An empirical model of carbon flow through marine viruses and microzooplankton grazers

Viruses and microzooplankton grazers represent major sources of mortality for marine phytoplankton and bacteria, redirecting the flow of organic material throughout the world's oceans. Here, we investigate the use of nonlinear population models of interactions between phytoplankton, viruses and grazers as a means to quantitatively constrain the flow of carbon through marine microbial ecosystems. We augment population models with a synthesis of laboratory-based estimates of prey, predator and viral life history traits that constrain transfer efficiencies. We then apply the model framework to estimate loss rates in the California Current Ecosystem (CCE). With our empirically parameterized model, we estimate that, of the total losses mediated by viruses and microzooplankton grazing at the focal CCE site, 22 ± 3%, 46 ± 27%, 3 ± 2% and 29 ± 20% were directed to grazers, sloppy feeding (as well as excretion and respiration), viruses and viral lysate respectively. We identify opportunities to leverage ecosystem models and conventional mortality assays to further constrain the quantitative rates of critical ecosystem processes.     

Acute inhibition of protease and suppression of growth in zooplankter,Moina macrocopa,by Microcystis blooms collected in Central India

Cyanobacterial blooms consisting of Microcystis spp., collected from 14 water-bodies in Central India, and an adapted culture, were studied for likely impact on zooplankton community. When fed with single cells of Microcystis from several locations, in mixtures with Chlorella, population growth of the cladoceran Moina macrocopa was suppressed. Microcystis alone was unsuitable as food. In three cases, bloom extracts enhanced mortality of starved zooplankton. Extracts from several sources inhibited protease activity when trypsin or a crude extract from zooplankton served as enzyme source. Upon fractionation by solid-phase extraction, the C-18 passed extract contained the anti-protease and toxic substances for zooplankton, whereas a methanol eluted fraction retained the trypsin inhibitory substance. The study suggests that production of protease inhibitors by cyanobacteria is a factor responsible for feeding inhibition and mortality in zooplankton, which in turn could regulate the community structure of grazers.     

Do transparent exopolymer particles (TEP) inhibit grazing by the euphausiid Euphausia pacifica?

The hypothesis that ubiquitous, sticky transparent exopolymerparticles (TEP) formed from phytoplankton exudates will adhereto and coat the feeding structures of marine zooplankton grazers,and thus depress feeding on phytoplankton, was tested usingthe euphausiid, Euphausia pacifica, as a model organism. Duringtwo feeding experiments, E.pacifica were offered cells of thediatom Thalassiosira weissflogii, TEP, or both TEP and T.weissflogiicells. Ingestion rates on cells were lower in the presence ofTEP. However, contrary to the hypothesis, grazing on cells wasnot inhibited by TEP. Rather, TEP-clusters, aggregates whichformed from TEP and nano-sized particles normally too smallfor the filtering apparatus of E.pacifica to retain, servedas an alternative food source for E.pacifica, reducing theiringestion of cells. These clusters were very similar in formto the TEP actually available to marine grazers in nature. TEP-clusterswere similar to cells in size and food quality, and were grazedat similar rates. When feeding on TEP-clusters, euphausiidsshort circuit the food web by feeding on nano- and picoplanktondirectly, bypassing the microbial loop. Thus, the presence ofTEP appears to enhance, rather than depress, macrozooplanktongrazing.     

Zooplankton feeding on seston in the Rhone River plume area (NW Mediterranean Sea) in May 1988

In the Rhone delta area the seston concentration was maximum in the plume waters where small-sized particles dominated, while marine waters were characterized by large particles. The food conditions were optimal for grazers in the interface layer, where the high chlorophyll concentration contrasted with the low values found under the plume.The mesozooplankton specific composition did not show marked difference in and outside the plume. No special pattern for particle size selection by grazers appeared in the plume, contrary to marine waters, where largest particles were the most intensively grazed. The calculations of specific ingestion and filtration rates show that the plume waters (particularly at the salinity interface) were more favourable to zooplankton feeding than the marine ones (outside and under the plume). Nevertheless, taking into account the high level of seston biomass in the whole area studied, the daily grazing pressure of mesozooplankton was very low.     

