Plankton community of a polyhumic lake with and without Daphnia longispina (Cladocera)

The impact of Daphnia longispina (Cladocera) on the plankton food web was studied in a polyhumic lake where this species comprised almost all zooplankton biomass. Plastic enclosures (volume 7 m³) were inserted into the lake retaining the initial water stratification except that in one enclosure zooplankton was removed. After the removal of Daphniaa rotifer, Keratella cochlearis, ciliates and heterotrophic nanoflagellates increased markedly and the density and biomass of bacteria decreased. Edible algal species, Cryptomonas rostratiformisand three small chrysophytes,Ochromonas, Pedinella and Spinifermonas, took advantage of the removal of Daphnia, while more grazing-resistant species declined. In spite of the changes in the species composition of phytoplankton, the removal of Daphnia did not affect the biomass, primary production or respiration of plankton. The results implied that the density of heterotrophic flagellates and ciliates was controlled by Daphnia, but in its absence the former took its role as the bacterial grazers.     

Copepods act as a switch between alternative trophic cascades in marine pelagic food webs

A recent meta‐analysis indicates that trophic cascades (indirect effects of predators on plants via herbivores) are weak in marine plankton in striking contrast to freshwater plankton ( Shurin et al. 2002 , Ecol. Lett., 5, 785–791). Here we show that in a marine plankton community consisting of jellyfish, calanoid copepods and algae, jellyfish predation consistently reduced copepods but produced two distinct, opposite responses of algal biomass. Calanoid copepods act as a switch between alternative trophic cascades along food chains of different length and with counteracting effects on algal biomass. Copepods reduced large algae but simultaneously promoted small algae by feeding on ciliates. The net effect of jellyfish on total algal biomass was positive when large algae were initially abundant in the phytoplankton, negative when small algae were dominant, but zero when experiments were analysed in combination. In contrast to marine systems, major pathways of energy flow in Daphnia‐dominated freshwater systems are of similar chain length. Thus, differences in the length of alternative, parallel food chains may explain the apparent discrepancy in trophic cascade strength between freshwater and marine planktonic systems.     

Contrasting effects of a cladoceran (Daphnia galeata) and a calanoid copepod (Eodiaptomus japonicus) on algal and microbial plankton in a Japanese lake, Lake Biwa

Macrozooplankton may affect algal and microbial plankton directly through grazing or predation and indirectly through nutrient regeneration. They may also affect potential prey positively by removing alternative predators. Here, we examined the effects of a cladoceran (Daphnia) and a calanoid copepod (Eodiaptomus) on algal and microbial plankton in a Japanese lake using in situ experiments in which we manipulated the nutrient supply and biomass of these macrozooplankton. The response of algal and microbial plankton to macrozooplankton was diverse and varied depending on the level of nutrient supply. Eodiaptomus seemed to feed mainly on large algae (>20 µm) and microzooplankton, while direct grazing by Daphnia on algae, bacteria, heterotrophic nanoflagellates (HNF), and microzooplankton (ciliates, heliozoa, and rotifers) was pronounced. Trophic linkages within these microbial plankton was also suggested; bacteria were grazed by HNF and these in turn were grazed by microzooplankton. When the nutrient supply was high, both HNF and microzooplankton were exposed to higher amounts of algae and lower bacterial abundance. Moreover, nutrient regeneration by daphnids and Eodiaptomus copepods seemed to differentially stimulate the growth of algae and bacteria. The results suggest that the relationship between macrozooplankton and microbial plankton cannot be fully understood without taking into consideration not only the feeding characteristics of the macrozooplankton, but also the food web structure, the subsidized algal resource, and nutrient regeneration from the macrozooplankton.     

