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Feeding of tropical cladocerans (Moina micrura, Diaphanosoma excisum) and rotifer (Brachionus calyciflorus) on natural phytoplankton: effect of phytoplankton size–structure MARC PAGANO* IRD, UR
167,
´ ANOLOGIE DE MARSEILLE, STATION MARINE D’ENDOUME, RUE DE LA BATTERIE DES CENTRE D’OCe
LIONS, 13007 MARSEILLE, FRANCE
*CORRESPONDING AUTHOR:
[email protected] Received April 15, 2007; accepted for publication January 10, 2008; published online February 11, 2008 Corresponding editor: Roger Harris
The proliferation of large phytoplankton in tropical shallow freshwater ecosystems may be attributable to inefficient feeding by the dominant zooplankton (small cladocerans and rotifers) on large particles, but more information on the feeding behavior of tropical organisms is required to explore this hypothesis. In this study, food size selectivity and functional feeding responses of three major tropical freshwater zooplankton species (Moina micrura, Diaphanosoma excisum and Brachionus calyciflorus) were studied to test their ability to control phytoplankton. Eleven grazing experiments were performed, using natural phytoplankton assemblages as a food source. Moina micrura fed efficiently on a wide range of sizes of phytoplankton particles, from unicellular picoplankton Chlorella sp. (2– 4 mm equivalent spherical diameter, ESD) to large Coelastrum reticulatum coenobia (20 – 40 mm ESD), but the selectivity depended on the nature and size distribution of the phytoplankton. Diaphanosoma excisum ingested only very small particles (Monoraphidium, Chlorella). Brachionus calyciflorus fed on a wide size range but showed a clear preference for the largest algae (Cyclotella sp, Scenedesmus opoliensis). These three species increased their ingestion rate linearly with the food concentration and the saturation point was reached for M. micrura within the range of experimental conditions. The results suggest a strong food partitioning between these three species and showed that B. calyciflorus and M. micrura were better able to exploit and control algal blooms than D. excisum, which was a more selective feeder controlled by the availability of small food particles.
I N T RO D U C T I O N The relationships between phyto and zooplankton are key information to understand and predict the planktonic events in freshwater ecosystems (Porter, 1977; McQueen et al., 1986; Sommer et al., 1986; Sterner, 1989). The importance of phytoplankton abundance and composition in structuring zooplankton community through resource competition and food limitation of
herbivorous organisms is well known (Lampert, 1985; Rothhaupt, 1990a). On another hand, the effects of zooplankton grazing on photosynthetic production and composition of phytoplankton have been evidenced since the study by Gliwicz (Gliwicz, 1975). Studies of the functional feeding response and food selectivity of planktonic animals are helpful to better understand these mechanisms (Peters, 1984). The size of food
doi:10.1093/plankt/fbn014, 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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particles is considered an important criterion for food selection by zooplankton grazers (Bern, 1987 and 1994), although other characteristics (e.g. shape, chemical cues, taste, food concentration, phosphorus content, etc.) can also be very important (Butler et al., 1989; Mayzaud et al., 1998; Martinez, 1999). Although the size of the smallest algae captured is a function of the distance between the setules on the filtering appendages (Geller and Mu¨ller, 1981), the maximum size of ingestible particles is generally considered to depend on the grazer body size (Burns, 1968). Small organisms are generally considered to be inefficient grazers of large particles. As there is a scarcity of large cladocerans (Daphnia) and calanoids in tropical shallow freshwater ecosystems, the main zooplankton grazers are small cladocerans and rotifers (Aka et al., 2000; Fernando, 2002). In these ecosystems, the small size of the dominant zooplankton organisms and their grazing inefficiency on large particles may explain the abundance, and sometimes the proliferation, of large phytoplankton particles, such as filamentous or colonial Chlorophycea and Cyanobacteria (Carpenter et al., 1985; Boon et al., 1994; Lazzaro, 1997; Pinel Alloul et al., 1998). Although the feeding behavior of temperate crustacean or rotifer zooplankton is well documented, the literature on tropical or subtropical zooplankton feeding is very scarce (Hart and Jarvis, 1993; Hart, 1998) and it is thus difficult to explore this hypothesis. In this study, I analyzed the feeding responses of major tropical species (the cladocerans Moina micrura and Diaphanosoma excisum and the rotifer Brachionus calyciflorus) fed on natural phytoplankton, to test their ability to control phytoplankton particles over a wide size range.
METHODS Eleven grazing experiments were performed with three zooplankton taxa (the cladocerans Moina micrura and Diaphanosoma excisum and the rotifer Brachionus calyciflorus) fed on different natural phytoplankton assemblages (Tables I and II). Zooplankton and water containing natural phytoplankton were collected from concrete aquaculture tanks (10 m3) at the CRO laboratory in Abidjan, Ivory Coast (Pagano et al., 2000). Before each feeding experiment, water for incubation was prefiltered through various mesh sizes (as specified in Table II) to provide a variety of size structures of the phytoplankton assemblages. For each food assemblage, a sample for phytoplankton analysis was preserved using Lugol’s iodine solution and stored in dark, cold conditions.
