ABSTRACT

The Arabian Sea is important in global C and N budgets because of its high rates of annual primary production, its extensive zone of oxygen depletion and denitrification, and its expected strong response to global warming via ocean-atmosphere feedback to monsoon winds and upwelling intensity, especially the Oman Upwelling, driven by the SW Monsoon. The paradox of the Arabian Sea is that it produces a very weak and delayed phytoplankton response compared to other physically dynamic upwelling systems. Thick blooms of large diatoms are uniquely not evident in the Arabian Sea despite apparently abundant nutrients, seed stocks and favorable hydrographic conditions. The subdued response shifts carbon export offshore of the coastal upwelling area and delays major flux events until the latter stages of the SW Monsoon, with important implications for the western boundary of the oxygen minimum zone. This work will test the hypotheses that grazing interactions, iron limitation, or both, control diatoms and, more generally, phytoplankton biomass, production and the delayed flux characteristics of the Arabian Sea. Grazer control was a major, but as yet untested, conclusion of the US JGOFS program.

In 2007, one of the investigators found that a large region of the central and southern Arabian Sea was Fe-limited during the SW Monsoon, consistent with recent modeling studies. Iron limitation may exacerbate grazing regulation, and integration of Fe and grazing experiments is a central part of the work plan. A cruise to the Oman upwelling region during the SW Monsoon in 2010 will include iron and nutrient biogeochemistry, plankton community structure and dynamics, taxon-specific estimates of phytoplankton growth and production, grazing contributions of micro- and mesozooplankton, with a focus on large copepods (Calanoides carinatus, Subeucalanus crassus, Paraeucalanus sewelli), whose life histories, abundance and size are consistent with a top-down regulatory role. The approach will involve assessment of net community rates of changes following a Lagrangian drift array and contemporaneous in situ studies of phytoplankton growth, production, micro- and mesozooplankton grazing. Shipboard experiments will address the response of phytoplankton to added Fe and Si and reproductive responses of dominant large grazers to food supply, their ability to modulate the grazing impacts of micro-herbivores and their direct and indirect effects on lower trophic levels. The sampling program will resolve the abundances, depth distribution and lipid contents of major grazers during a critical time of year (August-September) when there is little previous data and as the animals are preparing to leave surface waters for their seasonal diapause migration to mesopelagic depths. Grazing control in the Arabian Sea, if confirmed, will be a unique example of top-down regulation of a significant open-ocean ecosystem with important consequences for carbon and nitrogen cycling. Furthermore, understanding the potential limiting effects of Fe availability will allow us to better anticipate how the ocean ecology and biogeochemistry will respond to changes in climate and monsoon intensity that affect dust fluxes from the surrounding arid landmasses.

The most recent IPCC report cites changes in monsoon intensity as a likely impact of the present trend in planetary warming. This project seeks to understand ecological, biological and geochemical constraints on production, carbon cycling and export in the upwelling system so that the effects of climate changes can be better anticipated and modeled. This project will also contribute to the oceanographic infrastructure of India and Oman, both critical US partners in the Indian Ocean, as well as continue ongoing activities in public outreach, K-12 education, broadening undergraduate educational opportunities, for example through the MATE program, and graduate training of diversity students in marine science.

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