Structure of plankton communities in the Dogger Bank area (North Sea) during a stratified situation

Abstract

Plankton dynamics and hydrography were invest~gated in the Dogger Bank area, southern North Sea, In May 1990 after the onset of stratification Greatest phytoplankton biomass and production rates were observed withln a subsurface maximum around the pycnochne. At all 51 stations investigated, fluorescence maxima were found at depths > l 5 m and often at depths >30 m. Small flagellates (5 to 7 pm) dominated the phytoplankton. The vertical distributions of the smaller heterotrophs (bacteria, heterotrophic nanoflagellates and cihates) were closely related to the fluorescence distribution. Carbon budgets established for the southern North Sea suggest that, on and south of the Dogger Bank, ca 15 % of phytoplankton production was channelled directly into the larger zooplankton (copepods), while north of the Dogger Bank, ca 30 % was ingested by copepods. The production of the phytoplankton fraction > l 1 pm (i.e. potential copepod prey items) could not alone account for the daily carbon demand of the copepods. The carbon budgets suggest that ciliates could potentially have been of nutritional importance to the copepod populat~on. Calculations showed that copepods possessed the ability to clear a substantial fraction of ciliate production. The high biomass of bacteria, heterotroph~c flagellates and c~liates, as well as the fact that primary production of the larger (I.e. grazable) phytoplankton did not appear to be sufficient to meet copepod energy requirements, suggest the importance of the microbial loop in carbon cycling in the North Sea during the stratified period.