COLONY SIZE OF PHAEOCYSTIS ANTARCTICA (PRYMNESIOPHYCEAE) AS INFLUENCED BY ZOOPLANKTON GRAZERS1

The haptophyte Phaeocystis antarctica G. Karst. is a dominant phytoplankton species in the Ross Sea, Antarctica, and exists as solitary cells and mucilaginous colonies that differ by several orders of magnitude in size. Recent studies with Phaeocystis globosa suggest that colony formation and enlargement are defense mechanisms against small grazers. To test if a similar grazer‐induced morphological response exists in P. antarctica, we conducted incubation experiments during the austral summer using natural P. antarctica and zooplankton assemblages. Dialysis bags that allowed exchange of dissolved chemicals were used to separate P. antarctica and zooplankton during incubations. Geometric mean colony size decreased by 35% in the control, but increased by 30% in the presence of grazers (even without physical contact) over the 15 d incubation. The estimated colonial‐to‐solitary cell carbon ratio was significantly higher in the grazing treatment. These results suggest that P. antarctica colonies would grow larger in the presence of indigenous zooplankton and skew the carbon partitioning significantly toward the colonial phase. While these observations show that the colony size of P. antarctica was affected by a chemical signal related to grazers, the detailed nature and ecological significance of this signal remain unknown.     

Benthic and pelagic food resources for zooplankton in shallow high-latitude lakes and ponds

1. Shallow lakes and ponds are a major component of the northern landscape and often contain a high zooplankton biomass despite clear waters that are poor in phytoplankton. 2. In this study we quantified zooplankton food sources and feeding rates in the shallow waters of two contrasting high‐latitude biomes: subarctic forest tundra (Kuujjuarapik, Quebec) and high arctic polar desert (Resolute, Nunavut). Five substrate types were tested (beads, bacteria, picophytoplankton, filamentous plankton and microbial mats). Special attention was given to the role of benthos, a component that is usually poorly integrated into models of aquatic foodwebs. 3. Consistent with observations elsewhere in the circumpolar region, high concentrations of adult macrozooplankton occurred in all sites (up to 17 100 crustaceans m^?3) while phytoplankton concentrations and primary productivity were low. The communities were composed of multiple species, including Daphnia middendorfiana, Hesperodiaptomus arcticus, Leptodiaptomus minutus, Artemiopsis stefanssoni and Branchinecta paludosa. 4. Detritus made 89–98% of the planktonic resource pool and bacteria contributed the highest biomass (up to 29 mg C m^?3) of the planktonic food particles available to zooplankton. Benthic resources were dominated by microbial mats that grew in nutrient‐rich conditions at the base of the ponds and which dominated overall ecosystem biomass and productivity. 5. All species were flexible in their feeding but there were large, order of magnitude differences in clearance rates among taxa. These differences likely resulted from different grazing strategies among cladocerans, copepods and fairy shrimps, and possibly also from adaptation to specific food types and size ranges that occur locally in these waters. 6. The subarctic cladocerans Ceriodaphnia quadrangula and D. middendorfiana, and the arctic fairy shrimp B. paludosa were observed to graze directly on the microbial mats and the feeding experiments confirmed their assimilation of benthic substrates. The other zooplankton species showed a more pelagic feeding mode but were capable of using microbial mat filaments, thus may be indirectly linked to benthic processes via resuspension. 7. Our study indicates that the classical aquatic food web in which phytoplankton provide the sole production base for grazers does not apply to northern shallow lakes and ponds. Instead, microbial mats increase the physical complexity of these high latitude ecosystems and likely play a role in sustaining their high zooplankton biomass.     

Relative importance of inefficient feeding and consumer excretion to organic carbon flux from Daphnia

1. Sloppy and inefficient feeding by zooplankton is generally thought to make a major contribution to the regeneration of the dissolved organic carbon (DOC) pool in aquatic environments. In this study, we tested experimentally the regeneration of DOC by a freshwater zooplankter feeding on two species of phytoplankton at different food concentrations and C : P ratios. We separated the DOC production because of inefficient feeding (pre‐ingestive regeneration) and zooplankton excretion and faeces release (postdigestive regeneration). 2. Within a brief incubation period (10 min), DOC production in the presence of Daphnia was not significantly different from that in the control treatment without grazers. During a longer incubation period (4 h), the amounts of radiocarbon retained in the algal cells per se were constant or were not different from those in the control treatments. These experimental results strongly suggest that inefficient feeding did not contribute significantly to DOC production in the grazer–prey system. 3. During the 4‐h incubation, calculations of the DOC per ingestion rate (i.e. DOC produced by Daphnia alone) showed that food concentration and algal species did not affect the relative DOC production, but there was considerable difference at different algal C : P ratios and grazer densities. We found that direct excretion of DOC by Daphnia occurred rapidly following food digestion and accounted for >65% of the total DOC production. Maximum DOC leakage from Daphnia faeces contributed less to DOC production than the grazer excretion, except under P‐limited conditions. 4. This study highlights the dominant role of postingestive process, especially the direct excretion by zooplankton, in DOC production in a grazer–prey system.     