Zooplankton interactions in an enclosure experiment: insights from stable isotope analyses

1. Density gradients of cladocerans and copepods were generated in an enclosure experiment to compare the impact on the plankton of a filter feeder (Daphnia hyalina × galeata) with that of more selective feeders (calanoid and cyclopoid copepods). The experiment was conducted in situ over 25 days during spring in a mesotrophic lake, Schöhsee, Germany. 2. The plankton community was monitored regularly. Daphniids were able to graze on the phytoplankton present, which mainly consisted of small (<1000 μm³) species, whereas copepods did not show any impact on algae. 3. At the end of the experiment, Daphnia and remaining cyclopoid copepods were harvested and sorted manually, prior to analyses for stable isotopes of carbon and nitrogen. Daphniids from mesocosms stocked purely with differing densities of Daphnia showed little variability in stable isotope values, whereas those that thrived in enclosure bags together with copepods exhibited lower δ¹³C values. 4. The change in Daphniaδ¹³C indicates a change of food sources, modified by the presence of the copepods: the higher the mean abundance of copepods in the enclosures, the more ¹³C‐depleted the daphniids. Increasing abundance of high nucleic acid (HNA) bacteria in the copepod bags may account for the trend in Daphniaδ¹³C via increased grazing on the bacteria themselves, or via grazing on phytoplankton utilising isotopically light CO₂ from respiratory release. 5. Cyclopoid copepod stable isotope signatures were related to Daphnia and copepod abundances in copepod bags, suggesting that cyclopoids preyed on the available zooplankton.     

Seasonal dynamics of crustacean zooplankton, heterotrophic nanoflagellates and bacteria in a shallow, eutrophic lake

1. The seasonal development of crustacean zooplankton, heterotrophic nanoflagellates (HNF) and bacteria was examined in Grosser Binnensee, a shallow, eutrophic lake in northern Germany. The grazing impact of Daphnia on bacteria and nanoflagellates was estimated from field data on population abundances and from clearance rates obtained in laboratory experiments. 2. The seasonal succession of zooplankton showed distinct peaks of Daphnia magna, cyclopopid copepods, Bosmina longirostris and Daphnia galeata and D. hynlina. The population dynamics of Dapfinia had the strongest impact on all sestonic components. Daphnia maxima coincided with clearwater phases, and were negatively correlated with particulate organic carbon (POC), HNF and phytoplankton. Bacterial abundance was only slightly affected although daphnids were at times more important as bacterial consumers than HNF, as estimated from measured bacterial clearance rates. Other crustaceans (copepods, Bosmina) were probably of minor importance as grazers of bacteria and nanoplankton. 3. HNF abundance varied from 550 ml^?1 to more than 30000 ml^?1. HNF appeared to be suppressed by daphnids and reached highest densities when copepods dominated the metazooplankton. The variation in HNF abundance was not reflected in the concentration of heterotrophic bacteria, which fluctuated rather irregularly between 5 and 20 ± 10⁶ ml^?1. Long filamentous bacteria which were probably resistant to protozoan grazing, however, appeared parallel to the development of HNF. These bacterial cells, although small in number, could comprise more than 30% of the total bacterial biomass.     

Response of fish and plankton to nutrient loading reduction in eight shallow Danish lakes with special emphasis on seasonal dynamics

1. For 13 years the response of the plankton and fish community to a decline in external phosphorus loading was studied in eight lakes with a mean depth <5 m. We conducted chi‐square analyses of sign of slope (positive or negative) of bimonthly averages of plankton variables for the eight lakes versus time. For fish, we compared results from two periods, i.e. 1989–1994 versus 1994–2001 as less data were available. 2. Fish community structure tended to respond to the lowered concentration of total phosphorus (TP), although not all changes were significant. While catch per unit effort (multi‐mesh sized gill nets) of cyprinids (especially bream, Abramis brama and roach, Rutilus rutilus) was highest in the first 5‐year period, the quantitative importance particularly of perch (Perca fluviatilis), pike (Esox lucius) and rudd (Scardinius erythropthalmus), a littoral species, increased significantly after 1994. 3. No changes occurred in zooplankton biomass, except for an increase in November and December. Biomass of small cladocerans, however, declined during summer and autumn, and the proportion of Daphnia to cladoceran biomass also increased. Average body weight of Daphnia and that of all cladocerans increased. The proportion of calanoids among copepods decreased in summer and the average body weight of cyclopoids and calanoids decreased during summer and autumn/early winter. 4. Total biovolume of phytoplankton declined significantly in March to June and tended to decline in November and December as well, while no significant changes were observed during summer and autumn. Non‐heterocystous cyanobacteria showed a decreasing trend during summer and autumn, while heterocystous cyanobacteria increased significantly in late summer. An increase in late summer was also evident for cryptophytes and chrysophytes, while diatoms tended to decline during most seasons. 5. We conclude that phytoplankton, and probably also fish, responded rapidly to reduced loading, whereas the effect on zooplankton was less pronounced. However, increases in body weight of cladocerans and the zooplankton to phytoplankton biomass ratio during summer indicate reduced top‐down control on zooplankton and enhanced grazing on phytoplankton. This conclusion is supported by a tendency for fish biomass to decline and a shift towards greater dominance by piscivores and, thus, an increased likelihood of predator control of zooplanktivorous cyprinids.     