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Table I: Zooplankton numbers, biomass and individual weight in the experimental jars at the end of incubation periods; mean and standard deviation (SD) values of triplicates Ind. jar21 Species
mg C jar21 mg C ind.21
Exp. no. Mean SD
Mean SD Mean SD
1 2 3 4 5 6 7 8 9 10 11a
75 73 55 71 70 82 67 167 138 138 160
10 0 5 11 21 14 7 2 3 13 35
71 72 66 75 73 90 71 154 160 182 188
9 10 11a
83 184 160
9 61 25 193 8 187
9 10 11a
783 1545 1323
101 34 81 133 147 107
Moina micrura 7 1 4 3 13 16 5 7 5 17 21
0.95 0.99 1.20 1.09 1.08 1.11 1.06 0.93 1.16 1.31 1.20
0.03 0.02 0.06 0.13 0.17 0.14 0.03 0.05 0.06 0.00 0.13
16 0.73 12 1.06 19 1.17
0.11 0.08 0.06
Diaphanosoma excisum
Brachionus calyciflorus 5 0.043 0.001 6 0.086 0.001 11 0.081 0.001
a
Values after 8 h incubation.
Phytoplankton species were counted after sedimentation using an inverted microscope (Utermohl’s method). Cell volumes (V), based on measured dimensions, were estimated for each species using geometric shapes closely matching the cells’ morphology. Cell size, expressed as equivalent spherical diameters (ESD), was calculated using the cube root of the volume. Concentrations of particulate organic carbon were also estimated for a sub-sample. The sub-sample was filtered on precombusted Whatman GF/F filters, decarbonated, dried and analyzed using a Leco CHN analyzer. The first eight experiments were performed with M. micrura only and the last three experiments were performed with separate sets of M. micrura, D. excisum and B. calyciflorus offered the same phytoplankton assemblage as food. The grazers were divided into several equivalent sets and incubated in 500 mL flasks containing the experimental food assemblage. Three experimental flasks (with zooplankton) and three controls (without zooplankton) were prepared for each phytoplankton/zooplankton combination. The flasks were placed on a rotating wheel (1 rd mn21) to prevent the food particles settling in the ingestion flasks. Incubation took place in the dark (to limit algal growth) at ambient temperature (25 – 268C). For the first 10 experiments (experiments 1 – 10), the flasks were incubated for 8 h. For the last experiment (experiment 11), the total incubation time was 35 h,
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Table II: Particulate organic carbon (POC) seston volume, phytoplankton biomass (as algal volume) and size (as equivalent spherical diameter, ESD) and abundance percentage of the main phytoplankton species in the incubation water during the grazing experiments ESD mm Previous sieving (mm) POC (mg C mL21) Seston volume (109 mm3 L21) Total algal volume (109 mm3 L21) % algal volume of the main taxa
Exp1
Exp2
Exp3
Exp4
Exp5
Exp6
Exp7
Exp8
Exp9
Ex10
Exp11
,25 2.1 18.6 19.6
,25 3.6 33.3 36.2
,25 4.1 38.0 29.1
,25 5.7 53.7 51.3
25 –60 1.2 9.8 13.6
,40 6.7 63.7 49.6
25 –60 11.5 109.6 91.2
25 –60 2.3 20.2 12.8
,25 3.9 36.5 17.3
,25 1.8 15.9 15.0
,25 2.2 19.7 20.4
26.5 6.3
8.6 0.1 ,0.1
13.0
8.1
4.9
2.7
3.7
1.2
0.9
0.3
16.9
19.7
32.5
8.9
7.9
30.0
Chlorophycea Ankistrodesmius bibraianus Chlorella vulgaris Chlorella sp. Coelastrum microporum C. reticulatum (cenobes) Coenochloris sp. (cells) Coenochloris sp. (caenobes) Keratococcus suecicus Monoraphidium contortum Monoraphidium nanum Pediastrum duplex Scenedesmus acuminatus Scenedesmus acutus Scenedesmus lefevrii Scenedesmus opoliensis Scenedesmus sooi Scenedesmus var bicaudatus Tetraedron minimum Tetraselmis sp. Westella botryoides (cells)
7.3 4.6 2.1 27.1 32.3 14.2 51.8 8.7 3.1 5.1 33.5 9.3 11.3 11.1 21.0 4.6 17.0 19.6 8.0 5.0
30.2 14.3
Cryptomonophycaea Cryptomonas sp1. Cryptomonas sp2.
11.7 5.9
33.3 0.1
Cyanophycaea Lyngbia limnetica (cells) Microcystis sp. Oscillatoria sp.
2.7 14.0 17.5
Bacillaripohycaea Cyclotella sp1. Cyclotella sp2. Nitzschia palea Navicula sp. Euglenophycaea Euglena sp.