Microzooplankton feeding behaviour: grazing on the microbial and the classical food web of African soda lakes

We investigated the feeding behaviour of the dominant microzooplankton of saline lakes in the East African Rift Valley. A set of grazing experiments revealed high ingestion rates of the two euryhaline rotifers Brachionus dimidiatus and Brachionus plicatilis and of the large-sized omnivorous ciliates Frontonia sp. and Condylostoma magnum reflecting the unique nature of tropical saline systems. The size spectrum of ingested particles was broad and even included filamentous cyanobacteria such as the commonly dominating Arthrospira fusiformis. Feeding selectivity on cyanobacteria, however, was rather low showing higher values for cryptomonads and small ciliates. Bacterial biomass was favoured by the presence of grazers, as small bacterivorous predators were reduced at an average of 13.9%, showing the cascading effect of large zooplankton on the food web structure. Overall, based on this first-time study of the microzooplankton feeding behaviour in East African soda lakes, a strong structuring effect of rotifers and large ciliates on microbial plankton communities is assumed, especially in times of high consumer biomass.     

Mesozooplankton Grazing on Picocyanobacteria in the Baltic Sea as Inferred from Molecular Diet Analysis

Our current knowledge on the microbial component of zooplankton diet is limited, and it is generally assumed that bacteria-sized prey is not directly consumed by most mesozooplankton grazers in the marine food webs. We questioned this assumption and conducted field and laboratory studies to examine picocyanobacteria contribution to the diets of Baltic Sea zooplankton, including copepods. First, qPCR targeting ITS-1 rDNA sequence of the picocyanobacteria Synechococcus spp. was used to examine picocyanobacterial DNA occurrence in the guts of Baltic zooplankton (copepods, cladocerans and rotifers). All field-collected zooplankton were found to consume picocyanobacteria in substantial quantities. In terms of Synechococcus quantity, the individual gut content was highest in cladocerans, whereas biomass-specific gut content was highest in rotifers and copepod nauplii. Moreover, the gut content in copepods was positively related to the picocyanobacteria abundance and negatively to the total phytoplankton abundance in the water column at the time of sampling. This indicates that increased availability of picocyanobacteria resulted in the increased intake of this prey and that copepods may rely more on picoplankton when food in the preferred size range declines. Second, a feeding experiments with a laboratory reared copepod Acartia tonsa fed a mixture of the picocyanobacterium Synechococcus bacillaris and microalga Rhodomonas salina confirmed that copepods ingested Synechococcus, even when the alternative food was plentiful. Finally, palatability of the picocyanobacteria for A. tonsa was demonstrated using uptake of ¹³C by the copepods as a proxy for carbon uptake in feeding experiment with ¹³C-labeled S. bacillaris. These findings suggest that, if abundant, picoplankton may become an important component of mesozooplankton diet, which needs to be accounted for in food web models and productivity assessments.     

Fitness consequences for copepods feeding on a red tide dinoflagellate: deciphering the effects of nutritional value, toxicity, and feeding behavior

Phytoplankton exhibit a diversity of morphologies, nutritional values, and potential chemical defenses that could affect the feeding and fitness of zooplankton consumers. However, how phytoplankton traits shape plant–herbivore interactions in the marine plankton is not as well understood as for terrestrial or marine macrophytes and their grazers. The occurrence of blooms of marine dinoflagellates such as Karenia brevis suggests that, for uncertain reasons, grazers are unable to capitalize on, or control, this phytoplankton growth—making these systems appealing for testing mechanisms of grazing deterrence. Using the sympatric copepod Acartia tonsa, we conducted a mixed diet feeding experiment to test whether K. brevis is beneficial, toxic, nutritionally inadequate, or behaviorally rejected as food relative to the palatable and nutritionally adequate phytoplankter Rhodomonas lens. On diets rich in K. brevis, copepods experienced decreased survivorship and decreased egg production per female, but the percentage of eggs that hatched was unaffected. Although copepods showed a 6–17% preference for R. lens over K. brevis on some mixed diets, overall high ingestion rates eliminated the possibility that reduced copepod fitness was caused by copepods avoiding K. brevis, leaving nutritional inadequacy and toxicity as remaining hypotheses. Because egg production was dependent on the amount of R. lens consumed regardless of the amount of K. brevis eaten, there was no evidence that fitness costs were caused by K. brevis toxicity. Copepods limited to K. brevis ate 480% as much as those fed only R. lens, suggesting that copepods attempted to compensate for low food quality with increased quantity ingested. Our results indicate that K. brevis is a poor food for A. tonsa, probably due to nutritional inadequacy rather than toxicity, which could affect bloom dynamics in the Gulf of Mexico where these species co-occur.     