Zooplankton-mediated changes of bacterial community structure

Enclosure experiments in the mesotrophic Schöhsee in northern Germany were designed to study the impact of metazooplankton on components of the microbial food web (bacteria, flagellates, ciliates). Zooplankton was manipulated in 500-liter epilimnetic mesocosms so that either Daphnia or copepods were dominating, or metazooplankton was virtually absent. The bacterial community responded immediately to changes in zooplankton composition. Biomass, productivity, and especially the morphology of the bacteria changed drastically in the different treatments. Cascading predation effects on the bacterioplankton were transmitted mainly by phagotrophic protozoans which had changed in species composition and biomass. When Daphnia dominated, protozoans were largely suppressed and the original morphological structure of the bacteria (mainly small rods and cocci) remained throughout the experiment. Dominance of copepods or the absence of metazoan predators resulted in a mass appearance of bacterivorous protists (flagellates and ciliates). They promoted a fast decline of bacterial abundance and a shift to the predominance of morphologically inedible forms, mainly long filaments. After 3 days they formed 80–90% of the bacterial biomass. The results indicate that metazooplankton predation on phagotrophic protozoans is a key mechanism for the regulation of bacterioplankton density and community structure.Correspondence to: K. Jürgens.     

Grazer control and nutrient limitation of phytoplankton biomass in two Australian reservoirs

1. Grazer and nutrient controls of phytoplankton biomass were tested on two reservoirs of different productivity to assess the potential for zooplankton grazing to affect chlorophyll/phosphorus regression models under Australian conditions. Experiments with zooplankton and nutrients manipulated in enclosures, laboratory feeding trials, and the analysis of in-lake plankton time series were performed. 2. Enclosures with water from the more productive Lake Hume (chlorophyll a = 3–17.5 μg l^–1), revealed significant zooplankton effects on chlorophyll a in 3/6, phosphorus limitation in 4/6 and nitrogen limitation in 1/6 of experiments conducted throughout the year. Enclosures with water from the less productive Lake Dartmouth (chlorophyll a = 0.8–3.5 μg l^–1), revealed significant zooplankton effects in 5/6, phosphorus limitation in 5/6 and nitrogen limitation in 2/6 of experiments. 3. While Lake Hume enclosure manipulations of the biomass of cladocerans (Daphnia and Diaphanosoma) and large copepods (Boeckella) had negative effects, small copepods (Mesocyclops and Calamoecia) could have positive effects on chlorophyll a. 4. In Lake Hume, total phytoplankton biovolume was negatively correlated with cladoceran biomass, positively with copepod biomass and was uncorrelated with total crustacean biomass. In Lake Dartmouth, total phytoplankton biovolume was negatively correlated with cladoceran biomass, copepod biomass and total crustacean biomass. 5. In both reservoirs, temporal variation in the biomass of Daphnia carinata alone could explain more than 50% of the observed variance in total phytoplankton biovolume. 6. During a period of low phytoplankton biovolume in Lake Hume in spring–summer 1993–94, a conservative estimate of cladoceran community grazing reached a maximum of 0.80 day^–1, suggesting that Cladocera made an important contribution to the development of the observed clear-water phase. 7. Enclosure experiments predicted significant grazing when the Cladocera/Phytoplankton biomass ratio was greater than 0.1; this threshold was consistently exceeded during clear water phase in Lake Hume. 8. Crustacean length had a significant effect on individual grazing rates in bottle experiments, with large Daphnia having highest rates. In both reservoirs, mean crustacean length was negatively correlated with phytoplankton biovolume. The observed upper limit of its variation was nearly twice as high compared to other world lakes.     

