feeders do not visually
detect individual prey, but strain prey and particles from
engulfed water using gill rakers or other entrapment
structures (Lazzaro, 1987). Visual feeders directly suppress populations of large zooplankton, and may
indirectly enhance populations of phytoplankton and
small or highly evasive zooplankton (Gliwicz and
Pijanowska, 1989). In contrast, filter feeders directly suppress populations of less mobile zooplankton and large
phytoplankton, and indirectly enhance copepods and
small algae (Drenner et al., 1986, 1987). Therefore, predation on plankton by filter-feeding fish does not imply
doi:10.1093/plankt/fbn051, available online at www.plankt.oxfordjournals.org
# The Author 2008. Published by Oxford University Press. All rights reserved. For permissions, please email: [email protected]
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a nonselective feeding as larger particles are more likely
to be retained in the gill structures and less mobile prey
are more likely to be captured by the fish.
Experiments designed to compare the effects of these
two planktivore types on plankton communities and the
magnitude of their cascading trophic interactions are
only few (Lazzaro et al., 1992; Boveri and Quirós, 2007)
and yield contradictory results. Lazzaro et al. (Lazzaro
et al., 1992) investigated the impacts of two planktivorous fish with different feeding modes and concluded
that fish biomass was more important in regulating
plankton structure than planktivore type (i.e. fish
species). On the other hand, Boveri and Quirós (Boveri
and Quirós, 2007) manipulated the frequency of two
fish species belonging to different planktivore types and
found that the impacts of planktivorous fish on plankton
communities depend on planktivore type (Boveri and
Quirós, 2007). However, experimental manipulation of
different species of zooplanktivorous fish (same planktivore type) also yields contrasting effects on plankton
communities (Williams and Moss, 2003). Therefore,
there are confounding effects of using different fish
species with different planktivore types to investigate the
role of planktivore type on the strength of trophic cascades in pelagic communities. Here, this problem will
be overcome by using one fish species and manipulating
only its feeding mode.
To quantify how planktivore type affects plankton
dynamics and water transparency, we performed a pond
experiment manipulating the biomass and size/stage of
the Nile tilapia, Oreochromis niloticus. The Nile tilapia is a
planktivorous fish native to Africa but has been a significant component of inland fisheries and aquaculture in
tropical Asia and America for almost half a century
(Fernando, 1991). These fish have been introduced into
over 90 countries worldwide, with a global distribution
second only to common carp (De Silva et al., 2004). Both
visual and pump filter feeding are utilized by tilapias in
the ingestion of larger, crustacean zooplankton, while
pump filter feeding alone is employed in the ingestion of
phytoplankton and small zooplankton (Beveridge and
Baird, 2000). However, the profitability of visual, particulate feeding on zooplankton by adults should be highly
dependent upon prey size and prey density and under
most circumstances it might be more profitable for adult
O. niloticus to feed on phytoplankton (Yowell and Vinyard,
1993). Indeed, laboratory studies indicate that a distinct
ontogenetic shift from visual particle capture to pump
filter feeding occurs when tilapias are 6–7 cm of standard length (SL), which is consistent with field studies of
dietary preferences in a range of tilapias (Beveridge and
Baird, 2000). Therefore, because tilapias show such dramatic ontogenetic niche shifts, we expect differences in
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the biomass-related effects of juveniles (,5 cm SL) and
adults on plankton communities and water transparency.
To study how the competitive interactions within and
between juveniles and adults of Nile tilapia affect the
strength of their cascading trophic effects on plankton
communities, our experiment was designed to manipulate both the stocking biomass and proportion of the
two stages. Even though experiments with visually and
filter-feeding planktivorous fish are not new, our experimental design was unique and allowed us to investigate
simultaneously the effects of fish omnivory and
intra-specific competition on the strength of trophic cascades in plankton communities. Most studies addressing
the impacts of omnivory by planktivorous fish use an
absence of fish as the experimental control for omnivorous fish and, therefore, do not distinguish the effects of
fish omnivory from those of fish presence. We used a
zooplanktivorous fish ( juvenile tilapia) as a control treatment for an omnivorous fish (adult tilapia), and with
this design we were able to address the effects of fish
omnivory on plankton without the confounding effects
of fish presence (Fig. 1).
