LAUSR

Abstract Interactions between phytoplankton and zooplankton affect the overall functioning of lakes. Herbivores are habitually confronted with food of inferior quality, usually a result of low nutrient concentrations in plant material. Large-bodied cladocerans are better competitors for food than small-bodied species but they are more vulnerable to low food quality. Understanding the effects of food quality on zooplankton structure and competition between small - large bodied herbivorous is of considerable interest. We want to find out how differences in C:N:P ratios between phytoplankton and zooplankton communities affect their abundances in a freshwater food web. We want also to assess the role of phytoplankton and zooplankton as sinks of the phosphorus and nitrogen. Therefore, we conducted a 31-day mesocosms experiment with water from a mesotrophic and a eutrophic lake (with natural plankton communities). To simulate changes in the plankton communities large-bodied Daphnia magna and Daphnia pulicaria were added. Samples for zooplankton, phytoplankton and water chemistry were taken every 10 days. Samples for elemental analysis (C:N:P) of seston and zooplankton were collected on the first, and on the final day of the experiment. Our mesocosms experiment showed mismatch in C:P between seston (high) and zooplankton (low), which suggests that most of the phosphorus is incorporated in zooplankton biomass. This evidenced that zooplankton is an effective sink of phosphorus, while nitrogen is accumulated mainly by primary producers. Our results also indicated more stability in stoichiometry with increasing trophic levels of organisms. However, there were significant changes in the zooplankton structure. The increasing dominance of large Daphnia resulted in reduction of C:P ratio in zooplankton. Low food quality (C:P) did not limit the growth of large Daphnia in the experimental conditions, which competed effectively with small planktonic cladocerans and with Rotifera. Over time, inedible algae began to dominate resulting in increase of relative biomass of periphyton grazers, which suggests that plankton community is transformed into littoral system in mesocosms for about 30 days.