Tropical Diversity (2019) 1(2): 32-47.
ISSN: 2596-2388
DOI: 10.5281/zenodo.11094933
RESEARCH ARTICLE
© 2019 The Authors
32
Does predation by planktonic organisms influence the size structure of
phytoplanktonic algae in a black water lake in the Amazon?
Predação por organismos planctônicos influencia a estrutura de tamanho das algas
fitoplanctônicas em um lago de água preta na Amazônia?
Raize Castro-Mendes1* https://orcid.org/0000-0003-1215-7702, Edinaldo Nelson dos Santos-Silva1
https://orcid.org/0000-0002-3340-4541, Bruno Machado Leão1 https://orcid.org/0009-0001-8162-9323, Renan
Gomes do Nascimento1 https://orcid.org/0000-0002-0818-0347, Maiby Glorize da Silva Bandeira1
https://orcid.org/0000-0002-0534-2611, Luis José de Oliveira Geraldes-Primeiro1 https://orcid.org/0000-0002-
3892-8969
1Laboratório de Plâncton – Instituto Nacional de Pesquisas da Amazônia – Manaus, Amazonas Av. André Araújo,
2936, Aleixo, Manaus, Amazonas – CEP 69060-001.
*Email: raize.mendes@gmail.com
Received: 25, February 2019: / Accepted: 31, July 2019 / Published: 5, August 2019
Resumo Organismos do fitoplâncton podem pertencer
às categorias de tamanho pico, nano e microplâncton e
organismos do zooplâncton ao micro, meso e
macroplâncton. Por terem tamanhos diferentes, os
organismos zooplanctônicos podem se alimentar de
diferentes tamanhos do fitoplâncton. O objetivo foi
avaliar se microcrustáceos e rotíferos planctônicos
consomem o pico, nano e microfitoplâncton de forma
homogênea nos períodos de seca enchente do lago
Tupé. Um experimento foi colocado durante 24 horas
no período de seca e enchente no lago Tupé. Amostras
de zooplâncton e fitoplâncton foram coletadas com um
tubo de PVC de 4 m de comprimento e os organismos
zooplanctônicos foram contados e medidos. A amostra
de fitoplâncton foi fracionada em pico, nano e
microfitoplâncton para ser medida a biomassa de cada
fração. No período de seca, a biomassa inicial total foi
1,92µg/L, especificamente pico 0,82, nano 0,55 e
micro 0,55µg/L, sendo o valor da biomassa final
1,09µg/L correspondente ao pico 0,55, nano 0,27 e
micro 0,27µg/L. No período de enchente, a biomassa
inicial foi 2,91µg/L, especificamente pico igual a zero,
nano 0,54 µg/L e micro 2,37 µg/L, sendo o valor da
biomassa final 0,81 µg/L correspondente apenas ao
picoplâncton. A maior densidade de organismos foi
encontrada no experimento do período de seca.
Concluímos que a pressão de predação do zooplâncton
não influencia a estrutura de tamanho do fitoplâncton
no ambiente estudado, uma vez que atua de forma
similar sobre as diferentes classes.
Palavras-Chave: Fitoplâncton, nanoplâncton,
microplâncton, mesocosmo, biomassa.
Abstract Phytoplanktonic organisms may be
categorized as pico, nano and microplankton, and
zooplanktonic organisms as micro, meso and
macroplankton. Because they are different sizes,
zooplanktonic organisms can feed on varying sizes of
phytoplankton. The study objective was to test whether
microcrustaceans and planktonic rotifers consumed
pico-, nano- and microphyoplankton non-selectively
during low- and high-water periods in Lake Tupé,
Amazonian Brazil. An experiment was carried out
across 24 hours in the low- and high-water periods,
with zooplankton and phytoplankton samples collected
from the lake with a PVC tube 4 m in length.