METHODS
The field experiment was conducted at the DNOCS
fish hatchery station at Itans Reservoir, which is located
in Caicó city, Rio Grande do Norte State, Brazil.
Fifteen ponds were used and were 40 m2 in area with
1 m depth (40 m3 in volume). Water containing a
natural plankton assemblage was pumped from Itans
Reservoir into the ponds. Three replicate ponds were
randomly assigned to each of five treatments: treatments
with 20 g/m3 (J) and 40 g/m3 (JJ) of juveniles, treatments with 20 g/m3 (A) and 40 g/m3 (AA) of adults
Fig. 1. Trophic interactions between phytoplankton (P), zooplankton
(Z), juvenile (J) and adult (A) tilapias in our experimental treatments
with (a) only juveniles (J and JJ), (b) only adults (A and AA) and (c)
with both juveniles and adults (JA).
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and a treatment (JA) with 20 g/m3 of juveniles plus
20 g/m3 of adults. We selected the stocking biomass of
20 and 40 g/m2 (200 and 400 kg/ha) to ensure that
tilapia were stocked across biomass gradients found in
natural lakes and reservoirs. Tilapia were stocked at
total length of 30– 50 mm ( juveniles) and 120– 160 mm
(adults). The adult fish were manually sorted and only
adult males were stocked to avoid spawning during the
experiment. Fish stocked in each pond were counted
and weighed at the beginning and at the end of the
experiment, when ponds were drained and all fish collected. Fish received no food during the experiment
except for plankton naturally available in the pond
water.
The experiment ran from 26 February 2002 to 29
April 2002. Ponds were filled on 26 February and the
first sampling occurred on 27 February. Fish were
added on the same date soon after the first sampling,
and subsequent samples were taken every 2 weeks (Days
16, 32, 47 and 62) for zooplankton and chlorophyll a
analyses. Secchi depth was measured in all ponds
before each sampling. Itans reservoir was also sampled
simultaneously for chlorophyll a analysis and Secchi
depth measures to describe the trophic state of the
reservoir during the experiment (mean + SD of Secchi
depth and total chlorophyll a concentration were
0.80 + 0.35 m and 14.68 + 12.90 mg/L, respectively).
Water was sampled with a PVC tube (10 L) from the
four corners of each pond and pooled into a 40 L
bucket. From each pooled sample, a 500 mL subsample
was taken for chlorophyll analysis. The remaining
volume of the pooled sample (39.5 L) was filtered
through a plankton net with 50 mm of mesh size for
zooplankton analysis.
Zooplankton samples were preserved with Lugol’s
solution, and organisms were counted and sorted into
categories of rotifers, cladocerans, calanoid and cyclopoid copepods and copepod nauplii. The zooplankton
were counted in five subsamples from each sample
under a stereomicroscope. The average of the subsamples was taken for each group of organisms
counted, multiplied by the sample volume (mL) and
divided by the subsample volume (mL) to estimate the
total number of individuals in the sample. Afterwards,
the number of individuals in the sample was divided by
the water volume sampled in the field (39.5 L) to calculate the original density (ind. L21) of organisms in the
sample.
Chlorophyll a was determined with a Turner TD-700
fluorometer after filtration on a GF/C filter and extraction with ethanol at 288C for 18 h (Jespersen and
Christoffersen, 1988). Prior to filtration, each chlorophyll sample was equally divided and one half was
directly filtered on a GF/C filter for determination of
total chlorophyll a, while the other half was first filtered
through a plankton net with 20 mm mesh size for the
determination of chlorophyll a in the nanoplankton size
fraction. Chlorophyll a concentration was considered a
reasonable approximation of phytoplankton biomass.