Can tropical freshwater zooplankton graze efficiently on cyanobacteria?

Zooplankton may at times graze cyanobacteria. However, their top-down effects are considered to be low, particularly in tropical regions dominated by small-size grazers that may be unable to consume efficiently filamentous or colonial species. Recently, cyanobacteria blooms were reported in the Senegal River hydrosystem. We conducted feeding experiments to assess the ability of copepods (Pseudodiaptomus hessei and Mesocyclops ogunnus), cladocerans (Moina micrura and Ceriodaphnia cornuta), and rotifers (Brachionus angularis, B. falcatus, and Keratella sp.) to control different cyanobacteria (Cylindrospermopsis raciborskii, Anabaena solitaria, A. flos-aquae, and Microcystis aeruginosa). None of the zooplankton species ingested M. aeruginosa. Mesocyclops ogunnus did not consume any of the cyanobacteria. Both cladocerans consumed the smallest filaments of cyanobacteria, whereas all the rotifers and P. hessei consumed a broader food-size spectrum. The functional feeding responses suggest that the concentration and size of the filaments are not the sole criteria for food consumption. The high zooplankton community grazing rates, estimated by applying the clearance rates measured in the laboratory to the in situ zooplankton abundance, indicate that grazing by zooplankton potentially constitutes an important controlling factor for the filamentous cyanobacteria in the tropics.     

Pollution effects on the phytoplankton-zooplankton relationships in an inshore environment

The quantitative and qualitative relationships between the phytoplankton and zooplankton populations were examined for one year in an inshore environment (Elefsis Bay) of the Aegean Sea. It was found that quantitative relationships among these populations were influenced by the trophic level of the environment and their stocks were significantly correlated only in non-polluted conditions. The quantitative relationship between the principal grazers in the zooplankton population (Acartia clausi, Oithona nana) and the phytoplankton dominant species Exuviaella baltica, upon which these copepods were believed to graze preferentially, was tested and the results were discussed.     

舟山海域夏、秋季浮游动物的分布特征及其与环境因子的关系

为更好地了解舟山海域浮游动物的群落结构、生物量和丰度的时空分布特征及其与主要环境因子的关系,分别于2014年7月和10月进行了夏季、秋季两次生态综合调查,并用多维尺度分析法、典范对应分析法对浮游动物群落结构进行了研究。结果表明:夏季舟山海域调查的浮游动物有13类,64种,优势种为背针胸刺水蚤(Centropages dorsispinatus)、圆唇角水蚤(Labidocera rotunda)、中华哲水蚤(Calanus sinicus)、精致真刺水蚤(Euchaeta concinna)、百陶带箭虫(Zonosagitta bedoti)和真刺唇角水蚤(Labidocera euchaeta);秋季鉴定到浮游动物12类,45种,优势种为背针胸刺水蚤(Centropages dorsispinatus)、百陶带箭虫(Zonosagitta bedoti)、双生水母(Diphyes chamissonis)、瓜水母(Beroёcucumis)和中华哲水蚤。夏季浮游动物平均丰度及平均生物量(144.0 ind/m3和176.3 mg/m~3)都分别高于秋季(21.4个/m3和86.3 mg/m3);Shannon-Wiener多样性指数夏季(3.03)高于秋季(2.82),Pielou均匀度指数则是秋季(0.83)高于夏季(0.64);夏季不同区域浮游动物群落之间具有明显的差异,而秋季大部分站位群落之间差异不显著;温度、盐度、叶绿素a浓度和营养盐含量是影响舟山海域浮游动物分布的主要环境因子;与历史资料相比,舟山海域浮游动物丰度及生物量呈下降趋势,其优势种保持较稳定。     

Chemical Response of the Toxic Dinoflagellate Karenia mikimotoi Against Grazing by Three Species of Zooplankton

