Characterisation of mesoscale features and phytoplankton variability in the Mozambique Channel

Variability of hydrographic characteristics and phytoplankton distribution associated with mesoscale eddies in the Mozambique Channel was investigated during four cruises in September 2007, December 2008, November 2009, and April/May 2010. Satellite altimetry was used to map the location of mesoscale features, and together with ETOPO1 bathymetry, was used in a Discriminant Function Analysis to classify in situ sampling stations into five categories, namely cyclonic (C), anti-cyclonic (A), frontal (F), divergence (D), and shelf (S). Fluorescence profiles were integrated through four depth ranges in the upper 200 m and used to determine the depth of the maximum chlorophyll a concentration and to model the euphotic zone depth. At a depth of 100 m, distinctly different hydrographic characteristics were observed between mesoscale features with cyclonic eddies consisting of Subtropical Surface Water and anti-cyclonic eddies containing Tropical Surface Water from the Indian Ocean. Hydrographic properties at divergence and frontal stations reflected a mixture of these water masses, while shelf stations showed considerable variability as a result of the interaction of eddies with the continental slope. Chlorophyll a concentrations in the surface waters were found to be low, with subsurface levels being significantly greater. Phytoplankton biomass in cyclonic and anti-cyclonic eddies was relatively low and not significantly different. The interaction of mesoscale eddies with the continental slope on the western side of the Channel caused upwelling of cooler, nutrient-rich water, which resulted in elevated phytoplankton biomass in the shelf regions. Strong currents at the perimeters of these eddies produced offshore advection of the high biomass into the frontal regions.