Despite our effort for selecting only adult males to
avoid spawning, fish larvae were found in some ponds
stocked with adults at the end of the experiment. From
their small size (,5 mm), we could assume that spawning occurred during the last week of the experiment.
Therefore, we excluded the data of the last sampling
date from the statistical analysis since the presence of
these larvae in the adult treatments would have confounded the analysis. Data from the first sampling date
were analyzed separately by a one-way ANOVA to test
for differences in the initial pre-treatment conditions.
Time-related differences among treatments in the
response variables from Day 16 to 47 were tested using
repeated-measures ANOVA. Tukey tests were used a
posteriori to test whether treatment levels were significantly distinct. Data were log10 transformed to stabilize
the variances. Normal probability plots were used to
check for the normality of the residuals.
R E S U LT S
The number of fish in the ponds decreased while fish
biomass increased in all treatments, indicating that individual fish gained weight on average during the experiment (Table I). The rate of change or growth rate in
individual fish weight (unitless) clearly demonstrates that
resource limitation occurred. Growth rates of juvenile
and adult tilapia were inversely related to density
stocked, indicating strong density-dependent growth.
Table I shows that doubling the density of juveniles
from 20 g/m3 (J) to 40 g/m3 (JJ) resulted in a 39%
reduction in their mean growth rate (from 2.98 in J to
1.83 in JJ). Likewise, increasing the density of adults
from 20 g/m3 (A) to 40 g/m3 (AA) resulted in a 34%
reduction in their mean growth rate (from 1.98 in A to
1.30 in AA). However, the growth of juvenile tilapias
was more affected by competition with juveniles (R ¼
1.83 + 0.52 in JJ) than with adults (R ¼ 2.52 + 0.74 in
JA). In contrast, the growth of adults was more affected
by competition with juveniles (R ¼ 0.98 + 0.09 in JA)
than with adults (R ¼ 1.30 + 0.05 in AA). This suggests
that competition among size classes of tilapias is highly
asymmetric and that juveniles were better competitors
than adult tilapias in our pond experiment.
The major source of fish mortality in the ponds was
predation by birds. At the beginning of the experiment,
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Table I: Mean and standard deviation of abundance N (number of individuals), biomass g (in g) and
mean individual body weight (in g/N) of tilapia for each treatment level at the beginning and at the end
of the experiment
Initial (day 1)
Treatment
N0
J
J (JA)
JJ
A
A (JA)
AA
524
537
1072
14
12
24
Final (day 62)
g0
(13)
(3)
(2)
(2)
(2)
(3)
800
800
1600
800
800
1600
(0)
(0)
(0)
(0)
(0)
(0)
g0/N
N0
N
g
1.52 (0.04)
1.49 (0.01)
1.49 (0.00)
59.0 (6.87)
65.51 (7.86)
67.69 (10.66)
383
427
814
12
11
21
(44)
(53)
(96)
(1)
(3)
(2)
1717
1583
2183
1400
697
1860
g/N
N
(115)
(351)
(431)
(250)
(167)
(215)
4.53
3.76
2.73
116.67
64.49
87.86
(0.72)
(1.08)
(0.77)
(18.05)
(12.96)
(14.24)
R (g
g/N
N)/
N0)
(g
g0/N
Survival
%
2.98
2.52
1.83
1.98
0.98
1.30
73.09
79.52
75.93
85.71
91.67
87.50
(0.54)
(0.74)
(0.52)
(0.23)
(0.09)
(0.05)
Fish growth rate R (no unit), as change in mean individual body weight during the experiment, was assessed by the end/beginning g/n ratio. Values
are mean (+SD).
some fish also died due to handling stress and were
removed from the ponds without replacement. Besides
bird predation and mortality due to handling stress, survival remained high, higher for adults than for juveniles
(Table I), and bird predation was randomly allocated to
treatment levels.