0.7 15.1
0.3 94.8
30.8
45.1 0.7
,0.1
,0.1
0.9
8.0 4.7 ,0.1 25.3 0.3
1.6 0.5
98.8
0.2 11.1
0.3
0.3
1.6
,0.1 0.5 ,0.1
1.7 0.6 0.5 18.5
31.6 41.3 11.6 86.1
0.4 11.0
7.6
4.7 0.1 0.4
1.2
,0.1 0.2
7.9
0.8
,0.1
0.1
0.1 14.1
0.2
12.9 15.7 9.3 15.3
0.2 0.4
74.5
25.2
3.8
22.8
7.1
68.0
37.7
2.0
1.0
1.7
3.0
25.3
The pre-filtration mesh size (mm) is indicated.
which made it possible to study the changes in the food size spectrum under the influence of grazing with time. Sub-samples (5 mL) were siphoned from the flasks, after homogenization, several times (4, 8, 12, 22, 30 and 35 h after the start) during experiment 11 and at the end of the incubation period for all experiments to determine the particle volume and size spectra using a Coulter Counter Multisizer II with 70, 110 or 280 mm aperture tubes, depending on the phytoplankton assemblage. Where necessary, the samples were diluted with 0.22 mm filtered Isoton (Millex-GS) to keep the coincidence level below 2% (maximal dilution factor = 5). Six replicates were analyzed for each sample. The analytical
volumes were 2, 4 and 15 mL for the 70, 110 and 280 mm aperture tubes, respectively. After incubation, zooplankton from the experimental flasks was collected and fixed with formalin for subsequent counting and measuring. For experiment 11, rotifers were also counted in the sub-samples collected during incubation (see above). Individual weights of the grazers were estimated from their size measured under a dissecting microscope (objective 50, ocular 10). The size of the rotifers was converted into volume using the formulae established by Ruttner-Kolisko (Ruttner-Kolisko, 1977) and volume was converted into dry weight using a 10% ratio (Doohan, 1973, in Bottrell et al., 1976). For
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cladocerans, the length (L)/dry weight (DW) relationships from Saint-Jean and Bonou (Saint Jean and Bonou, 1994) were used for M. micrura and those from unpublished data of Pagano and Saint-Jean were used for D. excisum (DW = 4.42 L2.55). For all taxa, a carbon/ dry weight ratio of 0.45 was used (Pagano and Saint-Jean, 1993). Ingestion rates (I, in mm3 ng C21 h21) and clearance rates (F, mL of water cleared of particles mg C21 h21) were calculated from the difference between control and experimental flasks, assuming zero algal growth in a flask, owing to darkness: ðCc Ce Þ V B t lnCc lnCe V I¼ t B
I¼
Fig. 1. Relationship between phytoplankton and particle biomasses (both expressed in volumetric units) in the incubation water during the feeding experiments.
where Cc and Ce are the particle biovolume (mm3 mL21) at the end of the incubation in the control and experimental flasks, respectively (Cc is taken to be the initial concentration), V is the experimental flask volume (mL), B is the zooplankton carbon biomass in the flask and t is the incubation time (hours). In experiment 11 (time series), I and F were estimated only once after 8 h incubation for comparison with other experiments. The particle concentration and ingestion rates were also expressed in carbon units using the sestonic carbon to volume ratio determined for the experimental water.
R E S U LT S The zooplankton density in the experimental flasks is presented in Table I and the abundance and composition of the various phytoplankton assemblages offered as food during the experiment are presented in Table II. The phytoplankton biomass (expressed as algal volume) varied between 15 and 51 109 mm3 L21 and showed a linear relationship with the particle volume measured using the Coulter Counter (Fig. 1). According to the experiments, 2 – 12 species were identified in the incubation water, of which between 1 and 4 constituted 80– 100% of the phytoplankton biomass. Of the small algae (,10 mm ESD), the most important taxa were Chlorella vulgaris (experiments 1, 2 and 3), Chlorella sp. (experiments 10 and 11), Tetraselmis sp. (experiments 1 and 2), Westella botryoidal (experiments 1, 6, 9, 10 and 11) and Cryptomonas sp. (experiment 1). The main large algae (15– 30 mm ESD) were Scenedesmus opoliensis (abundant in experiments 3, 4, 9, 10 and 11 and
dominant in experiment 5 and the coenobial species Coelastrum reticulatum (dominant in experiments 7 and 8). In experiments 1–8, a comparison of the algae size distribution between the control and experimental flasks showed that M. micrura fed on a wide range of particle sizes (Figs 2 and 3, Table III). As shown by the t-test for control-treatment differences (Table III), the presence of cladocerans in the flasks significantly reduced the particle volume in different size-classes corresponding to peaks of various algae, namely: Chlorella vulgaris (experiment 1, Fig. 2A), Tetraselmis sp. and Cryptomonas sp. (experiment 1, Fig. 2A), Coenochloris (experiment 3, Fig. 2C), Cyclotella (experiment 4, Fig. 2D), Scenedesmus (experiments 5 and 6, Figs 3A and B), Westella botyoides (experiments 1 and 6, Figs 2A and 3B) and Coelastrum (experiment 8, Fig. 3D). In experiment 8, the cladocerans also caused a smaller secondary peak, probably due to the dislocation of Coelastrum coenobia during feeding (Fig. 3D). In other cases, the peaks were not significantly modified, due to high initial phytoplankton density in the incubation flasks (experiments 2 and 7, Figs 2B and 3C). In experiments 9 and 10, a comparison of the size structure modification by M. micrura, D. excisum and B. calyciflorus showed that M. micrura and B. calyciflorus were able to remove phytoplankton particles over the whole size range, whereas D. excisum only significantly modified the peaks of small algae, i.e. Monoraphidium, Chlorella and Westella (Figs 4 and 5, Table IV). In experiment 11, the same comparison based on time series measurements (Fig. 6, Table V) confirmed that D. excisum only modified the peak of small algae (mainly Chlorella sp) which decreased with time during the incubation period, whereas the peak of larger algae (Cyclotella sp. and Scenedesmus opoliensis) increased (Fig. 6C), as also observed in the control flasks (Fig. 6A).
