Grazing impact of filter-feeding zoobenthos in a Danish fjord

The grazing impact has been evaluated of filter-feeding animals in Odense Fjord (Denmark) that is characterized by large biomass of filter feeding polychaetes (Nereis diversicolor), clams (Mya arenaria) and cockles (Cerastoderma glaucum). The filtration rate of each population (Ftot) was related to the total water volume (Vtot) in the different areas of the fjord and expressed as ‘the grazing impact’ Q=Vtot/Ftot.

This quantity is related to the half-time for phytoplankton by t1/2=Q×ln 2, representing the time to reduce the concentration by a factor of 2 in the water column above the filter-feeding populations, assuming efficient vertical mixing. Based on recorded population densities it was found that the potential grazing impact exerted by the three dominating species could be expressed as Q=0.17 d and t1/2=0.12 d.

This indicates a strong filter-feeder control of the phytoplankton although it should be emphasised that the potential grazing impact may only be realised when the filter-feeding zoobenthos is exposed to the whole water column by effective vertical mixing of the water on windy days.