Qualitative importance of the microbial loop and plankton community structure in a eutrophic lake during a bloom of cyanobacteria

Plankton community structure and major pools and fluxes of carbon were observed before and after culmination of a bloom of cyanobacteria in eutrophic Frederiksborg Slotssø, Denmark. Biomass changes of heterotrophic nanoflagellates, ciliates, microzooplankton (50 to 140 μm), and macrozooplankton (larger than 140 μm) were compared to phytoplankton and bacterial production as well as micro- and macrozooplankton ingestion rates of phytoplankton and bacteria. The carbon budget was used as a means to examine causal relationships in the plankton community. Phytoplankton biomass decreased and algae smaller than 20 μm replacedAphanizomenon after the culmination of cyanobacteria. Bacterial net production peaked shortly after the culmination of the bloom (510 μg C liter^?1 d^?1 and decreased thereafter to a level of approximately 124 μg C liter^?1 d^?1. Phytoplankton extracellular release of organic carbon accounted for only 4–9% of bacterial carbon demand. Cyclopoid copepods and small-sized cladocerans started to grow after the culmination, but food limitation probably controlled the biomass after the collapse of the bloom. Grazing of micro- and macrozooplankton were estimated from in situ experiments using labeled bacteria and algae. Macrozooplankton grazed 22% of bacterial net production during the bloom and 86% after the bloom, while microzooplankton (nauplii, rotifers and ciliates larger than 50 μm) ingested low amounts of bacteria and removed 10–16% of bacterial carbon. Both macro-and microzooplankton grazed algae smaller than 20 μm, although they did not control algal biomass. From calculated clearance rates it was found that heterotrophic nanoflagellates (40–440 ml^?1) grazed 3–4% of the bacterial production, while ciliates smaller than 50 μm removed 19–39% of bacterial production, supporting the idea that ciliates are an important link between bacteria and higher trophic levels. During and after the bloom ofAphanizomenon, major fluxes of carbon between bacteria, ciliates and crustaceans were observed, and heterotrophic nanoflagellates played a minor role in the pelagic food web.     

Responses of cyanobacteria to herbivorous zooplankton across predator regimes: who mows the bloom?

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Seasonal and vertical distribution of the ciliated protozoa and micrometazoa in Kaštela Bay (central Adriatic)

Seasonal and vertical distribution of tintinnids, non-loricate ciliates and micrometazoa were studied in Kaštela Bay (central Adriatic Sea) throughout 1995. The species composition of tintinnids and copepods were studied as well. This is the first estimation of non-loricate ciliate biomass in the coastal area of the central Adriatic. Non-loricate ciliates were quantitatively the best represented ciliated protozoa, whereas nauplii were the most numerous micrometazoan organisms. Temperature affected the distribution of most micrometazoan components of the zooplankton and that of non-loricate ciliates. The temperature-dependent presence of individual size categories of non-loricate ciliates was also established. Apart from the interaction between microzooplankton groups, the influence of biotic factors, such as phytoplankton, bacteria, non-pigmented nanoflagellates (NNF) and mesozooplankton, was also discussed. The abundance of ciliates was controlled by both food supply (phytoplankton and NNF) and micrometazoan grazing. The results point to very complex trophic relationships within the planktonic community, suggesting that microzooplankton could be an important link between the microbial food web and higher trophic levels. Received in revised form: 8 November 2000 Electronic Publication     

