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

Zooplankton abundance, biomass (biovolume) and taxonomic composition were studied within an annual cycle (August 1995-October 1996) in the Bay of Blanes (northwest Mediterranean). Weekly zooplankton sampling included oblique tows made with a 200 mum Juday-Bogorov net, and vertical tows made with a 53 mum net, so adequately sample both mesoplankton and the smaller zooplankton fractions. Total zooplankton abundance showed high variability, lacking any clear seasonal pattern. However, the different species within the zooplankton community displayed a clear succession throughout the year. In general, 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 typicus) were predominant. The zooplankton annual cycle in the Bay of Blanes does not resemble those of other Mediterranean littoral areas, probably due to the inherent particularity and variability associated with open coastal environments. On average, the abundance of organisms estimated with a traditional 200 mum Juday-Bogorov net was 8.1 times lower than the values obtained with a 53 mum net. Even if only organisms >200 mum collected in the 53 mum 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 results suggest the need for accurate samplings of the entire zooplankton assemblage when characterizing the structure and dynamics of zooplanktonic communities.