The one-way ANOVA results show that there were no
significant differences in the response variables between
the treatments at the beginning of the experiment,
except for total phytoplankton biomass (F ¼ 6.128; d.f.
4,10; P ¼ 0.009). The Tukey post hoc test showed that
treatment J had higher total chlorophyll a concentrations than treatment JJ at the start of the experiment.
Since treatments were randomly allocated to the 15
ponds, we attribute these initial differences to random
effects.
The dynamics of each response variable shows that
total zooplankton, cyclopoid copepods, nano- and total
phytoplankton increased, while the Secchi depth
decreased in all treatments from the start to the end of
the experiment (Fig. 2). Rotifers increased in treatments
with juveniles while cladocerans increased in treatments
with adults over the course of the experiment, while the
remaining was more or less constant in the other treatments (Fig. 2). The repeated measures ANOVA results
show that there were highly significant time effects (P ,
0.001) on all variables excepting total zooplankton, rotifers and copepod nauplii. However, significant treatment effects were found only on the abundances of
rotifers and cladocerans and these effects did not interact with time (Table II). The results of the Tukey post hoc
tests showed that treatments with juvenile tilapia had
significantly (a ¼ 0.05) higher densities of rotifers and
lower densities of cladocerans than treatments A and
AA, with only adult tilapias (Fig. 2). However, there
were no significant differences between the treatments
with different stocking biomass of juveniles or between
the treatments with different stocking biomass of adults.
DISCUSSION
Because small tilapia are visual-oriented predators
feeding on larger zooplankton (Elhigzi et al, 1995), but
then exhibit a dramatic shift to filter-feeding on both
zooplankton and phytoplankton when they are about
60– 70 mm in SL (Beveridge and Baird, 2000); we
hypothesized that the impacts of juvenile and adult tilapias on plankton communities would be different.
Nevertheless, we found no significant differences in the
effects of juveniles and adults during our experiment.
The only exception was that treatments with juveniles
had significantly higher densities of rotifers and lower
densities of cladocerans than treatments with only
adults. This could actually be predicted from the two
feeding modes of tilapias. Visual predation by juvenile
tilapia should have stronger effects on larger planktonic
prey such as cladocerans, whereas suction-filter feeding
by adult tilapia should have stronger effects on less
mobile planktonic prey-like rotifers, ciliates or algae
(Drenner et al., 1982, 1984). On the other hand, copepods were largely unaffected by the feeding mode of
tilapias. This is probably due to their higher swimming
mobility and ability to escape from both types of planktivory (Drenner et al., 1982, 1984). Our results then indicate that the structure of zooplankton communities is
affected by the feeding mode of tilapias but this does
not seem to affect total phytoplankton biomass and
water transparency. This is surprising since cladocerans
are known to have larger grazing impacts on phytoplankton and recycle nutrients at different rates and
ratios compared with rotifers (Sterner, 1989).
Significant cascading effects of omnivorous fish on
phytoplankton have been found in most experimental
studies manipulating the abundance of filter-feeding fish
(Drenner et al., 1996). These studies found that the presence of omnivorous fish often results in an enhancement of total phytoplankton biomass or primary
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Fig. 2. Mean values (þ1 SD) of the response variables during the experiment in the treatments with 20 g/m3 (J) and 40 g/m3 (JJ) of juveniles,
with 20 g/m3 (A) and 40 g/m3 (AA) of adults and with 20 g/m3 of juveniles plus 20 g/m3 of adults (JA). Data from the last sampling date (Day
62) were removed from the statistical analyses due to spawning in the adult treatments in the last week of the experiment.
productivity. However, our study suggests that these
effects might be attributed to the presence of planktivorous fish and not to their feeding mode, since we did not
observe any difference in phytoplankton biomass
between treatments with juvenile (zooplanktivorous) and
adult (omnivorous) tilapia. To our knowledge, only two
previous studies have compared the strength of trophic
cascades mediated by the two types of planktivores
(Lazzaro et al., 1992; Boveri and Quirós, 2007). Lazzaro
et al. (Lazzaro et al., 1992) also found no differences in
the strength of the trophic cascade effects mediated by a
visual feeding fish and a filter-feeding fish. On the other
hand, Boveri and Quirós (Boveri and Quirós, 2007)
found that visually feeding planktivores produced stronger cascading trophic effects on phytoplankton than
filter-feeding planktivores.
