Relationships between frontal structures and zooplankton communities along a cross-shelf transect in the Bay of Biscay (1995 to 2003)

A high-resolution cross-shelf transect in front of the mouth of the Gironde estuary (45.62° N, and from 1.36 to 4.27° W, with 3 nautical miles between stations) is carried out once a year during the spring stratification period. In this study, we analysed the zooplankton samples in 1995, 1997, 1999, 2001 and 2003 in order to elucidate the effects that mesoscale structures exert over the zooplankton community. The Gironde river plume and a shelf-break front due to internal wave generation over the slope are the main frontal structures described along the transect. Their position and magnitude determine zooplankton community changes both qualitatively and quantitatively. Two peaks of zooplankton abundance and distinct zooplankton assemblages were found to be related to the river plume and the frontal structure associated with the shelf-break. However, the distribution of biomass was more homogenous, suggesting that other factors apart from enhancement of primary production related to the location of frontal structures are involved in the determination of the pattern of secondary production along the transect.