Zooplankton were counted and measured, while the
phytoplankton sample was divided into pico-, nano-
and microphytoplankton and the biomass of each
fraction measured. During low water, total initial
biomass was 1.92 μg/L and, by fraction, contained pico
0.82, nano 0.55 and microphytoplankton 0.55 μg/L.
Total biomass was1.09 μg/L, corresponding to pico-
0.55, nano- 0.27 and microphytoplankton 0.27 μg/L.
During high water, total initial biomass was 2,91µg/L
and by fraction, contained pico- equal to zero, nano-
0.54µg/L and micro- 2.37 µg/L. Total biomass was
0.81µg/L corresponding only to picophytoplankton.
The highest density of organisms occurred in the low-
water sample. We conclude that predation pressure
from zooplankton does not influence phytoplankton
size structure in the studied environment, since it
impacts the different size classes equally.
Keywords: Picoplankton; Nanoplankton;
Microplankton; Mesocosm; Biomass.
Castro-Mendes et al. (2019)
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Introduction
In continental aquatic environments
cladocerans, copepods and rotifers are the major
abundant zooplanktonic organisms. These
organisms are endowed with morphological and
chemical devices to propitiate their success in
these environments (Tian et al. 2019). For
feeding, the cladocerans have as devices, the
filtering bristles and, it has the capacity to select
their prey by size (Lampert 1994; Dumont &
Negrea 2002; Holynska et al. 2003). The
copepods present buccal appendages such as jaws,
maxilla, maxilula and maxillipeds, which gives
them the ability to capture individual food
particles and to select the appropriate food
(Dussart & Defaye 1995; Dussart & Defaye 2001;
Dumont & Negrea 2002; Holynska et al. 2003).
The rotifers have a filtering mouthpiece called
corona ciliata and, it has a specialized muscular
pharynx called mastax. In the presence of food,
rotifers perform movements with the cilia of the
corona ciliata creating a flow, and then the food is
sent into the organism body. In the mastax a
chitinous jaws (trophi) process, the food particles
ingested (Nogrady et al. 1993).
These organisms present different shapes
and sizes in aquatic environments. In freshwater
environments, rotifers are approximately 100 to
500 μm in size and microcrustaceans (cladocerans
and copepods) between 200 to 3000 μm (Dumont
& Negrea 2002).So, differences in body length
can have great effects on the filtration rate and
food size.
Phytoplankton also presents different
sizes and, it’s the main zooplankton’s feeding
item (Round 1983; Raven et al. 1996; Lourenço
2006; Frau et al. 2019). Sieburth et al. (1978)
classified by size the planktonic organisms
(including the phytoplankton and the
zooplankton). Algae were included in the
categories of picoplankton (0.02 to 2 μm),
nanoplankton (2.1 to 20 μm) and microplankton
(20.1 to 200 μm). Thezooplankton was included
in the categories of microplankton (20.1 to 200
μm), mesoplankton (200.1 to 2000 μm) and
macroplankton (> 2000 μm).
In this context, algae with smaller sizes
may be more consumed, since large and small
sizes of zooplanktonic organisms can consume
them. As an example, in the study by Filetto et al.
(2004) it was observed that nanoplanktonic algae
are the most suitable for feeding cladocerans,
from newborn to breeding stage, and that the limit
of particle sizes ingested by these herbivores
depends on body size and filtering bristles.
Studies on the phytoplankton-zooplankton
relationship have been carried out in laboratory
experiments (Lampert 1994; Diaz-Castro & Hardy
1998; Hardy & Castro 2000; Pagano 2008; Chen
et al. 2015) and in natural environments (Frau et
al., 2019). However, for the natural environments
of the Amazon region, there are few that
approached the size structure of the organisms
and, neither, those that approached the predation
of the zooplankton on the biomass of the different
phytoplankton size classes. According to Rai
(1982) and Romero & Arenas (1990) studying the
populations of the phytoplanktonic community
starting from their size allows a deeper
understanding about the participation and
efficiency of these fractions in total community
biomass and environments dynamics.
