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.

The distributions of bacteria, phytoplankton, protozooplankton and copepod biomass and activity were examined in relation to hydrographic characteristics of the water column on 2 cruises in the North Sea (August 1991 and May 1992). On both cruises, the greatest phytoplankton biomass concentrations were associated with subsurface chlorophyll peaks. The 480:665 nm absorption ratio suggested that phytoplankton located in surface waters were nutrient depleted and the ratio of phytoplankton carbon to bacterial carbon was generally low, suggesting that bacteria played an important ro!e ir! n u t~e n t turnever !n snrfzce x!ate:s a! these tLTes !a t h e subsu:facc ck!~:sphy!! pc;ks, :he pattern was variable with respect to the apparent nutrient status of the phytoplankton and the phytoplankton to bacteria carbon ratios On the basis of oceanographic features, we identified sites where we predicted the formation of subsurface chlorophyll peaks. At these sites, the phytoplankton to bacteria carbon ratios in the subsurface peak were generally hlgh and the 480:665 nm absorption ratio suggested that the phytoplankton were not nutrient limited. Also at these sites, the greatest absolute values of copepod production as well as the greatest percentage of total water column primary production being channelled into copepods were recorded. The regions where subsurface phytoplankton peaks were predicted to form were, thus, characterised by a 'classical' food web in which energy is efficiently transferred into larger zooplankters. We argue that heterogeneity in the nutrient status of phytoplankton in the subsurface peak can be important in controlling the type ('classical' or 'regenerated') of planktonic food web found in the water column as a whole.