Epipelagic mesozooplankton dynamics around the Mascarene Plateau and Basin, Southwestern Indian Ocean

It has been suggested that the obstruction of the South Equatorial Current by the Mascarene Plateau might cause upwelling, nutrient enrichment and enhanced chlorophyll and secondary production levels downstream. A study conducted in April and May 2001 showed variability in biomass and community structure which appeared to support this hypothesis but, in the absence of supporting physical and biochemical measurements, we were unable to confirm it. In June and July 2002 the sampling was repeated with the supporting environmental measurements available from a large research vessel. In this paper we present the results from this sampling programme, compare them with the 2001 results, and examine both datasets in the light of physical and other environmental data gathered during the 2002 programme in order to evaluate the evidence for significant upwelling around the Mascarene Plateau. The evidence is inconclusive: the 2002 dataset shows only a little evidence of topographic upwelling. However, the mesozooplankton and other physical and biochemical data from the 2002 sampling programme indicate support for the theory of an open–ocean upwelling between 5 and 10○ S across the central and western Indian Ocean from 50 to 90° E, due to Ekman divergence along the northern edge of the South Equatorial Current. It is possible that these two separate sources of upwelling may coexist and combine at times, producing the very high levels of biomass found during 2001.