On the paradox of high mesozooplankton biomass, throughout the year in the western Arabian Sea: Re-analysis of 1I0E data and comparison with newer data

Abstract

The mesozooplankton data from the northwestern Indian Ocean collected during 1962-1965 by the International Indian Ocean Expedition (HOE; displacement volumes from vertical net catches, 330 11m mesh, upper 200 m) showed highest biomass in the summer upwelling areas off Oman and Somalia but also suggested that zooplankton stocks were relatively large outside the SW monsoon. Catches by 200 Ilm nets off Oman (1963/64), Yemen (1984/85) and Somalia (1992193) confirmed that mean biomass during winter is considerably above the level that is found in wat~rs with the typical tropical structure. In the Gulf of Aden the NE monsoon is even the richest zooplankton season . This is due to winter cooling, with entrainment of nutrients into the upper layer, producing phytoplankton blooms. Th'rel atively high zooplankton biomass outside the upwelling season seems less paradoxical since seasonal differences in phytoplankton production and stock are less extreme than previously thought. On the one hand the early 14C data underestimated primary production outside the SW monsoon, and on the other hand the CZCS images exaggerated the chlorophyll concentration during the SW monsoon. The productivity in the strong Somali Current upwelling is below potential due to large advection and mixing, and zooplankton stocks in July are probably much smaller than off Oman. Results from an Arabian Sea model [McCreary et ai., Prog, Oceanog. 37 (1996) 193] , including zooplankton biomass and grazing, support the observation that zooplankton development differs between subregions and that entrainment blooms produce zooplankton peaks during the NE monsoon as well . The HOE data set seems still useful for comparison with existing and recently collected data from smaller-meshed nets, and for validation of JGOFS Arabian Sea modeling. Displacements from 330 and 230 11m catches were strikingl y similar (RV Discovery 1963/64). In catches from hauls with a 50 Ilm net, the 50-300 11m fraction only added about 25% to the biomass> 300 11m (samples without contamination by phytoplankton, RV Baldrige 199 5). Early studies l . s on zooplankton biomass and copepod