Annual zooplankton succession in coastal NW Mediterranean waters: the importance of the smaller size fractions

Zooplankton abundance, biomass (biovolume) and taxonomic composition were s tudied within an annual cycle (August 1995-October 1996) in the Bay of Blan es (northwest Mediterranean). Weekly zooplankton sampling included oblique tows made with a 200 mum Juday-Bogorov net, and vertical tows made with a 5 3 mum net, so adequately sample both mesoplankton and the smaller zooplankt on fractions. Total zooplankton abundance showed high variability, lacking any clear seasonal pattern. However, the different species within the zoopl ankton community displayed a clear succession throughout the year. In gener al, cyclopoid copepods (Oithona spp.) and cladocerans (Penilia avirostris) dominated the summer and autumn communities, whereas in winter and spring, calanoid copepods (Clausocalanus spp., Paracalanus sp. and Centropages typi cus) were predominant. The zooplankton annual cycle in the Bay of Blanes do es not resemble those of other Mediterranean littoral areas, probably due t o the inherent particularity and variability associated with open coastal e nvironments. On average, the abundance of organisms estimated with a tradit ional 200 mum Juday-Bogorov net was 8.1 times lower than the values obtaine d with a 53 mum net. Even if only organisms >200 mum collected in the 53 mu m tows were considered, the total abundance within the 53 mum net was still 4.4 times higher than the estimates from the Juday-Bogorov net. These resu lts suggest the need for accurate samplings of the entire zooplankton assem blage when characterizing the structure and dynamics of zooplanktonic commu nities.