Diversity and structure of pelagic copepod populations in the frontal zone of the eastern Alboran sea

During the oceanographic campaign, ALMOFRONT I (April 24–May 26, 1991) a total of 50 vertical zooplankton hauls was carried out in the Alboran Sea using a triple net. Leg A (18 stations; 18 hauls) described a widely spaced grid in the region of the Atlantic current and some northern eddies, while Leg B (16 stations; 32 hauls, 6 hydrodynamically characterized sites sampled during a two day period) covered a more restricted area in the frontal zone. Ninety-one species of copepods were found in 100 samples collected by either 200 µm or 500 µm mesh size nets, including 32 genera and 26 families. Copepod abundance, structure indexes (species richness, evenness, Shannon species diversity index, standardized for unequal sample counts) and species abundance patterns (as rank-frequency diagrams) are presented and compared among the sites of leg-B. Copepod abundance was found to be the highest in the more productive sites of the Atlantic current. Structure indexes values and the changing shape of rank-frequency diagrams give a coherent view of the ecological succession stages of the copepod community. Juvenile stage (1) develops from the Atlantic divergence zone, left side of the jet. More mature stages (1 and 2) occur on the right side of the jet influenced by cross frontal flow. Both abundance and structure indexes decrease within an anticyclonic gyre south to the current. A situation more influenced by oligotrophic conditions was observed in a Mediterranean anticyclonic gyre north of the current. The importance of lateral displacement and meandering of the Atlantic current across the Alboran Sea to the community structure is discussed.