Summary of Abstract Submission

Abstract Submission No. IO50-02-0020PresentationOral

THE HYDROGRAPHY AND BIOGEOCHEMISTRY OF THE MASCARENE PLATEAU

Denise Smythe-Wright*1

1 National Oceanography Centre, United Kingdom

ABSTRACT :

It has been suggested that the Mascarene Plateau obstructs the South Equatorial Current (SEC) causing upwelling, nutrient enrichment and enhanced chlorophyll and secondary production levels downstream. But, results from a detailed hydrographic and biogeochemical survey in June and July, 2002, show limited evidence of direct topographic upwelling. The data do, however, support the theory of open ocean upwelling between 5-10 S across the central and western Indian Ocean from 50-90 E, due to Ekman divergence along the northern edge of the SEC. The SEC also forms a sharp boundary between southerly water masses, which are low in nutrients, and relatively nutrient-rich waters to the north. The SEC crosses the plateau through a series of channels and banks and this determines the supply of enhanced nutrients to the near-surface waters of the central and northern regions of the plateau. These higher nutrient levels support increased biomass. This is not only important for biological production, but also has implications for air-sea gas exchange. While phytoplankton are important for the drawdown of carbon dioxide, the data show that the phytoplankton in the region also produce halogenated gases. On reaching the troposphere and stratosphere these gases participate in chemical reactions that have implications for ozone depletion and climate change. Understanding the sources and sinks of climatically-active gases is vital in establishing global budgets and is particularly important in an area such as the central Indian Ocean, where hitherto this assumed oligotrophic area was thought to have limited effect on air-sea exchange.