Variations of zooplankton in the frontal area of the Alboran sea (Mediterranean sea) in winter 1997

Abstract

Zooplankton from the upper 200 m of the Almeria-Oran frontal region, (east of the Alboran sea, western Mediterranean) was sampled during winter 1997, using two nets equipped with 200 and 80 μm mesh. Eight sites representative of the different hydrodynamic structures (Mediterranean water, geostrophic Atlantic jet, associated anticyclonic gyre) were investigated over 2 d, using two day- and two night-samples per site. Zooplankton biomass was correlated to chlorophyll abundance, with lowest values in the oligotrophic Mediterranean water and highest in Atlantic water. The high biomass resulted from the enhancement of the secondary production, as indicated by the greater proportion of nauplii and the increase of zooplankton metabolism and feeding. Total abundance and biovolume of organisms varied like their biomass. Maximum quantitative values were recorded in the material collected by the 80-μm net which sampled more efficiently young developmental stages such as nauplii. Copepods accounted for 60–71 % of the zooplankton, and their nauplii, scarce in the 200-μm net material, up to 33 % in the 80-μm samples. Compared to the average repartition of total zooplankton at the different sites, nauplii, siphonophora, chaetognatha, ostracoda and meroplankton were preferentially distributed in the Atlantic sites. The general aspect of the copepods assemblage was homogeneous, the bulk of the population being constituted of Clausocalanus and Oithona spp. Nevertheless, the specific diversity was greater in the Atlantic waters, with the addition of some scarce species absent from the Mediterranean water. Several other species, present at every site were significantly more abundant in the Atlantic waters, e.g. Pleuromamma borealis and Eucalanus crassus. The distribution of Centropages typicus and Clausocalanus furcatus was more specifically linked to the front limit where these species could find favourable feeding conditions.