We investigated the toxicity of Karenia mikimotoi toward three model grazers, the cladoceran Moina mongolica, the copepod Pseudodiaptomus annandalei, and the crustacean Artemia salina, and explored its chemical response upon zooplankton grazing. An induction experiment, where K. mikimotoi was exposed to grazers or waterborne cues from the mixed cultures revealed that K. mikimotoi might be toxic or nutritionally inadequate toward the three grazers. In general, direct exposure to the three grazers induced the production of hemolytic toxins and the synthesis of eicosapentaenoic acid (EPA). Both EPA and the hemolytic toxins from K. mikimotoi decreased the survival rate of the three grazers. In addition, the survival rates of M. mongolica, P. annandalei, and A. salina in the presence of induced K. mikimotoi that had previously been exposed to a certain grazer were lower than their counterparts caused by fresh K. mikimotoi, suggesting that exposure to some grazers might increase the toxicity of K. mikimotoi. The chemical response and associated increased resistance to further grazing suggested that K. mikimotoi could produce deterrents to protect against grazing by zooplankton and that the substances responsible might be hemolytic toxins and EPA.     

Zooplankton interactions with toxic phytoplankton: Some implications for food web studies and algal defence strategies of feeding selectivity behaviour, toxin dilution and phytoplankton population diversity

This study focuses on the interactions between toxic phytoplankton and zooplankton grazers. The experimental conditions used are an attempt to simulate situations that have, so far, received little attention. We presume the phytoplankton community to be a set of species where a population of a toxic species is intrinsically diverse by the presence of coexisting strains with different toxic properties. The other species in the community may not always be high-quality food for herbivorous zooplankton. Zooplankton populations may have developed adaptive responses to sympatric toxic phytoplankton species. Zooplankton grazers may perform a specific feeding behaviour and its consequences on fitness will depend on the species ingested, the effect of toxins, and the presence of mechanisms of toxin dilution and compensatory feeding. Our target species are a strain of the dinoflagellate Alexandrium minutum and a sympatric population of the copepod Acartia clausi. Mixed diets were used with two kinds of A. minutum cells: non-toxic and toxic. The flagellate Rhodomonas baltica and the non-toxic dinoflagellate Alexandrium tamarense were added as accompanying species. The effect of each alga was studied in separate diets. The toxic A. minutum cells were shown to have negative effects on egg production, hatching success and total reproductive output, while, in terms of its effect on fitness, the non-toxic A. minutum was the best quality food offered. R. baltica and A. tamarense were in intermediate positions. In the mixed diets, copepods showed a strong preference for toxic A. minutum cells and a weaker one for A. tamarense cells, while non-toxic A. minutum was slightly negatively selected and R. baltica strongly negatively selected. Although the level of toxins accumulated by copepods was very similar, in both the diet with only toxic A. minutum cells and in the mixed diet, the negative effects on fitness in the mixed diet could be offset by toxin dilution mechanisms. The implications of these findings are the fact that mesozooplankton may not play an important role in phytoplankton blooms development. Phytoplankton endotoxin production does not seem to be an evolutionary stable strategy as a defence against some herbivores.     

Investigations on the feeding habits of the rocky-shore miteHyadesia fusca (Acari: Astigmata: Hyadesiidae): diet range, food preference, food quality, and the implications for distribution patterns

Within the food web of estuarine and marine rocky shore ecosystems phytophagous mites of terrestrial and marine origin constitute an important part as grazers on algae and as a food source for certain arthropods, especially zoophagous mites. This investigation deals with the feeding biology ofHyadesia fusca taking as an example a population located on an artificial rocky shore of the middle Weser estuary in Northern Germany. The species is characterized by a broad diet range; in feeding experiments diatoms, lichens, detritus as well as blue, red and green algae were accepted. Even analyses of faecal pellets produced by field specimen suggest a non-specific feeding habit. However, the influence of certain diets on mortality, offspring number and rearing success showed that the food quality differs significantly. The most suitable food, the UlvaceaeBlidingia, was clearly preferred in a series of pairwise choice tests. These findings correlate with the vertical zonation of the field population i.e.: higher population densities in the vegetation zone dominated byBlidingia. It can be concluded that in addition to abiotic factors food supply could play an important role for distribution patterns of phytophagous mites.     

