Zooplankton standing stock, copepod faecal pellets and particulate detritus in Kiel Bight

The annual cycle of metazooplankton biomass has been compared with that of particulate organic detritus in the 20m water column of Kiel Bight. Zooplankton biomass was high from April through September and small neritic copepods were the dominant organisms. The particulate detritus levels fluctuated with greater frequency than zooplankton stocks during this period and no quantitative relationship between the two could be discerned.

The contribution of whole calanoid copepod faecal pellets to the detritus pool in terms of carbon was below 10% on an average from April to September and well below 5% during the rest of the year. The ratio by numbers of copepod faecal pellets to copepods showed a marked seasonal variation with low values in spring and summer when copepods were most abundant and high values during autumn and winter. Calculated residence times of pellets in the water column were a few hours in summer and two orders of magnitude longer in winter. The latter residence times indicate that in situ sinking rates of pellets must be considerably lower than the high values given in the literature derived from in vitro studies. Zooplankton feeding and microbial breakdown in the water column rather than sedimentation to the sea-bed appear to be the important mechanisms determining removal rate of faecal pellets from the water column in Kiel Bight, particularly in summer. Results from sediment traps support this conclusion, as the annual minimum in sedimentation occurs in late spring and summer and coincides with the season of largest zooplankton stocks. Grazing by small neritic zooplankton will thus tend to conserve essential nutrients in the surface layer.