Spatial heterogeneity in the structure of the planktonic food web in the North Sea

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 role in nutrient turnover in surface waters at these times. In the subsurface chlorophyll peaks, the 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 sue predicted the formation of subsurface chlorophyll peaks. At these sites, the phytoplankton to bacteria carbon ratios in the subsurface peak were generally high and the 480:665 nn 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