The importance of Chironomus plumosus larvae on benthic metabolism and nutrient exchange across the sediment-water interface was evaluated in a shallow eutrophic lake (Lake Arreskov, Denmark) following a phytoplankton sedimentation. Chironomus plumosus larvae were added to laboratory sediment microcosms, corresponding to a density of 2825 larvae m-2. Non-inhabited microcosms served as controls. A sedimentation pulse of organic matter was simulated by adding fresh algal material (Chlamydomonas reinhardii) to sediment cores (36 g dry weight m-2). The mineralization was followed by measuring fluxes of O2, CO2, dissolved inorganic nitrogen and phosphate. A rapid clearance of algae from the water column in faunated microcosms suggested that chironomids may be of major importance in controlling phytoplankton concentrations in shallow eutrophic lakes. Chironomids increased the sediment O2 uptake ≈ 3 times more than what would be expected from their own respiration, indicating a stimulation of microbial activity and decomposition of organic matter in the sediment. Addition of algae enhanced the release of CO2, NH+ and o-P. The excess inorganic C, N and P released in amended non-inhabited sediment after 36 days corresponded to 65, 31 and 58% of the C, N and P in the added algae. In sediment inhabited by Chironomus plumosus the corresponding numbers were 147, 45 and 73%, indicating that mineralization of organic matter also from the indigenous sediment pool was stimulated by chironomids. © 1998 Kluwer Academic Publishers.
