THE IMPACT OF CHIRONOMUS-PLUMOSUS LARVAE ON ORGANIC-MATTER DECAY AND NUTRIENT (N,P) EXCHANGE IN A SHALLOW EUTROPHIC LAKE SEDIMENT FOLLOWINGA PHYTOPLANKTON SEDIMENTATION

The importance of Chironomus plumosus larvae on benthic metabolism and nutrient exchange across the sediment-water interface was evaluated i n a shallow eutrophic lake (Lake Arreskov, Denmark) following a phytop lankton sedimentation. Chironomus plumosus larvae were added to labora tory sediment microcosms, corresponding to a density of 2825 larvae m( -2). Non-inhabited microcosms served as controls. A sedimentation puls e of organic matter was simulated by adding fresh algal material (Chla mydomonas reinhardii) to sediment cores (36 g dry weight m(-2)). The m ineralization was followed by measuring fluxes of O-2, 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 sha llow eutrophic lakes. Chironomids increased the sediment O-2 uptake ap proximate to 3 times more than what would be expected from their own r espiration, indicating a stimulation of microbial activity and decompo sition of organic matter in the sediment. Addition of algae enhanced t he release of CO2, NH4+ and o-P. The excess inorganic C, N and P relea sed 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 inhabit ed by Chironomus plumosus the corresponding numbers were 147, 45 and 7 3%, indicating that mineralization of organic matter also from the ind igenous sediment pool was stimulated by chironomids.