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The hypothesis of size-differential control of phytoplankton was tested by sampling stations on a transect from the Dogger Bank to the Shetland isles in the North Sea on cruises in March/April and July/August 1994. On this transect Light limitation was expected in spring, and an increase in nutrient limitation was expected from north to south in summer. The Shetland area, where nutrient levels and primary production rates were highest in summer, did not show the correspondingly higher relative abundances of large algae which would have been predicted by the hypothesis. The question addressed in this study is how the highly abundant zooplankton in the stratified North Sea in summer affect the structure of the plankton community of interacting algae, heterotrophic protists and metazoans. Variations in zooplankton species distributions revealed 4 latitudinal subregions. The Calanus finmarchicus population in the central North Sea reproduced in March. In August, reproduction rates were highest in the 2 northernmost regions. Carbon specific ingestion based on in vitro egg production rates led to an estimate of 10.4% for these stations in summer. It was concluded that the stratified central and northern North Sea can vary with latitude in zooplankton species composition, population structure and vertical distribution. In summer, adjacent subregions can differ widely in vertical distribution of C. finmarchicus biomass. High copepod abundance and grazing rates can promote the dominance of small algae by causing mortality of large algae and heterotrophic protists (grazers of small algae). During summer in the North Sea, egg production rates indicated an increased grazing activity of C, finmarchicus towards the north. This could have obscured the negative effect of decreasing nutrient Limitation on the dominance of small algae predicted by the hypothesis of size-differential control of phytoplankton.

Metazoan plankton and the structure of the plankton community in the stratified North Sea