Space and time variations in zooplankton distribution south of Marseilles

The mesozooplankton distribution in the upper water layer (up to 200 m) off Marseilles (NW Mediterranean Sea) was studied during 22 cruises performed between March 1992 and February 1995. Four stations (M1, M3, M5 and M7) were investigated along a coast-open sea transect. Spatial and seasonal variations of zooplankton were described using different quantitative parameters: biomass (dry weight, carbon, nitrogen), displacement volume (biovolume) and abundance of total organisms. C/N ratio, dry weight per individual and volume per individual were also calculated. The seasonal quantitative variations occurring at the four stations were not well synchronized. Annual maximum biomass was observed during spring and summer at M1 but only in early spring at the other stations. Abundance and biovolumes followed the same general pattern of variation. The mean values of the different parameters were maximum near the coast, at M1, and minimum at the most distant station (M7), but the decrease towards the open sea was not regular: the values found at M5 were higher than at M3 and markedly exceeded those at M7. This seems to be related to the presence of the oligotrophic Northern Mediterranean Current flowing parallel to the coast. In most cases M3 was in the core of the current whereas M5 seemed to be frequently influenced by its external boundary. Locally, this frontal situation enhanced the primary production and consequently favoured an increase in zooplankton biomass or production as suggested by the strong temporal correlation between chlorophyll and zooplankton at this station. Comparison between stations demonstrated the specificity of M5 zooplankton which showed the lowest variability in its specific dry weight and biovolume and the highest C\N ratios.