BIOLOGICAL STRUCTURE IN A SHALLOW COVE (KERTINGE-NOR, DENMARK) - CONTROL BY BENTHIC NUTRIENT FLUXES AND SUSPENSION-FEEDING ASCIDIANS AND JELLYFISH

In 1991 Kertinge Nor was characterized by a high benthic autotrophic biomass dominated by the filamentous macroalga Chaetomorpha linum which reduced the flux of inorganic nitrogen and phosphorus from the sediment to the water column. A dense population of filter-feeding ascidians Ciona intestinalis (up to 270 individuals m(-2)) had a significant controlling impact on the phytoplankton biomass. Also, small jellyfish Aurelia aurita were present in extremely high densities (up to 300 individuals m(-3)) controlling the zooplankton population. During a long period of calm and warm weather in the summer of 1992 the filamentous algal mat rose from the sediment surface. High amounts of nutrients were then released to the water column causing a phytoplankton bloom (up to 120 mu g chlorophyll a l(-1)), but the zooplankton biomass remained low due to intense predation by the A. aurita population. It is concluded that Kertinge Nor is an unstable eutrophic ecosystem in which interactions between suspension-feeding organisms and mobilization of nutrients from the sediment pool (enhanced by a former sewage disposal) control the biological structure.