Summer distribution of micro- and small mesozooplankton in the Red Sea and Gulf of Aden, with special reference to non-calanoid copepods

From the Gulf of Aden along a transect to the central-northern Red Sea the abundance and taxonomic composition of metazoan plankton was studied during the southwest monsoon period (summer 1987). Samples were taken with 0.055 mm mesh nets down to a maximum depth of 1050 m. In the epipelagic zone, a distinct decrease in total plankton abundance was observed from south to north, which was much more pronounced in biomass (by a factor of up to 10) as compared to numbers (by a factor of 2). This could partly be explained by differences in the taxonomic and/or size composition of the planktonic fauna. Among non-calanoid copepods, 40 out of 75 species or taxa investigated decreased in abundance from south to north. Sixteen of these species were completely absent in the central-northern area Nineteen species or taxa, ho'ivever, showed the opposite feature of a higher abundance in the central-northern Red Sea. The stations were grouped according to sim~larities in the taxonomic composition of non-calanoid copepods in the epipelagic zone. The following 3 geographical regions could be separated: (1) Gulf of Aden and Strait of Bab a1 Mandab; (2) southern Red Sea, and (3) central-northern Red Sea. In the meso-and bathypelagic zones, regional differences were not evident. The results are discussed in relation to hydrographic conditions during summer 1987.