The effects of fish omnivory on plankton communities are expected to be variable and should depend on
the relative strength of the direct and indirect effects of
the omnivorous predators (Diehl, 1993). Omnivorous
filter-feeding fish can dampen trophic cascades as they
can also feed on phytoplankton and, therefore, prevent
an increase in algal biomass after suppression of
zooplankton herbivores (Diana et al., 1991).
Omnivorous filter-feeding fish can affect phytoplankton
directly, by selectively consuming larger phytoplankton
species, but also indirectly, by suppressing herbivorous
zooplankton, resuspending settled phytoplankton or
excreting nutrients in dissolved form into the water
column (Stein et al., 1995; Drenner et al., 1996; Vanni,
2002). Therefore, while the abundance of large phytoplankton should be inhibited by tilapia grazing, the
abundance of small phytoplankton should be enhanced
by tilapia due to: (i) suppression of their competitors
(large phytoplankton); (ii) reduction of their predators
(herbivorous zooplankton); and (iii) nutrient recycling
and/or translocation from benthic habitats to the water
column (Drenner et al., 1996; Attayde and Hansson,
2001; Vanni, 2002; Figueredo and Giani, 2005; Okun
et al., 2008; Rondel et al., 2008).
Nevertheless, we found no differences in the effects of
juvenile and adult tilapia on the biomass of small algae.
This was probably due to the size structure of the phytoplankton community in our experimental ponds.
Particle ingestion by filter-feeding tilapia is a function of
particle size and greatly decreases when the particle
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Table II: Results of the repeated-measures ANOVA testing for time-related differences in the response
variables among the treatments over the three sampling dates (Days 16, 32 and 47)
Variable
21
Total zooplankton (ind. L
)
Rotifers (ind. L21)
Cladocerans (ind. L21)
Nauplii (ind. L21)
Calanoids (ind. L21)
Cyclopoids (ind. L21)
Total chlorophyll a (mg L21)
Nano chlorophyll a (mg L21)
Secchi depth (m)
Source
SS effect
MS effect
F
P-level
Treatment
Time
Treat time
Treatment
Time
Treat time
Treatment
Time
Treat time
Treatment
Time
Treat time
Treatment
Time
Treat time
Treatment
Time
Treat time
Treatment
Time
Treat time
Treatment
Time
Treat time
Treatment
Time
Treat time
0.077
0.143
0.320
7.495
0.389
0.964
8.880
0.603
0.715
0.577
,0.001
0.602
2.985
4.184
1.659
1.176
5.762
0.706
0.170
1.394
0.069
0.091
0.949
0.108
0.088
0.104
0.075
0.019
0.072
0.040
1.874
0.194
0.120
2.220
0.302
0.089
0.144
,0.001
0.075
0.746
2.092
0.207
0.294
2.881
0.088
0.042
0.697
0.009
0.023
0.474
0.013
0.022
0.052
0.009
0.271
1.406
0.786
10.504
2.505
1.553
15.048
3.703
1.098
1.323
0.001
0.556
1.415
10.321
1.023
0.997
13.023
0.399
1.726
49.001
0.611
1.006
43.144
1.224
0.703
10.254
1.846
0.890
0.268
0.621
,0.001
0.107
0.201
,0.001
0.043
0.404
0.327
0.999
0.801
0.298
,0.001
0.451
0.453
,0.001
0.908
0.220
,0.001
0.758
0.449
,0.001
0.335
0.608
,0.001
0.127
Degrees of freedom from each source of variation are: 4 (treatment), 2 (time) and 8 (treatment time).
diameter is ,20– 30 mm (Drenner et al., 1984, 1987a).