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Fig. 2. Size distribution of phytoplankton particles (Coulter counter measurement) in control (continuous line) and experimental (black circles) flasks and difference between control and experimental flasks (white circles) at the end of incubation period, in experiments 1–4 (A, B, C and D) with M. micrura. The main phytoplankton species, explaining particle peaks, are indicated.
The intermediate size-class (4.5– 10 mm, W. botryoides) was significantly modified only after 24 h (Table V). In the flasks with M. micrura, the various peaks decreased simultaneously (Fig. 6D) and were all significantly different from their equivalents in the controls after 8 h (Table V). In the flasks with B. calyciflorus, large particles (Cyclotella sp. and Scenedesmus opoliensis) decreased sharply during the first hours of incubation and the small particles (Chlorella) decreased later, i.e. when largest particles were rarefied (Fig. 6B). This trend was confirmed by the t-test analysis, the differences between control and experimental jars in the smallest size-class (,4.5 mm) being significant only after 12 h of incubation (Table V). During this experiment, the total algal biomass in the control flasks decreased slightly during the first 8 h of incubation and stabilized afterwards (Fig. 7A). An exponential decrease of the algal biomass was observed in the flasks with B. calyciflorus or M. micrura. This decrease was particularly marked with B. calyciflorus, whose population increased exponentially during the incubation (Fig. 7B). As a consequence, at the end of the experiments, the rotifers had markedly cleared the water of algal particles. In flasks with D. excisum, there was a
marked decrease of algal biomass during the first 8 h, with the disappearance of the peak of Chlorella sp. The total algal biovolume then remained stable around 18.109 mm3 L1. The three species displayed a classic linear increase of their specific ingestion rates and a decrease of clearance rates with increasing food concentration (Fig. 8). Moina micrura reached saturation point at 50 109 mm3 L21 (experiments 4, 6 and 7), whereas B. calyciflorus and D. excisum did not reach saturation within the range of experimental concentrations (experiments 9, 10 and 11). At corresponding food concentrations (experiments 9, 10 and 11), B. calyciflorus displayed the highest specific ingestion and clearance rates and D. excisum the lowest rates.
DISCUSSION Methodological aspects Laboratory determination of feeding and grazing rates by measuring the loss of particles from suspension in
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Fig. 3. Size distribution of phytoplankton particles (Coulter counter measurement) in control (continuous line) and experimental (black circles) flasks and difference between control and experimental flasks (white circles) at the end of incubation period, in experiments 5–8 (A, B, C and D) with M. micrura. The main phytoplankton species, explaining particle peaks, are indicated.