Grazing by rotifers and crustacean zooplankton on nanoplanktonic protists

Predation on nanoflagellates by metazoan zooplankton was investigated using a radioactively labeled flagellate, Poterioochromonas malhamensis, as a tracer cell in laboratory incubations of freshly collected plankton assemblages. Experiments conducted in the fall, winter and spring indicated that rotifers dominated the grazing on nanoflagellates by metazoans in the winter (68%) and spring (92%). Rotifer grazing was not determined in the autumn. It is likely that the greater impact of rotifer grazing in the spring was due to the occurrence of abundant filamentous cyanobacteria and gelatinous colonial phytoplankton which selectively depressed feeding rates of crustaceans compared to rotifers. Crustacean predation on nanoflagellates was highest in the autumn when cladocerans (primarily Daphnia spp.) were abundant. Predation by metazoan zooplankton in this lake appeared capable of removing the total standing stock of heterotrophic and phototrophic nanoplankton in < 1 d. Impacts of ciliated protozoa on nanoplankton, calculated from abundances and literature feeding rates, ranged from approximately one-third to four times that of metazoan predation depending on season and method of calculation. The relative importance of the different groups of predators appears to vary seasonally which is expected to alter the transfer of energy, carbon and nutrients from bacteria to higher trophic levels.     

Metabarcoding and metabolome analyses of copepod grazing reveal feeding preference and linkage to metabolite classes in dynamic microbial plankton communities

In order to characterize copepod feeding in relation to microbial plankton community dynamics, we combined metabarcoding and metabolome analyses during a 22‐day seawater mesocosm experiment. Nutrient amendment of mesocosms promoted the development of haptophyte (Phaeocystis pouchetii)‐ and diatom (Skeletonema marinoi)‐dominated plankton communities in mesocosms, in which Calanus sp. copepods were incubated for 24 h in flow‐through chambers to allow access to prey particles (<500 μm). Copepods and mesocosm water sampled six times spanning the experiment were analysed using metabarcoding, while intracellular metabolite profiles of mesocosm plankton communities were generated for all experimental days. Taxon‐specific metabarcoding ratios (ratio of consumed prey to available prey in the surrounding seawater) revealed diverse and dynamic copepod feeding selection, with positive selection on large diatoms, heterotrophic nanoflagellates and fungi, while smaller phytoplankton, including P. pouchetii, were passively consumed or even negatively selected according to our indicator. Our analysis of the relationship between Calanus grazing ratios and intracellular metabolite profiles indicates the importance of carbohydrates and lipids in plankton succession and copepod–prey interactions. This molecular characterization of Calanus sp. grazing therefore provides new evidence for selective feeding in mixed plankton assemblages and corroborates previous findings that copepod grazing may be coupled to the developmental and metabolic stage of the entire prey community rather than to individual prey abundances.     

Planktonic food chains of a highly humic lake

The development and metabolism of epilimnetic plankton from a highly humic lake was followed in late summer, when the predominant zooplankton species, Daphnia longispina, was very abundant (ca. 200 ind. l^?1). The experiment was made in two tanks: one with an unaltered plankton assemblage and one with larger zooplankton removed. The scarce phytoplankton community was also simple, consisting mainly of one Cryptomonas and two Mallomonas species. The abundance and species composition of smaller plankton was heavily influenced by grazing of Daphnia. In particular, the biomass, of heterotrophic flagellates increased after the removal of Daphnia. The biomass and production of bacterioplankton were not affected, and remained several times higher than that of phytoplankton. Bacterial production and grazing on bacteria were balanced, and when Daphnia was removed its grazing activity was compensated by flagellates. The removal of Daphnia did not affect the respiration or community net production of plankton. Among organisms smaller than zooplankton, bacteria seemed to be responsible for most of the respiration. The community net production was consistently negative even at the water surface, indicating an allochthonous carbon source. The results suggest that phytoplankton primary production was insufficient for the secondary production in the epilimnetic water of the study lake. The food requirements of bacteria and zooplankton, as well as of flagellates, each exceeded that supplied by phytoplankton primary production. The simple food chains in this experiment made it possible to reveal the functioning of the community so completely that dissolved organic matter is certainly comparable to or exceeds the importance of phytoplankton primary production as an energy and carbon source for food webs in this humic lake.     