Role of infochemical mediated zooplankton grazing in a phytoplankton competition model

Infochemicals released by marine phytoplankton play important roles in food web interactions by influencing the feeding behavior and selectivity of zooplanktonic predators. Recent modeling efforts have focused on the role of such chemicals as toxic grazing deterrents in phytoplankton competition. However, infochemicals may also be utilized as grazing cues, leading predators to profitable foraging patches. Here we investigate the role of infochemical mediated zooplankton grazing in a standard 3-species phytoplankton competition model, with the aim of further elucidating the ecological role of phytoplankton derived infochemicals. We then extend this to consider a more realistic 4-species model. The models produce a range of solutions depending on the strength of competition and microzooplankton grazing selectivity. Our key result is that infochemical chemoattractants, which increase the susceptibility of the producer to grazing, can provide a refuge for both competing phytoplankton species by attracting carnivorous copepods to consume microzooplankton grazers in a multi-trophic interaction. Our results indicate that infochemicals potentially have important consequences for the dynamics of marine food webs.     

Response of zooplankton to nutrient enrichment and fish in shallow lakes: a pan-European mesocosm experiment

1. Responses of zooplankton to nutrient enrichment and fish predation were studied in 1998 and 1999 by carrying out parallel mesocosm experiments in six lakes across Europe. 2. Zooplankton community structure, biomass and responses to nutrient and fish manipulation showed geographical and year‐to‐year differences. Fish had a greater influence than nutrients in regulating zooplankton biomass and especially the relative abundances of different functional groups of zooplankton. When fish reduced the biomass of large crustaceans, there was a complementary increase in the biomasses of smaller crustacean species and rotifers. 3. High abundance of submerged macrophytes provided refuge for zooplankton against fish predation but this refuge effect differed notably in magnitude among sites. 4. Large crustacean grazers (Daphnia, Diaphanosoma, Sida and Simocephalus) were crucial in controlling algal biomass, while smaller crustacean grazers and rotifers were of minor importance. Large grazers were able to control phytoplankton biomass even under hypereutrophic conditions (up to 1600 μg TP L^?1) when grazer biomass was high (>80–90 μg dry mass L^?1) or accounted for >30% of the grazer community. 5. The littoral zooplankton community was less resistant to change following nutrient enrichment in southern Spain, at high temperatures (close to 30 °C), than at lower temperatures (17–23 °C) characterising the other sites. This lower resistance was because of a greater importance of nutrients than zooplankton in controlling algal biomass. 6. Apart from the reduced role of large crustacean grazers at the lowest latitude, no consistent geographical patterns were observed in the responses of zooplankton communities to nutrient and fish manipulation.     

Divergence in host use ability of a marine herbivore from two habitat types

We studied ecological divergence of host use ability in a generalist marine herbivore living in two distinct host plant assemblages. We collected Idotea balthica isopods from three populations dominated by the brown alga Fucus vesiculosus and three dominated by the seagrass Zostera marina. In two reciprocal common garden feeding experiments for adult and laboratory‐born juvenile isopods, we found that isopods from both assemblages performed better with their sympatric dominant host species than did isopods allopatric to this host. This indicates parallel divergence of populations according to the sympatric host plant assemblage. Furthermore, initial body size and body size‐dependent mortality differed between populations from the two assemblages. In nature, this may result in lower fitness of immigrants compared with that of residents and consequently reinforce divergence of the populations. Finally, we discuss how phenotypic plasticity and maternal and random effects may associate with the results.     

Species-specific phytoplankton responses to nutrients and zooplankton manipulations in enclosure experiments

1. In situ enclosure experiments were performed in the mesotrophic Bermejales reservoir to evaluate the algal response to changes in the nutrient supply and in the zooplankton size structure and density in a 2 × 2 factorial design. The experiments were conducted during the spring bloom of nanoplanktonic diatoms in 1989. 2. Nutrient enrichment promoted a great increase of phytoplankton biomass indicating a strong nutrient limitation on phytoplankton growth. Total phytoplankton biomass was significantly lower in the Daphina-added enclosures at a given nutrient level and strong direct an indirect effect of zooplankton on phytoplankton community structure and nutrient availability were observed. 3. Most of the nanoplanktonic species were effectively grazed but species with protective coverings and large size colonies were favoured by grazers and small chlorococcales were unaffected probably because of their compensatory high growth rates. The decrease in total biomass imposed by grazers is attributable mainly to the decrease of Cyclotella ocellata, the most abundant species. This taxon suffers two net effects of zooplankton: direct grazing and the indirect decrease of Si availability caused by the growth of C. ocellata which was promoted by P excretion by zooplankton. Indirect effects of grazers on Si availability should, therefore, be taken into account in explaining phytoplankton succession and community structure. 4. In this experiment grazers affected considerably the nanoplanktonic community in Bermejales reservoir. The extent which they were affected, however, depended not only on the algal size as a determinant of edibility but also greatly on the specific nutrient requirements and taxonomic features of the algal species.