which planktonic animals have been introduced is submitted to several methodological problems, so that uncritical use can confuse the interpretation of results (Peters, 1984). The Coulter Counter method, used in the present study, gives only partial information on the food particles (i.e. volume and ESD), but microscopic observations performed simultaneously with the Coulter measurements permitted to associate the main phytoplankton species and the main peaks of particle in the food assemblages. The good relationship between phytoplankton and particle volumes (Fig. 1) also reinforces the reliability of the results. The incubation method is subjected to several biases, the main of which being a re-introduction of particles within the counted particle size range through feces and possible cell breakage during the ingestion mechanical processes which can lead to an underestimation of ingestion rate, and to errors in the selective pattern (Harbison and Mc Alister, 1980; Peters, 1984). In this study, I found no evidence of particle “production” in the experimental jars (i.e. significant negative difference between experimental and control flasks), except in experiment 8 where the significant increase of ,19 mm particles was probably due to
the dislocation of Coelastrum coenobia by the cladocerans during feeding. Other biases are linked to classical incubation effects, i.e. bottle volume and animal density in the jars. High density of animals may cause scramble competition and accelerate change in food-suspension, also leading to flock formation and declining availability of algae to herbivores. This can be especially problematic in high ambient temperatures (25 –268C in this study) as—due to reduced viscosity—the interference by larger and filamentous particles becomes sever enough, up to stop ingestion when the appendage filters are clogged completely (Abrusan, 2004). In this study, the problems linked to zooplankton density and the interference of large phytoplankton species with the filtering combs of animals may not have been that important. The densities of plankton animals in the jars (130 – 330 ind. L21 for M. micrura, 166– 368 ind. L21 for D. excisum and 1566– 2646 ind. L21 for B. calyciflorus, see details in Table I) were lower than the maximal densities observed in the tanks from which they were collected (500 ind. L21 for M. micrura, 1200 ind. L21 for D. excisum and 5000 ind. L21 for B. calyciflorus; Pagano et al., 2000 and unpubl. data). In addition, the
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Table III: t-Test to compare the difference in particle volume between control and experimental bottles (triplicates), within different size-classes corresponding to the phytoplankton peaks, after 8 h incubation during experiments 1 – 8 with M. micrura Exp. no. 1
Size-class Total ,5 mm 5– 10 mm .10 mm
2
3
Total 0– 4 mm 4– 8 mm 8– 13 mm .13 mm Total ,4 mm 4– 9 mm 9– 16 mm 16 –26 mm
4
5
6
7
8
.26 mm Total ,13 mm 13 –18 mm 18 –22 mm .22 mm Total ,10 mm 10 –33 mm .33 mm Total ,5 mm 5– 12 mm 12 –17 mm .17 mm Total ,19 mm 19 –49 mm .49 mm Total ,19 mm 19 –49 mm .49 mm
Main phytoplankton species
Chlorella vulgaris, Westella botyoides Tetraselmis sp., Cryptomonas sp. Euglena sp., Scenedesmus opoliensis Chlorella vulgaris, Chlorella sp. Tetraselmis sp. Coenochloris sp. Euglena sp. Chlorella vulgaris Scenedesmus acuminatus, S. acutus Coenochloris sp. S. opoliensis, Euglena sp. ? ? Cyclotella sp S. opoliensis Euglena sp., Navicula sp. Scenedesmus lefevrii Scenedesmus opoliensis ? Westella botryoides S. acuminatus, Keratococcus suecicus S. acutus S. acutus ? Coelastrum reticulatum ? ? Coelastrum reticulatum ?
T-value
Significance
7.08 7.26
** **
3.95
*
2.02
ns
0.98 1.71
ns ns
20.18 1.53 0.73 3.19 0.45 2.55
ns ns ns * ns ns
4.77 1.79
** ns
1.29 3.07 1.33 4.02 0.77 1.42 2.77 3.20 3.09 0.52 2.95 3.63 4.14
ns * ns * ns ns * * * ns * * *
2.90 0.75 1.03 2.55 0.96 1.79 14.35 225.23 22.64 20.99
* ns ns ns ns ns *** *** *** ns
Fig. 4. Size distribution of phytoplankton particles (Coulter counter measurement) in control (continuous line) and experimental (black circles) flasks and difference between control and experimental flasks (white circles) at the end of incubation period, in experiment 9 with M. micrura (A), D. excisum (B) and B. calyciflorus (C). The main phytoplankton species, explaining particle peaks, are indicated.
Degree of freedom ¼ 4. ***P , 0.001; **P , 0.01; *P , 0.05; ns, non-significant (P . 0.05). T-values in bold characters, significant positive differences (i.e. particle removing), underlined T-values, significant negative differences (i.e. particle “production”).
phytoplanktonic assemblages offered as food included no filamentous species, except Oscillatoria sp. and Lyngbia limnetica present only in some experiments at very low density (Table I). Finally, observations of zooplankton immediately after incubations (before fixing with
formaldehyde) revealed that most animals were healthy and very active and no cladocerans were observed with clogged appendages.
Food selectivity In the experiments, M. micrura fed efficiently on a wide size range of phytoplankton particles, from the
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Fig. 5. Size distribution of phytoplankton particles (Coulter counter measurement) in control (continuous line) and experimental (black circles) flasks and difference between control and experimental flasks (white circles) at the end of incubation period, in experiment 10 with M. micrura (A), D. excisum (B) and B. calyciflorus (C). The main phytoplankton species, explaining particle peaks, are indicated.
