MEPS 168:197-211 (1998)  -  doi:10.3354/meps168197

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

Katherine Richardson1,*, Torkel Gissel Nielsen2, Flemming Bo Pedersen3, Jens Peter Heilmann1, Bo Løkkegaard4, Hanne Kaas2

1Danish Institute for Fisheries Research, Department of Marine and Coastal Ecology, Kavalergården 6, DK-2920 Charlottenlund, Denmark
2National Environmental Research Institute, Department of Marine Ecology and Microbiology, Frederiksborgvej 399, PO Box 358, DK-4000 Roskilde, Denmark
3ISVA, The Technical University of Denmark, Building 115, DK-2800 Lyngby, Denmark
4DIS, Vestergade 7, DK-1456 Copenhagen K, Denmark
*Present address: Dept Marine Ecology, Aarhus University, Finlandsgade 14, DK-8200 Aarhus N, Denmark. E-mail:

ABSTRACT: 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 we 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 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 Œclassical¹ 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 (Œclassical¹ or Œregenerated¹) of planktonic food web found in the water column as a whole.

KEY WORDS: Phytoplankton · Zooplankton · Food web · Vertical and horizontal heterogeneity

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