Food chain structure of the North Sea plankton communities: seasonal variations of the role of the microbial loop

Abstract

Carbon flow in the plankton community was examined in various regions of the North Sea from 24 February to 4 March 1988. At this time, the spring phytoplankton bloom had begun in southern but not in northern regions. Bacterial abundance and production were highest in the Dogger Bank region where the spring bloom was under development, but there was no evidence of substantial carbon flow through the 'microbial loop' at this or any other station sampled. However, comparisons between ratios of bacterial carbon to phytoplankton carbon obtained on thls and a subsequent (May/June) cruise, show that the 'microbial loop' contributes substantially to carbon cycling during summer. During the Februarymarch cruise, copepod production rates were substantially higher in the southern than in the northern North Sea. For all stations it was estimated that < 5 % of total daily primary production was grazed by the copepod community. The bulk of the primary production occumng in the North Sea at this time of the year is, therefore, transferred directly to the benthos.