unicellular picoplankton Chlorella sp. (2 – 4 mm ESD, Fig. 5A) to the large Coelastrum reticulatum coenobia (20 – 40 mm ESD, Fig. 3D). However, its feeding preference varied according to the type of phytoplankton assemblage. When the food provided included two (or more) distinct size peaks, sometimes the largest particles were consumed more intensively (Fig. 2A) and sometimes less intensively (Fig. 4A) than the smallest, whereas
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sometimes large and small particles were consumed at about the same rate (Fig. 5A). These differences could not result from differences in grazer size or age between experiments, as observed for Daphnia by Lehman and Sandgren (Lehman and Sandgren, 1985) or for Ceriodaphnia reticulata and M. micrura by Hart and Jarvis (Hart and Jarvis, 1993). All experiments were started with adult females and there was little variation in the average individual weight of animals at the end of incubation (Table II). These results show that M. micrura fed selectively and that this selectivity was variable. Most studies on cladoceran selectivity have dealt with Daphnia species and there does not appear to be any example of comparable work for Moina species. Hart and Jarvis (Hart and Jarvis, 1993) showed that M. micrura feed selectively on very small particles (natural bacteria, Chlorella) in a hypertrophic subtropical reservoir, but they did not evaluate its selectivity on a wide range of particle sizes. Cladocerans are now considered to be relatively selective (Sterner, 1989). The morphology of food particles, especially size, is the characteristic most associated with selective cladoceran feeding, although other criteria, such as taste, presence of flagella and surface properties (e.g. electrostatic charge), play some role so far as some species are concerned (references summarized by Sterner, 1989). In the present experiments with M. micrura, the particle size (ESD) was probably not the sole food selection criterion. The Chlorella vulgaris included in the assemblages in experiments 1, 2 and 3 was never consumed unlike other species with similar particle size (e.g. Monoraphidium contortum, in experiment 9 or Chlorella sp.). Similarly variable selectivity has been reported for several species, such as Sida (Downing, 1981), and, according to Gliwicz and Siedlar (Gliwicz and Siedlar, 1980), this selective plasticity may be linked to the ability of cladocerans to adjust their gape depending on the abundance and the size of particles, to avoid entanglement and the clogging of their filtering apparatus. Unlike M. micrura, D. excisum appeared to have constant selectivity in the experiments. This species was unable to consume large particles such as Scenedesmus or Cyclotella and was restricted to ingesting small unicellular algae such as Monoraphidium and Chlorella. No similar examples of experimental results for D. excisum were found in the literature. Hart and Jarvis (Hart and Jarvis, 1993) showed that this species fed at similar rates on small particles (natural bacteria and Chlorella), but they did not evaluate its selectivity for larger particles. The work by Bodgan and Gilbert (Bodgan and Gilbert, 1987) on the food size preference of several freshwater zooplankton, including Diaphanosoma leuchtenbergianum, showed that this species had uniformly high efficiency
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Table IV: t-Test to compare the difference in particle volume between control and experimental bottles (triplicates), within different size-classes corresponding to the phytoplankton peaks, after 8 h incubation in experiments 9 and 10 with B. calyciflorus, D. excisum and M. micrura Exp. no.
Size-class
9
Total ,5 mm 5 – 11 mm 11 –18 mm .18 mm Total ,5 mm 5 – 10 mm 10 –18 mm .18 mm
10
Main phytoplankton species
B. calyciflorus 4.81 20.88 4.48 2.85 0.57 51.79 24.25 28.84 80.36 22.89
Monoraphidium contortum Westella botryoides Cyclotella sp.
Chlorella sp., M. contortum Westella botryoides Microcystis sp., Cyclotella sp.
D. excisum ** ns ** * ns *** *** *** *** ***
3.67 3.75 2.26 0.21 0.69 36.65 68.04 1.73 1.29 1.11
M. micrura * * ns ns ns *** *** ns ns ns
12.43 4.75 19.21 4.56 1.81 41.24 19.90 22.07 18.93 5.36
*** ** *** * ns *** *** *** *** **
Degree of freedom ¼ 4. ***P , 0.001; **P , 0.01; *P , 0.05; ns, non-significant (P . 0.05). T-values in bold characters, significant positive differences (i.e. particle removing).
Fig. 6. Time variation, during experiment 11, of the size distribution of phytoplankton particles (Couler counter measurement) in controls (A) and in experimental flasks with B. calyciflorus (B), D. excisum (C) and M. micrura (D). The main phytoplankton species, explaining particle peaks, are indicated.
on small cells (Coccoid bacterium, 0.45 mm ESD) and larger cells (Cryptomonas erosa, 12 mm ESD). The restriction of D. excisum to small particles is in agreement with
the observations by Geller and Mu¨ller (Geller and Mu¨ller, 1981) on the filtration apparatus of cladocerans. They showed that Diaphanosoma species (unlike other
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Table V: t-Test to compare the difference in particle volume between control and experimental bottles (triplicates), within different size-classes corresponding to the phytoplankton peaks, after 4, 8, 12, 24 and 32 h incubation times in experiment 11 with B. calyciflorus, D. excisum and M. micrura Total B. calyciflorus 0–4 h 0–8 h 0–12 h 0–24 h 0–32 h
10 mm (Cyclotella Cyclotella sp. S. opoliensis) opoliensis
6.29 18.91 27.02 28.66 39.13
** *** *** *** ***
1.43 1.89 4.18 5.64 7.99
ns ns * ** **
3.91 6.23 17.13 14.42 45.03
* ** *** *** ***
7.11 34.40 14.86 27.77 51.16
** *** *** *** ***
D. excisum 0–4 h 0–8 h 0–12 h 0–24 h 0–32 h
2.59 1.41 2.04 3.79 6.90
ns ns ns * **
8.62 5.51 9.30 5.93 7.34
*** ** *** ** **
1.76 0.46 1.51 3.41 13.35
ns ns ns * ***
1.33 2.37 2.00 1.14 0.30
ns ns ns ns ns
M. micrura 0–4 h 0–8 h 0–12 h 0–24 h 0–32 h
1.32 4.45 7.14 11.61 21.15
ns * ** *** ***
8.81 4.28 6.18 5.61 7.28
*** * ** ** **
2.42 3.86 14.05 15.76 35.89
ns * *** *** ***
0.24 4.03 5.27 21.70 40.97
ns * ** *** ***
Degree of freedom = 4. ***P , 0.001; **P , 0.01; *P , 0.05; ns, non-significant (P . 0.05). T-values in bold characters, significant positive differences (i.e. particle removing).
