Mesozooplankton grazing during the Phaeocystis globosa bloom in the southern bight of the North Sea

During spring blooms 1998 and 1999, three complementary methods were used t o evaluate the in situ feeding activities of the dominant copepod species o f the Belgian coastal zone: gut pigment content analysis using HPLC, the C- 14 tracer method, and cell count experiments. The results obtained by all t hree methods consistently showed that Phaeocystis globosa is not an adequat e food source for the spring copepods in the Belgian coastal zone. Our resu lts demonstrated that, among the potential prey, copepods strongly selected diatoms and microzooplankton, and that these types of prey accounted for t he major part of the ingested carbon. However, diatoms and microzooplankton ingestion did not always seem sufficient in terms of carbon to avoid food limitation. Comparison of clearance rates exerted on different potential pr ey types during the P. globosa peak with those before and after the P. glob osa peak showed that the copepods' feeding pressure on diatoms was reduced during the P. globasa peak while that on microzooplankton was not. The low grazing pressure on P. globosa, together with the preferential grazing on d iatoms, which reduces the competition for nutrients, and the predation on m icrozooplankton organisms, which reduces the microzooplankton grazing press ure on P. globosa cells, are likely to favour the P. globosa bloom in the S outhern Eight of the North Sea. (C) 2000 Elsevier Science B.V. All rights r eserved.