Specifically, in the phytoplankton-
Castro-Mendes et al. (2019)
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34
zooplankton relationship, studies of this nature
provide elements to understand the complexities
of trophic chains for the various types of aquatic
environments of the Amazon. Especially for black
water environments, such as Tupé Lake, studies
on the taxonomy, composition and abundance of
these organisms are great (Melo et al. 2005a;
2005b; Previattelli et al. 2005; Brandorff & Hardy
2009; Ghidini & Santos-Silva 2009; Pereira 2009;
Calixto et al. 2011; Leão 2012; Souza 2012),
however almost nothing or none about the
influence of zooplankton on the specific
categories of algal size.
Thus, the objective of this study is to
understand the relationship and influence of
zooplankton on phytoplankton size fractions in
different periods of a blackwater Amazon lake,
Tupé Lake. The hypothesis tested was that neither
predation by zooplankton nor the river regime
phases studied affect the size structure of the
phytoplankton community populations.
Materials and Methods
Study Area
Lake Tupé (3° 2'36 "S and 60° 15'18" W)
is located in the Tupé Sustainable Development
Reserve (Tupé RDS), left bank of the Rio Negro,
25 km from the port of Manaus, Amazonas State,
Brazil (Figure 1). It is a black water lake, into
which eight streams flow and it is connected to
the Rio Negro by a channel that, during the dry
season, is some 20 m wide, 0.5 m deep and 150 m
long. When the level of the Rio Negro, in the port
of Manaus, is below 19 m a.s.l. (above sea level),
the river has no influence on the lake, and, for the
lake, this period is considered low-water. When
the level of the Rio Negro at the port of Manaus is
exceeds 19 m m.s.l., then river waters have an
influence on those of the lake, flowing in and
causing the water level of the lake to rise,
flooding its banks. This is considered to be the
high-water period. Maximum depths of the lake
vary between 4.5 m in the low-water season to 15
m in the high-water season. During high-water
temperature in Lake Tupé varies between 27.8° C
and 30.9° C, oxygen saturation 0.4 and 88.5% (4.6
mg.L-1) and pH between 3.05 and 4.67. During
low-water the temperature varies between 24.8
and 32.0° C, oxygen saturation 0.8 and 135, 6%
and pH 3.89 to 5.95 (Darwich et al. 2005).
Figure 1 - Map of Tupé RDS, showing Lake Tupé and collection points within it.
Castro-Mendes et al. (2019)
Predation by planktonic orhganisms
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35
In situ experiments
Phytoplankton
In order to evaluate the consumption of
phytoplankton by planktonic microcrustaceans
and rotifers, an in situ experiment was carried out
in the low-water (December 2015), and high-
water seasons (May 2016). The criterion for
choosing a mesocosm for each period studied was
due to already established information about the
zooplankton in Tupé Lake, which consists of the
spatial distribution of the wealth of cladocerans,
copepods and rotifers to be homogeneous in the
lake (Calixto et al. 2011).
The experiment was performed once in
each period, for 24 hours at a single point on the
lake.
The mesocosm consisted of a sturdy 60 L
plastic bag, secured between two hoops of
floating material. This assemblage was placed
between three wooden poles from which it was
suspended by ropes (Figure 2). The plastic bag
contained 60 L of lake water, and so contained the
planktonic organisms present at that moment in
the lake. Collection occurred using the same
procedure used to collect phytoplankton and
zooplankton (see below). Simultaneously, a
sample of zooplankton and phytoplankton were
collected from the lake, and this was considered
representative of the organisms present in the
environment at the beginning of the experiment.
After 24 hours, the volume of water in the
mesocosm was measured and 1 L was withdrawn
to measure the final biomass of the size fractions
used in this study. The remainder was filtered
through a 55 μm plankton sieve. Samples were
then prepared using canonical methods and
transported to the National Institute of Amazonian
Research (INPA) Plankton Laboratory for
processing.