cladoceran genera) have only one filter screen per thoracic limb with a fairly constant filter mesh, which restricts the size range of potentially edible particles. The results presented here also agree with the experimental results of Bern (Bern, 1994) which show that when Diaphanosoma brachyurum was fed with six algae, it ingested the two smallest (Chlorella sp., 2 mm, and Chlorella homosphaera, 6 mm), most intensively and showed very low clearance rates for algae .20 mm (Peridiniopsis borgei, 38 mm, and Peridinium cinctum, 61 mm). Bern argued that the characteristic selection of very small particles was probably caused by a rejection of the largest particles from the filtering chamber, as suggested by his previous experimental work on Daphnia cucullata fed different sized polystyrene beads (Bern, 1990). Feeding by rotifers is usually concentrated on small cells, 20 mm in diameter or less (Pourriot, 1977). However, the results of the present study show that B. calyciflorus has a clear preference for the largest algae (Cyclotella sp., Scenedesmus opoliensis) as shown in experiment 10 (Fig. 5C) and in the time series experiment (experiment 11, Fig. 6) where rotifers cleared these particles during the first hours of incubation, before attacking smaller particles. These results agree with those of Rothhaupt (Rothhaupt, 1990b) whose experiments with polystyrene spheres or algae provided separately or in mixtures, showed that B. calyciflorus always preferred the biggest food particles and whose clearance rate
increased with the size of particle ( provided at the same concentration). In the present study, B. calyciflorus also consumed very small algae (Chlorella sp., 2 mm diameter) efficiently when it had depleted the largest and intermediate sized particles. These results suggest great adaptability in the feeding behavior of this species which was able to shift between particle peaks depending on the changes in concentration and size composition of the food assemblage. They also show that B. calyciflorus feeds efficiently on picoplankton-sized cells as already observed by Rothhaupt (Rothhaupt, 1990b) on the same species. According to Gilbert and Bodgan (Gilbert and Bodgan, 1984), B. calyciflorus can be considered as a suspension feeder able to collect a number of very small particles (such as yeast) simultaneously, whereas larger cells are processed individually. More generally, these authors consider that the feeding apparatus of brachionids permits the efficient collection of a wide variety of particles. It was shown that these rotifers, particularly B. calyciflorus, are able to select or reject suspended particles according to their quality and quantity (Gilbert and Starkweather, 1977). This is related to the presence of chemoreceptors and mechanoreceptors and the possibility of pseudotrochal screening and particle rejection caused by reverse ciliary beating. These observations are consistent with the eclectic feeding of B. calyciflorus and its preference for some large algae (Cyclotella, Scenedesmus) shown in my experiments.
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Fig. 7. Time series of the algal concentration (A) and the zooplankton biomass (B) during experiment 11. Each point represents mean + standard deviation of triplicates.
Feeding rates As for other cladocerans, particularly Daphnia species (Rigler, 1961; McMahon and Rigler, 1965), the functional feeding response of M. micrura (Fig. 8A) could be clearly described by a rectilinear (type 1) model with increased ingestion rate and relatively constant clearance rate up to a certain concentration called the incipient limiting level or ILL (Rigler, 1961). Diaphanosoma excisum and B. calyciflorus also increased their ingestion rate linearly with food concentration, but the saturation point was not observed within the range of experimental conditions for these species (10 –40 109 mm3 L21).
