SPATIAL HETEROGENEITY IN THE STRUCTURE OF THE PLANKTONIC FOOD-WEB IN THE NORTH-SEA

The distributions of bacteria, phytoplankton, protozooplankton and cop epod biomass and activity were examined in relation to hydrographic ch aracteristics of the water column on 2 cruises in the North Sea (Augus t 1991 and May 1992). On both cruises. the greatest phytoplankton biom ass concentrations were associated with subsurface chlorophyll peaks. The 480:665 nm absorption ratio suggested that phytoplankton located i n surface waters were nutrient depleted and the ratio of phytoplankton carbon to bacterial carbon was generally low, suggesting that bacteri a played an important role in nutrient turnover in surface waters at t hese times. In the subsurface chlorophyll peaks, the pattern was varia ble with respect to the apparent nutrient status of the phytoplankton and the phytoplankton to bacteria carbon ratios. On the basis of ocean ographic features, we identified sites where sue predicted the formati on of subsurface chlorophyll peaks. At these sites, the phytoplankton to bacteria carbon ratios in the subsurface peak were generally high a nd the 480:665 nn absorption ratio suggested that the phytoplankton we re not nutrient limited. Also at these sites, the greatest absolute va lues of copepod production as well as the greatest percentage of total water column primary production being channelled into copepods were r ecorded. The regions where subsurface phytoplankton peaks were predict ed to form were, thus, characterised by a 'classical' food web in whic h energy is efficiently transferred into larger zooplankters. We argue that heterogeneity in the nutrient status of phytoplankton in the sub surface peak can be important in controlling the type ('classical' or 'regenerated') of planktonic food web found in the water column as a w hole.