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In the Arabian Sea (AS), spatiotemporal nutrient limitation patterns of primary production and the possible role of nutrient inputs from the atmosphere are still not well understood. Using a biogeochemical model forced by modeled aerosol deposition, we show that without high atmospheric iron inputs through dust deposition during the summer monsoon, primary production over the AS would be reduced by half. Atmospheric iron deposition also supports most of the nitrogen fixation over the AS. However, our ocean biogeochemistry modeling results suggest that dinitrogen fixation constitutes a negligible fraction of the primary production. Finally, we show that atmospheric inputs of nitrogen, mostly from anthropogenic activities in India, have a negligible impact on primary production.

Plain-Language summary Phytoplankton are microscopic organisms that live in watery environments such as the ocean. Like land plants, phytoplankton need nutrients to survive, develop, and reproduce. In the surface ocean, nutrients come from one of several pathways: from the depths of the ocean, from the rivers, and from the atmosphere. In the Arabian Sea, there are two important sources of nutrients for the organisms living in the surface layer of the ocean: the nutrient-rich waters coming from below, which occurs along the coast of the Arabian Peninsula, and the desert dust deposited from above. In this study, we show that neither source brings all the necessary nutrients nor brings enough nutrients. Both of these types of inputs are required to understand the distribution of the phytoplankton. If there was no dust deposition in the Arabian Sea, these organisms that represent the first link in the ocean food chain would be half as abundant as they are currently.