Bottom-up effects of lake sediment on pelagic food-web compartments: a mesocosm study

1. Sediment plays a key role in organic matter (OM) and internal nutrient cycling in lakes. The role of sediment as a source of OM and its potential bottom-up effects on the pelagic food web have rarely been studied. Particularly, the influence of the biochemical composition of sediment OM on pelagic compartments remains largely unknown. 2. During a 5-month experiment, we studied the influence of two different sediments added at the bottom of large replicated mesocosms on the biomass of seston and zooplankton, and their elemental and lipid compositions. The influence of sediment treatments on sedimentation rates, elemental and biochemical compositions and potential biodegradability of recently sedimented OM (c. 1 week) was also examined. 3. The two added sediments (S-1 and S-2) presented contrasting elemental and biochemical compositions and potential biodegradabilities. According to their contents of organic carbon, nitrogen, proteins, sugars and polyunsaturated fatty acids, S-2 appeared to be much more biodegradable than S-1. Therefore, the S-2 sediment was expected to release more nutrients and OM to the water column than S-1, leading to changes in communities, stoichiometry and lipid compositions of pelagic compartments. 4. Probably due to its very poor content of labile compounds, the presence of S-1 at the bottom of the mesocosms did not induce changes in the biomass of seston and zooplankton. Only few changes in the stoichiometry of these compartments were observed. In contrast, S-2 sediment released more phosphorus and dissolved OM into the water column than S-1. As a result, the S-2 treatment induced an increase in seston biomass and therefore in zooplankton biomass via herbivory. 5. None of the sediment treatments affected the lipid composition of seston and zooplankton. Moreover, neither S-1 nor S-2 induced changes in the sedimentation rates, elemental and lipid compositions, and potential biodegradability of recent sediments. Our mesocosm experiment suggests that differences in the quality of lake sediments lead to moderate changes in the pelagic communities in the absence of planktivorous or omnivorous fish. 6. Our results could partly explain the efficiency of biomanipulation for improving water quality of eutrophic lakes despite potential nutrient release from sediment.