A large proportion (.80%) of the total phytoplankton
biomass in our experimental ponds was composed of
small algae that could pass through a 20 mm mesh and
would not be efficiently removed by tilapia filter
feeding. Therefore, adults probably consumed mainly
zooplankton during our experiment, indirectly affecting
phytoplankton in a way similar to juvenile zooplanktivorous tilapia. This suggests that the strength of trophic
cascades mediated by filter-feeding omnivorous fish
depends on the phytoplankton size structure and might
differ from that mediated by zooplanktivorous fish only
when large algae account for a greater proportion of
the total phytoplankton biomass. In eutrophic systems,
where phytoplankton is often dominated by large colonial and filamentous algae, grazing by tilapia should
become more important, allowing tilapia to enhance
indirectly the biomass of small algae and sometimes the
biomass of total phytoplankton (Drenner et al., 1996,
1998). However, a recent experimental study aimed at
comparing the impacts of visually feeding zooplanktivorous fry and filter-feeding omnivorous juveniles of
Nile tilapia found no cascading trophic effects of tilapia
on the total biomass of phytoplankton after a bloom of
large filamentous cyanobacteria (Rondel et al., 2008).
The lack of differential impacts between juvenile and
adult tilapias on phytoplankton found in our study may
also arise from juveniles being more omnivorous than
truly zooplanktivorous (Rondel et al., 2008). More
feeding selectivity experiments with different life-stages
of tilapias are clearly needed to elucidate how the
feeding behaviour and prey selectivity of tilapias change
during ontogenetic development, and how the relative
abundance and size structure of both zooplankton and
phytoplankton affect ontogenetic niche shift. A basic
understanding of the foraging behaviour of tilapias is
crucial to make appropriate predictions and explain the
results of the experimental studies on the impacts of this
planktivorous fish in tropical aquatic ecosystems.
The effects of planktivorous fish on plankton communities should depend on the fish biomass (Lazzaro et al.,
1992; Drenner et al., 1996; Starling et al., 1998), but we
found no effects of a 2-fold increase in tilapia biomass in
our experiment. The functional relationships between
planktivore biomass and plankton biomass appear to be
nonlinear, levelling off when fish stocking biomass exceeds
a certain threshold (Drenner et al., 1987; Threlkeld, 1988;
Lazzaro et al., 1992). Indeed, most effects of planktivorous
fish are found when treatments without fish and with low
abundance of fish are compared (Lazzaro et al., 1992,
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Okun et al., 2008). Competitive interactions among fish
can constrain their potential to further depress zooplankton abundance and increase algal biomass as fish stocking
biomass increases. Since fish growth decreased in our
experiment with increasing stocking biomass and prey
consumption is a function of fish growth, the overall prey
consumption in the fish ponds was probably not affected
by the 2-fold increase in fish biomass due to intraspecific
competition among fish.
One current challenge in freshwater ecology is to
understand why the response of phytoplankton to planktivorous fish is so variable (Brett and Goldman, 1996).
Omnivory by planktivorous fish and the size-structure
and species composition of plankton communities can be
important factors determining the strength of pelagic
trophic cascades. Our results support the hypothesis that
in tropical lakes and reservoirs with predominantly small
herbivorous zooplankton and omnivorous filter-feeding
fish, neither the type nor the biomass of planktivorous
fish present seems likely to produce strong cascading
effects on the total biomass of phytoplankton.
AC K N OW L E D G E M E N T S
We thank Roberto Menescal and the staff from the
DNOCS fish hatchery station at Itans reservoir (Caicó,
RN) for their field assistance during the experiment. We
also thank Lars-Anders Hansson, Erik Jeppesen and
Michael Vanni for valuable comments on a previous
version of the manuscript. Our acknowledgments to
two anonymous reviewers for valuable comments on a
previous draft of the manuscript.
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