Seasonal variability of infaunal community structures in three areas of the North Sea under different environmental conditions

Infaunal communities were studied on a monthly scale from September 2000 until May 2002 in order to investigate their seasonal variability in different areas of the North Sea: the German Bight, the Oyster Ground and the Dogger Bank. The stations were chosen to reflect a gradient in the hydrographic regime, temperature and organic matter supply. The seasonal variability in mean abundance, diversity and community structure of the infauna was mainly caused by recruitment in spring and summer. It was found to be highest in the German Bight, where fluctuations in environmental parameters were higher than in the Oyster Ground and at the Dogger Bank. Despite the dominant role of recruitment, the seasonal variability of the benthic communities seems to be a result of synergistic effects of numerous factors such as food availability, water temperature, predation and hydrodynamical stress. Among these effects, the input of high quality food to the benthic system seems to be of major importance for the seasonal variability of the benthic communities, since the faunal patterns in the German Bight and the Oyster Ground were correlated significantly with sediment parameters indicating inputs of fresh nitrogen rich organic matter, such as chlorophyll a and C/N ratio (German Bight) and fucoxanthin (Oyster Ground). This role of food quality might be intensified by high bottom water temperatures in areas such as the German Bight, where a mixed water column results in an increase of macrofauna abundance during summer, in contrast to a moderate increase of macrofauna in the Oyster Ground and the Dogger Bank which have a stratified water column. Low water temperatures in winter did not affect the mortality of the juvenile benthic fauna. It is suggested that food limitation and predation pressure caused the decrease in abundance. Thus, depending on the season, a shift from a bottom-up controlled community to a top-down controlled community might be the result at least for the juvenile macrofauna.