When collecting water for biomass
analysis of pico-, nano-, and
microphytoplanktonic and to obtain zooplanktonic
organisms for use in the experiment (initial time
T1), the limit of the euphotic zone was estimated
via water transparency using a Secchi disk. The
value obtained was multiplied by three to give a
final value that was then considered as the limit of
the Euphotic Zone, that is, the depth at which the
value of photosynthetically-active light in the
water column is 1% of the light incident on the
surface (Esteves, 1998). After euphotic zone
depth estimation, phytoplankton was collected
with a PVC tube 4.5 m in length and 5 cm in
diameter with a water-retaining valve coupled at
its far end. The tube was inserted vertically into
the water column, to the limit of the euphotic
zone. Once the desired depth was reached,
movement was paused and the valve activated,
thus collecting an integrated sample of the entire
euphotic zone. After this, the tube was pulled
back into the boat and its contents dumped into a
bucket. The sample volume of the drought period
was 47.1 m3 and the sample volume of the flood
period was 58.8 m³.
The collected water was homogenized and
2L (initial) sample withdrawn. After 24 hours, the
water from the experiment was again
homogenized and the final 2L sample was
withdrawn. Sample vials were wrapped in foil and
placed in black plastic bags to avoid any influence
of light. These were then labelled and packed in
an ice-filled expanded polystyrene chest, then
Castro-Mendes et al. (2019)
Predation by planktonic orhganisms
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36
transported to the laboratory where they were
refrigerated until analysed. At the INPA Water
Chemistry laboratory, 1L of each sample was
sequentially filtered through a graded filter battery
with porosities of 20μm, 2μm and 0.2μm. The
first was a sieve made of 20μm mesh. The other
two filters were fiberglass. With this procedure
three phytoplankton fractions (pico, nano and
micro) were obtained for chlorophyll-a extraction,
which was used to estimate biomass. Chlorophyll-
a concentration was used as an estimate of the
biomass of each size class, and was extracted and
measured using the spectrometric method
proposed by Lorenzen (1967), at wavelengths 663
nm and 750 nm. Calculation of chlorophyll-a
values followed Golterman et al. (1978).
Figure 2 - A) Mesocosm in the low-water period, and B) Mesocosm in the high-water period. Photos:
Castro-Mendes (2016); C) Diagram showing the mesocosm structure. Source: (Modified) Couto (2009).
Zooplankton
Zooplanktonic organisms were also
sampled with the PVC tube and placed in the
mesocosm previously mentioned above. At the
same time, an initial sample of zooplanktonic
organisms was collected with a 55 μm mesh net.
The net was drawn vertically through the water
column, and the sample filtered and packed in 100
ml vials and fixed with 6% buffered formalin.
After 24 hours, the mesocosm was broken down
and the zooplankton present were filtered out with
a 55 μm mesh net to obtain final zooplankton
sample. These samples were packed and
transported to the INPA Plankton Laboratory.
In counting zooplankton each sample was
fractionated in the Laboratory using a Folsom-
type sample fractionator, until 1/8 of the original
sample was obtained. This one-eighth fraction of
Castro-Mendes et al. (2019)
Predation by planktonic orhganisms
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37
the sample was concentrated and placed in Petri
dish with a millimeter-gridded underside to
facilitate counting. The taxonomic level used for
the zooplankton identification was at the order
level for microcrustaceans and phylum for
rotifers.
Rotifers, cladocerans and copepods in
each sample were then counted using a
stereoscopic microscope. From each sample, 15
subjects were randomly selected for measurement.
For this, a composite microscope equipped with a
millimeter eyepiece was used. The measured
individuals were classified by size according to
the classification of Sieburth et al., (1978).
Density was expressed as organisms/m3, with the
formula proposed by Tonolli (1971) used for its
calculation. To determine the filtered volume in
the trawl: Vf = π.r2.h. Where: r = radius of the
PVC tube (r = 2.5 cm), h = water column height.
