Filter feeding and plankton dynamics in a Danish fjord: a review of the importance of flow, mixing and density-driven circulation

Filter feeders are important components in coastal ecosystems where they remove suspended food particles from the water. A key to the understanding of spatial and temporal variations in pelagic biomass is knowledge of the circumstances under which filter feeding takes place. Frequently changing salinities caused by water exchange between the highly saline North Sea and the brackish Baltic create density-driven currents that are of great importance not only for the extent of grazing impact exerted by benthic filter-feeding macro-invertebrates (e.g. bivalves, ascidians and polychaetes), but also for the performance of pelagic filter feeders such as jellyfish and their zooplankton prey organisms. There is good evidence that the feeding demands and effectiveness of filter feeders explain variations in plankton biomass. In a recent project the hydrodynamic conditions in a Danish fjord (Kerteminde Fjord/Kertinge Nor) were brought into focus where it was found that the interaction between hydraulic processes and filter-feeding organisms (ascidians and jellyfish) could explain otherwise unaccountably large variations in the phytoplankton biomass. The interaction between hydrography and filter feeders is typically overlooked although of great importance for marine monitoring programmes using parameters such as phytoplankton biomass routinely in the assessment of environmental conditions.1998 Academic Press Limited