JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. . Wiley and Nordic Society Oikos are collaborating with JSTOR to digitize, preserve and extend access to Oikos. . 1985. Competition among Cladocera: effect of different levels of food supply. -Oikos 44: 243-252.A hypothesis has been suggested which explains the inverse relation between the abundances of large-and small-bodied cladoceran species in natural environments on the basis of competitive interactions and complete overlapping of resource utilization. The different survivorship of juveniles and adults under food limitation is assumed to be the most important trait in the exploitative competition. When food concentration and rate of its regeneration are high at the beginning of population development (eutrophic conditions), the large cladocerans dominate and then outcompete the small ones. The small-bodied species are superior competitors under oligotrophic conditions, when food concentrations vary insignificantly. Marked oscillations of food concentration with short periods of food depletion account for the coexistence of large-and small-bodied cladocerans. A mathematical model of exploitative competition between hypothetical "large" and "small" species has confirmed the resource-dependent competitive ability. Laboratory studies on competitive interactions between large Daphnia pulex and small Ceriodaphnia reticulate have shown similar results.

Summary We studied the fatty acid (FA) composition of six species of Cladocera and six species of Copepoda from five cold‐water lakes, situated in the tundra and/or in the mountains, and eight species of Cladocera and four species of Copepoda from eight warm‐water lakes (including one reservoir) in temperate regions. We asked whether the contrasting temperature would result primarily simply in changes in the percentages (i.e. percentage of total FAs) and absolute contents (quantities) of the long‐chain polyunsaturated fatty acids (PUFAs), eicosapentaenoic acid (20:5n‐3, EPA) and docosahexaenoic acid (22:6n‐3, DHA), or whether there are other FAs with various number of double bonds and/or chain lengths which could be responsible for a putative homeoviscous adaptation. We also aimed to reveal any consistent phylogenetic differences in FA percentages and contents between Cladocera and Copepoda, separable from any temperature effects. Both taxa in warm waters had greater percentages of 18:0, and lower percentages of 14:0 and 18:4n‐3, than in cold waters, but there were no differences in percentages of DHA. In addition, Cladocera, besides the lower percentage of EPA, had higher percentages of 20:0 and 22:0 in warm waters. These patterns in the percentages of 14:0, 18:0, 18:4n‐3, 20:0 and 22:0 are in a good agreement with the hypothesis of homeoviscous adaptation. Thus, the role of EPA, and particularly DHA, as unique regulators of the homeoviscous adaptation of the zooplankton may have been overestimated. Overall, we confirmed the known differences between Cladocera and Copepoda, namely higher percentages of EPA in Cladocera and higher percentages of DHA in Copepoda. However, there was c. 50% overlap in the ranges of the percentage of EPA in Cladocera and Copepoda, while the ranges in the content of EPA per unit organic carbon in Cladocera and Copepoda overlapped completely. Differences in the percentages and content of DHA between Cladocera and Copepoda were statistically significant and invariant with temperature, and therefore are probably due to phylogenetic factors, rather than any temperature adaptation. Contrasting temperature was not associated with significant differences in the contents of EPA and DHA per unit of organic carbon within the taxa studied. If this remained the case in a warming climate, such warming would be unlikely to reduce the accumulation of these important PUFAs in the zooplankton, at least if species composition was unchanged. However, if there were shifts in the proportions of Cladocera and Copepoda in the zooplankton, for example fewer copepods as temperature rises, a decrease of the flux of PUFA in the ecosystem is plausible, taking into account the phylogenetic (and temperature invariant) differences in DHA between the two groups.

Recent changes in climate and eutrophication have caused increases in oxygen depletion in both freshwater and marine ecosystems. However, the impact of oxygen stress on zooplankton, which is the major trophic link between primary producers and fish, remains largely unknown in lakes. Therefore, we studied 41 lakes with different trophic and oxygen conditions to assess the role of oxygen stress on zooplankton communities and carbon transfer between phytoplankton and zooplankton. Samples were collected from each lake at the peak of summer stratification from three depth layers (the epilimnion, metalimnion, and hypolimnion). Our results revealed that freshwater zooplankton were relatively tolerant to anoxic conditions and the greatest changes in community structure were found in lakes with the highest oxygen deficits. This caused a switch in dominance from large to small species and reduced the zooplankton biomass in lower, anoxic layers of water, but not in the upper layers of water where the oxygen deficits began. This upper anoxic layer could thus be a very important refuge for zooplankton to avoid predation during the day. However, the reduction of zooplankton in the lower water layers was the main factor that reduced the effectiveness of carbon transfer between the phytoplankton and zooplankton.

We evaluated how eutrophication and humification impacted nutrient cycles and the efficiency of carbon transfer in the planktonic food webs of 12 lakes in north-eastern Poland that differed in trophic state. Our results indicated that the effectiveness of carbon transfer between phytoplankton and zooplankton varied from 0.0005% to 0.14%, which is much lower than the theoretical 10%. The highest efficiency of carbon transfer occurred in the mesotrophic lakes due to the higher hypolimnetic zooplankton production, while the lowest efficiency was observed in the dystrophic lakes and in one eutrophic lake that was dominated by cyanobacteria. Inedible algae (e.g. Gonyostomum semen) and cyanobacteria appeared to be the main factors reducing the efficiency of the transfer of matter in pelagic food webs. The results of our study showed that plankton communities are a key component of the nutrient cycle in freshwater food webs. Phytoplankton were a very effective nitrogen sink, and in the mesotrophic lakes, up to 99% of the total nitrogen was sequestrated in phytoplankton. As a result, there was a depletion of inorganic nitrogen in the upper water layer. Furthermore, zooplankton were an important phosphorus sink, thus significantly influencing the nutrient cycles.

Mathematical modelling and laboratory experiments were used to study how exploitative competition and predation influence the species structure in cladoceran community. For five species of Cladocera (Sida crystallina, Daphnia magna, Simocephalus vetulus, Daphnia longispina, and Diaphanosoma brachyurum), representing a gradient of body size, population characteristics were described as functions of food concentration. Abundance dynamics were simulated in mixed species cultures and invasion experiments under different levels of food supply corresponding to oligo-, meso-, and eutrophic conditions. Separate simulations were also run including and excluding (fish) predation. The competitive ability of each species was estimated as the values of the population equilibrium food concentration. Simulation results showed that for the no-predator scenario, increases in the level of food supply promoted species coexistence while under lower food concentrations only one species remained at the end of the simulation runs. When predation was allowed, the number of species that coexisted at the end of the simulations increased up to four species, indicating that predator pressure facilitated species coexistence because it shortened periods of food depletion. Simulation results were verified in laboratory experiments which suggested that population equilibrium food concentration can be used as an estimate of competitive ability. Finally, species structure and relative abundance in Lake Naroch (Belarus) during the summer of 2004 was found to be consistent with our results from computer simulation and laboratory experiments with regard to competition and predation impacts on zooplankton community.