The volume of water filtered was used to calculate
the density of individuals per m3, via the formula:
Nº of indivíduals = n/Vf. Where: n = number of
individuals counted, Vf = filtered volume.
Data analysis
A G test was conducted to test whether
significant differences existed between the sizes
of collected zooplanktonic organisms, this being
an alternative to χ2 (Chi-squared). The G-test was
calculated based on the observed and expected
values, assuming a significance level of G>3.84
and P<0.05. Analysis was done using the BioEstat
statistical program (version 5.3).
Results
Size of zooplanktonic organisms
None of the sampled cladocerans,
copepods and rotifers classified into
microplankton and mesoplankton size categories
was larger than 2 mm. In the low-water season,
the smallest was a 33.2 μm rotifer, and the largest
size was 1079 μm adult Calanoid copepod. Most
zooplanktonic organisms occupied the
mesoplankton size-class (Table 1). There was a
statistically significant differences between the
microplankton and mesoplankton (G = 39.8613, P
<0.0001) for both the initial and final samples of
the experiment (G = 41.7093, P <0.0001). There
was no statistically significant difference between
the initial and final microplankton (G = 0.1023
and P> 0.05) samples, nor was there any
statistically significant difference between the
initial and the final mesoplankton samples (G =
0.2203 and P> 0.05).
Zooplankton (µm)
Initial
Last
Min.
Max.
Mean ± SD
Min.
Max.
Mean ± SD
Cladocera
91.3
499
301.6±138.7
141.1
464.8
264.4±84.9
Nauplius
107
298.8
184.2±67.8
83
332
207.5±81
Young-Copepods
232.4
747
417±127.4
323.7
572.7
441±63.8
Adults-Copepods
290.5
1079
691.6±301.1
415
921.3
488.5±122.1
Rotifera
33.2
307.1
146.6±86.3
33.2
415
135.5±104.5
Table 1 – Size of zooplanktonic organisms in the low-water season at Lake Tupé.
Castro-Mendes et al. (2019)
Predation by planktonic orhganisms
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38
For the high-water sample, the smallest recorded
size was for a nauplius (49.8 μm), and the largest
size was an adult Calanoida copepod (1120.5 μm).
Recorded sizes occupied both the micro- and
mesoplankton classes (Table 2). There were
differences between the micro-and mesoplankton
(G = 71.3441 and P <0.0001) for both the initial
and final samples of the experiment (G = 83.231
and P <0.0001). As in the low-water season, there
was no difference between the initial and final
microplankton (G = 0.196 and P> 0.05), nor
between the initial and final mesoplankton values
(G = 2.8263 and P> 0.5873).
Initial
Last
Zooplankton (µm)
Min.
Max.
Mean ± SD
Min.
Max.
Mean ± SD
Cladocera
132.8
664
253.8±148.9
166
224.1
253.9±19.8
Nauplius
99.6
232.4
146.6±40.1
49.8
182.6
133.9±38.6
Young-Copepods
207.5
456.5
350.2±72.1
207.5
498
393.4±77.7
Adults-Copepods
448.2
946.2
766.9±118
315.4
1120.5
650±210.6
Rotifera
99.6
141.1
121.1±13.2
91.3
149.4
120±15.3
Table 2 – Size of zooplanktonic organisms in the high-water season at Lake Tupé.
Zooplankton Density
The highest densities were found during
the low-water period experiment. At this time the
rotifers were the most abundant organisms, while
at high-water, the copepods had the greater
numbers. After 24 hours, in both low and high-
water experiments, there was an increase in the
density of the three studied zooplanktonic groups,
but during high-water the overall number of
organisms was lower than in low-water season
(Figures 3 and 4).
Figure 3 – Initial and final zooplanktonic densities in the low-water mesocosm.