Response of phytoplankton and bacteria to nutrients and zooplankton: a mesocosm experiment

Although both nutrient inputs and zooplankton grazing are importantto phytoplankton and bacteria in lakes, controversy surroundsthe relative importance of grazing pressure for these two groupsof organisms. For phytoplankton, the controversy revolves aroundwhether zooplankton grazers, especially large cladocerans likeDaphnia, can effectively reduce phytoplankton populations regardlessof nutrient conditions. For bacteria, little is known aboutthe balance between possible direct and indirect effects ofboth nutrients and zooplankton grazing. However, there is evidencethat bacteria may affect phytoplankton responses to nutrientsor zooplankton grazing through direct or apparent competition.We performed a mesocosm experiment to evaluate the relativeimportance of the effects of nutrients and zooplankton grazingfor phytoplankton and bacteria, and to determine whether bacteriamediate phytoplankton responses to these factors. The factorialdesign crossed two zooplankton treatments (unsieved and sieved)with four nutrient treatments (0, 0.5, 1.0 and 2.0 µgphosphorus (P) l^–1 day^–1 together with nitrogen(N) at a N:P ratio of 20:1 by weight). Weekly sieving with 300µm mesh reduced the average size of crustacean zooplanktonin the mesocosms, decreased the numbers and biomass of Daphnia,and increased the biomass of adult copepods. Nutrient enrichmentcaused significant increases in phytoplankton chlorophyll a(4–5x), bacterial abundance and production (1.3x and 1.6x,respectively), Daphnia (3x) and total zooplankton biomass (2x).Although both total phytoplankton chlorophyll a and chlorophylla in the <35 µm size fraction were significantly lowerin unsieved mesocosms than in sieved mesocosms, sieving hadno significant effect on bacterial abundance or production.There was no statistical interaction between nutrient and zooplanktontreatments for total phytoplankton biomass or bacterial abundance,although there were marginally significant interactions forphytoplankton biomass <35 µm and bacterial production.Our results do not support the hypothesis that large cladoceransbecome less effective grazers with enrichment; rather, the differencebetween phytoplankton biomass in sieved versus unsieved zooplanktontreatments increased across the gradient of nutrient additions.Furthermore, there was no evidence that bacteria buffered phytoplanktonresponses to enrichment by either sequestering P or affectingthe growth of zooplankton.     

Calanoid copepod grazing on phytoplankton: seasonal experiments on natural communities

Calanoid copepods are major components of most lacustrine ecosystems and their grazing activities may influence both phytoplankton biomass and species composition. To assess this we conducted four seasonal, in situ, grazing experiments in eutrophic Lake Rotomanuka, New Zealand. Ambient concentrations of late stage copepodites and adults of calanoid copepods (predominantly Calamoecia lucasi, but with small numbers of Boeckella delicata) were allowed to feed for nine days on natural phytoplankton assemblages suspended in the lake within 1160 litre polyethylene enclosures. The copepods reduced the total phytoplankton biomass of the dominant species in all experiments but were most effective in summer (the time of highest grazer biomass) followed by spring and autumn. In response to grazing pressure the density of individual algal species showed either no change or a decline. There were no taxa which increased in density in the presence of the copepods. The calanoid copepods suppressed the smallest phytoplankton species (especially those with GALD (Greatest Axial Linear Dimension) < µm) and there appeared to be no selection of algae on the basis of biovolume. Algal taxa which showed strong declines in abundance in the presence of the copepods include Cyclotella stelligera, Coelastrum spp., Trachelomonas spp., Cryptomonas spp., and Mallomonas akrokomos. Calanoid copepods are considered important grazers of phytoplankton biomass in this lake. The study supports the view that high phytoplankton:zooplankton biomass ratios and large average algal sizes characteristic of New Zealand lake plankton may, at least partly, be caused by year round grazing pressure on small algae shifting the competitive balance in favour of larger algal species.     

Possible food chain relationships between bacterioplankton,protozoans, and cladocerans in a reservoir

Ingestion of fluorescent particles by natural protozoan assemblage was studied in the Řimov Reservoir (Southern Bohemia) from April to October, 1987. Attached and free-living bacterial abundance, proportion of active bacteria, density of suspended particles and biomass of cladocerans were also monitored. Heterotrophic nanoflagellates (HNF; 5–12.8 10²ml⁻¹) were the dominant bacterial micrograzers during the spring period and consumed 3 to 9% of the total bacteria per day. After the spring phytoplankton bloom maximum densities of suspended particles and attached bacteria (up to 28% of the total counts) were found. Development of cladocerans in May sharply decreased the proportion of attached bacteria and kept them below 5% of the total counts. All the studied components of plankton except Cladocera decreased during the clearwater phase. The most significant drop was observed in the numbers of protozoans, and they were negligible for bacterial elimination. Bacterial losses during that time apparently were due to cladoceran grazing. During the summer period, ciliates (15–142 ml⁻¹) were mostly dominant micrograzers, and protozoan community grazing increased up to 21% of bacterial standing stock per day. The proportion of active bacteria was strongly correlated with protozoan grazing (r=0.83).     

