BG Biogeosciences BG Biogeosciences 1726-4189 Copernicus Publications Göttingen, Germany 10.5194/bg-7-2091-2010 The Arabian Sea as a high-nutrient, low-chlorophyll region during the late Southwest Monsoon Naqvi S. W. A. 1 Moffett J. W. 2 Gauns M. U. 1 Narvekar P. V. 1 Pratihary A. K. 1 Naik H. 1 Shenoy D. M. 1 Jayakumar D. A. 3 Goepfert T. J. 4 Patra P. K. 5 Al-Azri A. 6 Ahmed S. I. 6 National Institute of Oceanography (Council of Scientific & Industrial Research), Dona Paula, Goa 403 004, India Department of Biological Sciences, University of Southern California, Los Angeles, CA 90089, USA Department of Geosciences, Princeton University, Princeton, NJ 08544, USA Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Frontier Research Center for Global Change, JAMSTEC, Yokohama 236 001, Japan Department of Marine Science and Fisheries, College of Agricultural and Marine Sciences, Sultan Qaboos University, Al-Khod 123, Sultanate of Oman 05 07 2010 7 7 2091 2100 Copyright: © 2010 S. W. A. Naqvi et al. 2010 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://bg.copernicus.org/articles/7/2091/2010/bg-7-2091-2010.html The full text article is available as a PDF file from https://bg.copernicus.org/articles/7/2091/2010/bg-7-2091-2010.pdf

Extensive observations were made during the late Southwest Monsoon of 2004 over the Indian and Omani shelves, and along a transect that extended from the southern coast of Oman to the central west coast of India, tracking the southern leg of the US JGOFS expedition (1994–1995) in the west. The data are used, in conjunction with satellite-derived data, to investigate long-term trends in chlorophyll and sea surface temperature, indicators of upwelling intensity, and to understand factors that control primary production (PP) in the Arabian Sea, focussing on the role of iron. Our results do not support an intensification of upwelling in the western Arabian Sea, reported to have been caused by the decline in the winter/spring Eurasian snow cover since 1997. We also noticed, for the first time, an unexpected development of high-nutrient, low-chlorophyll condition off the southern Omani coast. This feature, coupled with other characteristics of the system, such as a narrow shelf and relatively low iron concentrations in surface waters, suggest a close similarity between the Omani upwelling system and the Peruvian and California upwelling systems, where PP is limited by iron. Iron limitation of PP may complicate simple relationship between upwelling and PP assumed by previous workers, and contribute to the anomalous offshore occurrence of the most severe oxygen (O<sub>2</sub>) depletion in the region. Over the much wider Indian shelf, which experiences large-scale bottom water O<sub>2</sub>-depletion in summer, adequate iron supply from reducing bottom-waters and sediments seems to support moderately high PP; however, such production is restricted to the thin, oxygenated surface layer, probably because of the unsuitability of the O<sub>2</sub>-depleted environment for the growth of oxygenic photosynthesizers.

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