0
50
100
150
200
Density (org/L)
Inicial
Final
Castro-Mendes et al. (2019)
Predation by planktonic orhganisms
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39
Figure 4 – Initial and final zooplanktonic densities in the high-water mesocosm.
Phytoplankton Biomass
In the low-water season, the initial
biomass was 1.92 μg/L, and the final biomass
1.09 μg/L. For high-water, the initial biomass was
2.91 μg/L, and the final 0.81 μg/L. Initial and final
biomass of the pico-, nano- and
microphytoplankton are shown in Table 3. Both in
the low and high-water samples there was
decrease in the biomass of all three
phytoplanktonic size fractions after 24 hours.
Possibly the phytoplankton in the three fractions
were consumed by zooplanktonic organisms.
Biomass (µg/L)
Low-water
High-water
Initial
Last
Initial
Last
Picophytoplankton
0.82
0.55
0
0.81
Nanophytoplankton
0.55
0.27
0.54
0
Microphytoplankton
0.55
0.27
2.37
0
Table 3 – Initial and final biomass of mesocosm phytoplankton fractions in the low- and high-water periods
at Lake Tupé.
Discussion
Zooplankton sizes
Freshwater cladocerans are generally 0.2
to 3 mm long, while rotifers are usually smaller,
length ranging from 100 to 1000 μm, although in
the current study rotifers less to 33.1 μm were
recorded. Copepods range from <1 mm to more
than 1 mm, and in this study the largest copepod
was an 1120.5 μm adult Calanoid. Micro- and
mesoplanktonic organisms sizes found in the Lake
Tupé agree with those reported by Gliwicz
(1977), Gliwicz (1990), Ghidini e Santos-Silva
(2009) and Trevisan & Forsberg (2007).
The fact that members of the of micr and
mesoplankton size-classes were found is related to
the presence of certain species that are dominant
in these time-periods. The most abundance
cladocerans species during the low-water season
are Bosminopsis deitersi, which occurs in both
micro- and mesoplankton classes, and the
mesoplanktonicspecies Moina minuta,
Ceriodaphnia cornuta and Diaphanosoma
polyspina. Bosminopsis deitersi is the most
abundant cladoceran in the transition period
between low and high waters (Ghidini 2007;
Brandorff & Hardy 2009; Ghidini & Santos-Silva
0
50
100
150
200
Density (org/L)
Inicial
Final
Castro-Mendes et al. (2019)
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40
2009; Calixto et al. 2011; Ghidini, 2011). Ghidini
& Santos-Silva (2009) analysed the biomass and
size of the four most abundant cladocerans during
the lake’s low and high-water seasons, and found
B. deitersi and M. minuta to be co-dominant at
low-water. These species also had the highest
biomass. While at high-water, B. deitersi was both
the most dominant species and had the higher
biomass. Both in low- and high-water seasons,
Oitona amazonica is the most abundant cyclopoid
species (Brandorff & Hardy 2009; Calixto et al.
2011; Segundo 2013; Raid 2015) and the calanoid
Aspinus acicularis (Raid 2015). For rotifers, 72
planktonic species have been recorded in Lake
Tupé (Calixto et al., 2011; Vásquez 2011).
Possibly, these same species may have formed
part of the assemblage sampled during the
mesocosm experiment. If so, then the sizes
recorded agree with those already reported for the
lake. It is known that most of the organisms
recorded belonged more to the mesoplankton than
the microplankton size class, so it is possible that
that most of the organisms within the mesocosm
were large-sized organisms, which may have fed
on all available food particles, so making it
difficult to ascertain if small species did, indeed,
feed on smaller particles.
