Biogeochemistry of iron in the Arabian Sea

Abstract

The Arabian Sea is a productive basin where seasonal upwelling and convective mixing result in high surface nutrient concentrations and widespread algal blooms. The factors controlling primary productivity in the Arabian Sea are of interest because the region contains an intense oxygen minimum zone (OMZ) that is a major sink for nitrate in the ocean. A survey of iron (Fe) distribution and redox chemistry was carried out in 2007 to assess its role in Arabian Sea biogeochemistry, including an investigation of the biological response to Fe additions. Results show that Fe is strongly enriched in the eastern Arabian Sea, associated with the OMZ. Much of the Fe within the OMZ is present as Fe(II), which enhances the residence time and accumulation of Fe. In contrast, Fe concentrations are lower in the western Arabian Sea, separated from the Fe-rich OMZ by a water mass boundary. Consequently, low surface values are associated with monsoon-driven upwelling off the Omani coast. Incubations revealed that primary production during the southwest monsoon was strongly limited by Fe over much of the study area. Incubation of surface waters with Fe resulted in rapid growth of Phaeocystis and up to sixfold increase in chlorophyll. This is the first demonstration of Fe limitation in the Arabian Sea, and the first high resolution zonal survey of dissolved Fe and Fe(II) across the basin. Our findings suggest that models developed to predict the response of the Arabian Sea biogeochemistry to global warming need to consider effects on Fe inputs.