The ILL for M. micrura (ca. 50 109 mm3 L21 or ca. 5 mg C mL21) and the food concentration at the sub-ILL maximum ingestion rate observed for D. excisum (40 109 mm3 L21 or ca. 4 mg C mL21) were much higher than the ILL determined by McMahon and Rigler (McMahon and Rigler, 1965) for Daphnia magna fed Chlorella vulgaris (ca. 2– 22 109 mm3 L21 depending on the size of food particles) or maximum reported values from the literature on Daphnia species cited by Walz (Walz, 1997) (0.3 mg C L21). The maximum ingestion rates of B. calyciflorus were observed at a food concentration (40 109 mm3 L21, or ca. 4 mg C mL21)
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Fig. 8. Variations of the specific ingestion (A) and clearance (B) rates of B. calyciflorus, D. excisum and M. micrura with particle concentration during the feeding experiments. Each point represents mean + standard deviation of triplicates. Symbol numbers close to M. micrura plots are the experiment numbers. The lines represent the visually adjustments of the ingestion rates for M. micrura.
much higher than the ILL for temperate rotifers including B. calyciflorus (0.03 – 2.5 mg C mL21) summarized from literature by Walz (Walz, 1997). They are also much higher than the ILL measured experimentally for B. calyciflorus fed different algae (0.5 – 2.5 mg C mL21) by Rothhaupt (Rothhaupt, 1990c). This comparison
suggests that the saturation point seems to occur at higher food levels for tropical freshwater rotifers and cladocerans than for their temperate counterparts. However, the difference in ILL could also result from difference in experimental temperatures (208C in Rothhaupt’s experiments, 25– 268C in this study) or
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from difference in food quality between experiments as suggested by Rothhaupt (Rothhaupt, 1990c). The specific ingestion rates calculated for M. micrura and D. excisum in this study (200–3000 mm3 ng C21 h21, i.e. 11–160 106 mm3 mg dry weight day21) overlap but are higher than the highest values reported in the literature as summarized by Peters (Peters, 1984) for cladocerans (0.01–100 106 mm3 mg dry weight day21, with median at 1.2, n = 259). Their clearance rates (0.2– 0.9 mL mg C21 h21, i.e. 1.1–4.9 mL mg dry weight day21) are toward the upper end of the range cited (0.001–10 mL mg dry weight day21, with median at 0. 35, n = 259). Unlike Daphnia, there are few values in the literature for Moina and Diaphanosoma species. The clearance rates observed for D. excisum (12–40 mL ind.21 h21) and M. micrura (11–114 mL ind.21 h21) are the same magnitude as those measured by Hart and Jarvis (Hart and Jarvis, 1993) for these species fed bacteria or Chlorella (10–220 mL ind.21 h21 depending on the size of grazers). Bodgan and Gilbert (Bodgan and Gilbert, 1987) found a much higher average maximum individual clearance rate for Diaphanosoma leuchtenbergianum (406 mL ind.21 h21). The clearance rates of B. calyciflorus (150 – 180 mL ng C21 h21, or 6 – 17 mL ind.21 h21) are within the range of values summarized by Wallace and Snell (Wallace and Snell, 2001) for temperate rotifers (commonly between 1 and 10 mL ind.21 h21 with a few examples exceeding 50 mL ind.21 h21) and the values reported by Rothhaupt (Rothhaupt, 1990c) for B. calyciflorus fed different algae (10 – 30 mL ind.21 h21). The maximum ingestion rate of B. calyciflorus in the present study (4500 mm3 ng C21 h21, i.e. 20 ng C ind.21 h21) is very close to the higher values reported by Rothhaupt for the same species (18 ng C ind.21 h21).
agrees with field observations showing that these species cohabit in several shallow tropical freshwater ecosystems (Aka et al., 2000; Bouvy et al., 2000; Kaˆ et al., 2006). These differences also illustrate the high capacities of the rotifer (typically r-strategist) and to a lesser extent M. micrura to exploit and control algal blooms, unlike D. excisum. This last species is more food-specific and appears to be controlled by the availability of small food particles, as also shown by Pagano et al. (Pagano et al., 2000) who observed that D. excisum, reared in 1 m3 tanks, quickly cleared small algae (Kirchneriella sp.) during the first days, owing to selective grazing pressure leading to food limitation, and subsequent population decrease. These characteristics are probably very important in natural conditions and should be considered when examining factors structuring zooplankton communities.
AC K N OW L E D G E M E N T S I want to thank Rob Hart and an anonymous reviewer for very helpful comments and suggestions on the manuscript.
FUNDING This work was supported by the IRD Research Unit 098 “FLAG” (“key factors and consequences of algal blooms in shallow systems”).
REFERENCES Abrusan, G. (2004) Filamentous cyanobacteria, temperature and Daphnia growth: the role of fluid mechanics. Oecologia, 141, 395 –401.
Comparison of the three species and ecological implications Of the three species studied, which are major zooplankton components in shallow tropical freshwater ecosystems, the smallest species B. calyciflorus was best able to harvest food over a wide size range. It also had the highest specific ingestion rates as well as the highest rates of ingestion increase (i.e. the slope of the I/C curve, Fig. 8A). The largest species, D. excisum, however, restricted its ingestion to small particles and showed the lowest clearance rates. Moina micrura had an intermediate position, being able to collect particles in a similar size range to B. calyciflorus but with lower specific feeding rates. The eclectic feeding behavior of B. calyciflorus and M. micrura and the specialization of D. excisum suggest weak competition and good food partitioning between the three species. This experimental result
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