Zooplankton Density
Zooplanktonic density was higher in the
dry season, a pattern already recorded in other
zooplankton studies at this site (Brandorff &
Hardy 2009; Ferreira & Robertson 2009; Calixto
et al. 2011; Ghidini 2011; Vasquez 2011;
Segundo 2013). This occurs because high-water
season is accompanied by changes in the
environmental characteristics of the lake, which
occur when the waters of the Rio Negro enter the
lake, increasing its volume, and causing a dilution
effect and so physically diminishing the
zooplankton populations (Brandorff & Andrade
1978; Hamillton et al. 1990; Aprille & Darwich,
2005). However, this high-water decrease in the
zooplanktonic density may also be related to
predation by Chaoborus sp. (Santa-Rita & Santos-
Silva, 2009), and plankton-feeding fish that enter
the lake during this period from the Rio Negro
(Previattelli et al. 2005; Soares & Yamamoto,
2005). Rotifers were denser in the dry season
because as it has a higher richness and abundance
in the lake compared to copepods and cladocerans
(Calixto et al. 2011; Vasquez 2011). Trevisan &
Forsberg (2007), studying three types of Amazon
lakes in both white and black water systems,
found that rotifers comprised 80% of
zooplanktonic abundance. Copepod numbers were
much higher at high-water, but this was due to the
greater number of nauplii. According to Melão
(1997) the developmental period from nauplii to
copepodite I is protracted in Amazonian
copepods.
In the smaller time scale of 24 hours,
two main factors are likely to have influenced the
development of organisms, temperature and food.
In the bag, food would have been concentrated
and zooplankton feed on algae. Cladocerans and
rotifers can reproduce by cyclic parthenogenesis
(Nogrady et al. 1993) and so can produce large
quantities of offspring very quickly. Ghidini &
Santos-Silva (2009) studying the most abundant
Cladocera species of Lake Tupé, Bosminopsis
deitersi, stated that the density of the species
increases in 24 hours mainly at 18 hours. On the
other hand, calanoid and cyclopoid copepods
Castro-Mendes et al. (2019)
Predation by planktonic orhganisms
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41
reproduce only by sexual reproduction (Dussart &
Defaye 2001), though with large numbers of eggs
per cycle. In the experimental samples egg-
bearing females were recorded from all three
groups, indicating that individuals that were
sampled and counted may have hatched during the
experimental period, thus increasing the density
within the sampled 24 hours.
Phytoplanktonic biomass in relation to
zooplankton
The mesocosm method used to provide an
in situ experiment was effective in allowing some
of the simpler of lacustrine environment plankton
dynamics to be simulated and investigated. This
method has already been used in other studies
involving phytoplankton and zooplankton, such as
those of Dodson (1974), Northcote et al. (1990),
Gliwicz & Lampert (1994), Armengol et al.
(2001), Arcifa & Guagnonil (2003), DeMott &
Donk (2004), Filleto et al. (2004), Castilho-Noll
& Arcifa (2007), Bukovinszky et al. (2012),
Hansson et al., (2013).
The decrease in biomass at the end of the
24 hours period indicates predation by
zooplanktonic organisms on the three
phytoplankton fractions had. This could happen
because of the variety of zooplankton sizes found
which, as discussed above, ranged from tiny
nauplii and small rotifers to large adult calanoids.
However, Geller & Muller (1981) argue that when
an organism grows in size this does not mean that
the filtering bristles and food collecting apparatus
always increase allometrically, pointing out that in
some species of cladocerans the swimming
bristles that filter food particles remain the same
size even as body size increases. However, they
also show that, in still other species, the size of
the filtering apparatus and bristles does increase in
size as the body grows. Predation by zooplankton
of phytoplanktonic organisms is corroborated by
the results found by Northcote et al. (1990) and
Gliwicz & Lampert (1994).
Northcote et al. (1990) report that
predation of phytoplankton by zooplankton occurs
more frequently when zooplankton are not
predated by fish. In their mesocosm-based
experiments with fish both the density and
biomass of phytoplankton increased, since it had
not been so heavily consumed by the zooplankton.
A second experiment without fish recorded a
decrease in phytoplankton density and biomass.
