Biomass, metabolism and nutrition of zooplankton in the Gulf of Lions (NW Mediterranean)

Zooplankton biomass was measured at 20 stations located along five coastal–offshore transects in the Gulf of Lions (NW Mediterranean) during spring 1998 and winter 1999, using two nets equipped with 200 and 80 μm mesh. Biomass was twice as abundant in spring. It increased toward the coast and the west of the gulf and was positively correlated to chlorophyll, primary production and organic particulate matter and negatively to temperature and salinity. The enrichment effect of Rhone river input and the oligotrophic influence of the north Mediterranean current (NMC) were the main distribution factors of zooplankton biomass. Higher biomass was obtained in coastal stations with the 80 μm net, which sampled more efficiently young larval stages and nauplii. Zooplankton metabolic activity (respiration, ammonium and phosphorus excretion) was studied at the inner and outer stations of each transect. Oxygen/nitrogen (O/N) ratio was lower in spring than in winter, indicating that more protein material was used for food, to complete the energy needed for the spring enhancement of secondary production. Nitrogen and phosphorus excreted by zooplankton accounted for 31% and 10% of the primary production requirements in spring and for 32% and >100%, in winter. Grazing impact and secondary production were calculated from respiratory requirements using a K₂ value of 0.4 and an assimilation rate of 0.8. Grazing impact ranged between 9% and 69% of the primary production in spring and between 6% and 83% in winter, depending on the stations. The average secondary production was 54 mg C m^–2 d^–1 in spring and 19 mg C m^–2 d^–1 in winter, which represents 11% and 12% of the primary production, respectively.