Zooplankton grazing impact in the plume of dilution of the Gironde estuary (France) prior to the spring bloom

Abstract

The Bay of Biscay is a coastal area intensively exploited for fishing and is submitted to important human actions (proximity of important industrial and agricultural areas). Thus, the understanding of the materials and energy transfers in such ecosystems is of great interest. However, investigations on zooplankton (an important component of the marine food web) are very scarce in this area. Our study concerns the grazing impact of the zooplankton in shelf waters in the plume of dilution of the Gironde estuary prior to the spring bloom. Samples were taken through the photic layer in April 1993. Our results have shown that Temora longicornis and Paracalanus parvus dominated the ‘herbivorous’ planktonic community. The grazing impact of the zooplankton community on the total phytoplankton stock and on the total primary production was low (9–14 and 17–21% day−1, respectively) during this period, which is in accordance with most results in coastal areas. However, due to the small size of the algae (∼70% of the algal stock <5 μm), the most important part of the phytoplankton was not available for these large particle feeders. Thus, the daily grazing pressure calculated on the available fraction indicates that 35–68% of this stock were removed daily.