During low-water season at Lake Tupé
zooplankivorous fish are not greatly abundant
(Soares & Yamamoto 2005), but larval
Chaoborus sp. are (Santa-Rita & Santos-Silva
2009). Such larvae were present in the mesocosm,
but not at densities enough to impact
zooplanktonic organisms and to influence their
biomass and the size-range profile of the
surviving population. During high-water,
zooplankton apparently did not impact the three
phytoplankton size-fractions. This may have
occurred because of low density and because
zooplanktonic organisms may have been
themselves predated by Chaoborus sp. larva
during the experiment. According to Castilho-
Noll & Arcifa (2007), during experiments with
mesocosms in lakes, some populations of
zooplankton, such as Daphnia gessneri, can be
regulated by the predation of invertebrates,
Castro-Mendes et al. (2019)
Predation by planktonic orhganisms
© 2019 The Authors
42
particularly by Chaoborus sp.
On the other hand, Trevisan & Forsberg
(2007), evaluating the predation pressure of
zooplanktonic organisms on phytoplankton in the
lacustrine systems, found highest zooplankton
densities for small-sized organisms, and
concluded that phytoplankton was free of high
predation pressure and was able to increase its
biomass to the limit of available resources. This is
considered a frequent occurrence in tropical
systems where small forms, such as Rotifera or
Bosmina sp., dominate zooplankton communities
(Nõges 1997). Such small individuals are not able
to regulate the relationships between nutrients and
phytoplankton biomass in the way that occurs in
temperate systems, where phytoplankton numbers
are suppressed by the larger, more competitive,
crustaceans (Trevisan & Forsberg 2007).
However, Filleto et al., (2004) tested the
influence of different phytoplankton size fractions
on the growth and reproduction of cladocerans in
Monte Alegre Lake, southeaster Brazil, by
feeding cladocerans from recently-hatched up to
breeding stage on different sizes of phytoplankton
(micro- and nanoplankton). They concluded that
nanoplankton was most suitable for most
cladocerans, with particle size ingestion by these
herbivores depended on body size and filtering
bristle dimensions. Caraballo (2011),
investigating the cladocerans Diaphanosoma
spinolosum and Ceriodaphnia cornuta, observed
that, although they grew when fed on
phytoplankton from a range of size-fractions,
population performance was best when fed on the
<30 μm size fraction. Consequently, they
suggested that the different fractions tested
produce different rates of population growth and
isotopic signatures in cladocerans.
For Tupé Lake, it was observed that
there is the presence of zooplankton species of
small and large sizes (micro and mesoplankton)
and that there are species of phytoplankton from
different sizes. It was also observed that the
zooplankton’s organisms acted in the biomass of
the three fractions of phytoplankton’s size, thus, it
isn’t possible to state that there is a dominance of
large or small species of phytoplankton since
zooplankton’s organisms equally prey the three
sizes fractions.
Conclusions
Cladocerans, copepods and rotifers did
not affect the size-structure of the phytoplankton
community or the total biomass of these
organisms. In addition, at the community-level,
they did not exert selective predation pressure on
any of the size fractions of the organisms studied.
It is possible that the result of predation
on phytoplankton organisms can be conserved
only when heavy predation pressure or selective
predation alters the size and/or density structure of
some zooplankton size fractions.
The size structure of the phytoplankton
was the same in low- and high-water samples, and
this may mean that these organisms are also not
influenced by the hydrological changes caused by
the water from the Negro River into the lake
during the flooding period.
Acknowledgements
This research was funded by the Biotupé
Project and the National Institute of Amazonian
Castro-Mendes et al. (2019)
Predation by planktonic orhganisms
© 2019 The Authors
43
Research (Instituto Nacional de Pesquisas da
Amazonia: INPA). We thank the residents of the
Tupé Reserve for their help and participation, and
CNPq for a master's degree grant. Adrian Barnett
helped with the English writing.
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