Physical influence on biological production along the western shelf of Madagascar

In September 2009, the R.V. Dr. Fridtjof Nansen surveyed the western coast of Madagascar. Environmental parameters of temperature, salinity, fluorescence and oxygen were profiled with a CTD probe and continuously underway at 5 m utilising a thermosalinograph equipped with a fluorescence sensor. A ship mounted Acoustic Doppler Current Profiler (ADCP) provided current profiles down to 250 m, while estimates of biomass were obtained from acoustics and trawling was used for species identification. In addition, visual whale observations were conducted. The survey revealed three areas that were identified as upwelling regions, namely the Southern Coast (26°S), offshore from Cap St. André (16°S) and near Nosy Be Island (13°S). In these upwelling regions, acoustic estimates, trawling and whale observations indicated high biological productivity. The total acoustic estimate for the whole western coast was as low as 62 000 t, typical for tropical waters. In addition to the upwelling areas, high biological productivity was also found outside river mouths. Ship born wind measurements, as well as re-analysed wind fields, indicated that the southern coast upwelling cell was wind-driven and had a larger extent than reported earlier. The wind conditions were not favourable for upwelling in the two northernmost upwelling cells. Here the ADCP showed high bottom velocities (>1 m s−1) oriented northeast. These currents were probably forced by the migrating eddies in the area as indicated by the remotely sensed Sea Level Anomaly (SLA). Such currents induce bottom friction layer transport oriented towards the coast, thus driving upwelling, although not necessarily penetrating all the way to the surface layer as was the case near Cap St. André.