ZOOPLANKTON GRAZING IMPACT IN THE PLUME OF DILUTION OF THE GIRONDE ESTUARY (FRANCE) PRIOR TO THE SPRING BLOOM

The Bay of Biscay is a coastal area intensively exploited for fishing and is submitted to important human actions (proximity of important in dustrial and agricultural areas). Thus, the understanding of the mater ials and energy transfers in such ecosystems is of great interest. How ever, investigations on zooplankton (an important component of the mar ine food web) are very scarce in this area. Our study concerns the gra zing impact of the zooplankton in shelf waters in the plume of dilutio n of the Gironde estuary prior to the spring bloom. Samples were taken through the photic layer in April 1993. Our results have shown that T emora longicornis and Paracalanus parvus dominated the 'herbivorous' p lanktonic community. The grazing impact of the zooplankton community o n the total phytoplankton stock and on the total primary production wa s low (9-14 and 17-21% day(-1), respectively) during this period, whic h is in accordance with most results in coastal areas. However, due to the small size of the algae (similar to 70% of the algal stock <5 mu m). the most important part of the phytoplankton was not available for these large particle feeders. Thus, the daily grazing pressure calcul ated on the available fraction indicates that 35-68% of this stock wer e removed daily.