Nutrient, Phytoplankton and Zooplankton Variability in the Indian Ocean

Abstract

International audienceThis chapter provides an overview of the physical and biogeochemical factors that control spatial and temporal variability in macro-and micro-nutrient concentrations, phytoplankton biomass and production, and zooplankton biomass and grazing in the Indian Ocean. As in other ocean basins, in the Indian Ocean there is a strong connection between the physics that drives (or suppresses) nutrient delivery to the photic zone and responses of phytoplankton and zooplankton. Wind forcing during the Southwest/Southeast Monsoon (boreal summer, austral winter) is a major biogeochemical and ecological driver with broad stimulatory effects throughout many regions, including the Arabian Sea, Bay of Bengal, Seychelles Chagos Thermocline Ridge, Java, and the central/eastern subtropical gyre. In contrast, substantial stimulatory response to the Northeast/Northwest Monsoon (boreal winter, austral summer) is manifested primarily in the Arabian Sea and Bay of Bengal. In addition, there are regionally specific processes that significantly modulate the biogeochemical and ecological responses. For example, strong advective impacts and Fe/Si limitation in the Arabian Sea; freshwater and stratification in the Bay of Bengal; the influence of Indonesian Throughflow nutrient inputs, poleward transport, downwelling and seaward-propagating eddies in the southeastern Indian Ocean; and alongshorepropagating eddies, meanders, upwelling and poleward transport in the southwestern Indian Ocean. The southern subtropical gyre is extremely oligotrophic except when/where the Southeast Madagascar bloom occurs. Recommendations for future work include the need to reassess the potential effects of climate change on the biogeochemistry and ecology of the Indian Ocean basin, and motivate more planktonic food web studies