Impacts of a submerged plant (Elodea canadensis) on interactions between roach (Rutilus rutilus) and its invertebrate prey communities in a lake littoral zone

1. Using 5‐m² field enclosures, we examined the effects of Elodea canadensis on zooplankton communities and on the trophic cascade caused by 4–5 year old (approximately 16 cm) roach. We also tested the hypothesis that roach in Elodea beds use variable food resources as their diet, mainly benthic and epiphytic macroinvertebrates, and feed less efficiently on zooplankton. Switching of the prey preference stabilises the zooplankton community and, in turn, also the fluctuation of algal biomass. The factorial design of the experiment included three levels of Elodea (no‐, sparse‐ and dense‐Elodea) and two levels of fish (present and absent). 2. During the 4‐week experiment, the total biomass of euplanktonic zooplankton, especially that of the dominant cladoceran Daphnia longispina, decreased with increase in Elodea density. The Daphnia biomass was also reduced by roach in all the Elodea treatments. Thus, Elodea provided neither a favourable habitat nor a good refuge for Daphnia against predation by roach. 3. The electivity of roach for cladocerans was high in all the Elodea treatments. Roach were able to prey on cladocerans in Elodea beds, even when the abundance and size of these prey animals were low. In addition to cladocerans, the diet of roach consisted of macroinvertebrates and detrital/plant material. Although the biomass of macroinvertebrates increased during the experiment in all Elodea treatments, they were relatively unimportant in roach diets regardless of the density of Elodea beds. 4. Euplanktonic zooplankton species other than Daphnia were not affected by Elodea or fish and the treatments had no effects on the total clearance rate of euplanktonic zooplankton. However, the chlorophyll a concentration increased with fish in all the Elodea treatments, suggesting that fish enhanced algal growth through regeneration of nutrients. Thus, our results did not unequivocally show that Elodea hampered the trophic cascade of fish via lowered predation on grazing zooplankton. 5. In treatments with dense Elodea beds (750 g FW m^?2), chlorophyll a concentration was always low suggesting that phytoplankton production was controlled by Elodea. Apparently, the top‐down control of phytoplankton biomass by zooplankton was facilitated by the macrophytes and operated simultaneously with control of phytoplankton production by Elodea.     

<Emphasis Type="Italic">Daphnia</Emphasis> versus copepod impact on summer phytoplankton: functional compensation at both trophic levels

Here we report on a mesocom study performed to compare the top-down impact of microphagous and macrophagous zooplankton on phytoplankton. We exposed a species-rich, summer phytoplankton assemblage from the mesotrophic Lake Schöhsee (Germany) to logarithmically scaled abundance gradients of the microphagous cladoceran Daphnia hyalina×galeata and of a macrophagous copepod assemblage. Total phytoplankton biomass, chlorophyll a and primary production showed only a weak or even insignificant response to zooplankton density in both gradients. In contrast to the weak responses of bulk parameters, both zooplankton groups exerted a strong and contrasting influence on the phytoplankton species composition. The copepods suppressed large phytoplankton, while nanoplanktonic algae increased with increasing copepod density. Daphnia suppressed small algae, while larger species compensated in terms of biomass for the losses. Autotrophic picoplankton declined with zooplankton density in both gradients. Gelatinous, colonial algae were fostered by both zooplankton functional groups, while medium-sized (ca. 3,000 µm³), non-gelatinous algae were suppressed by both. The impact of a functionally mixed zooplankton assemblage became evident when Daphnia began to invade and grow in copepod mesocosms after ca. 10 days. Contrary to the impact of a single functional group, the combined impact of both zooplankton groups led to a substantial decline in total phytoplankton biomass.