Copepod grazing potential in late winter in the Norwegian Sea - a factor in the control of spring phytoplankton growth?

Abstract

During a winter cruise in the Norwegian Sea, vertical distribution of zooplankton as well as grazing and defecation by calanoid copepods from different water layers were investigated. An enriched population of natural phytoplankton (> 4 pg chl a I-') served as food in grazing and defecation experiments. Results from chl a measurements and cell counts of the food medium before and after incubation with copepods as well as from HPLC analyses of copepod gut contents and scanning electron microscopy of the faecal pellets collected during these experiments revealed that copepods from the upper water layer (0 to 200 m) were potential active feeders. Copepods from the water layers of their maximum abundance (below 500 m) did not feed. Results implied that grazing of copepods already present in surface waters (surface overwinters) in late winter/early spring is a vital factor affecting the pelagic biological regime in spring INTRODUCTION