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Published December 2000 | Published
Journal Article Open

A revised nitrogen budget for the Arabian Sea


Despite its importance for the global oceanic nitrogen (N) cycle, considerable uncertainties exist about the N fluxes of the Arabian Sea. On the basis of our recent measurements during the German Arabian Sea Process Study as part of the Joint Global Ocean Flux Study (JGOFS) in 1995 and 1997, we present estimates of various N sources and sinks such as atmospheric dry and wet depositions of N aerosols, pelagic denitrification, nitrous oxide (N_2O) emissions, and advective N input from the south. Additionally, we estimated the N burial in the deep sea and the sedimentary shelf denitrification. On the basis of our measurements and literature data, the N budget for the Arabian Sea was reassessed. It is dominated by the N loss due to denitrification, which is balanced by the advective input of N from the south. The role of N fixation in the Arabian Sea is still difficult to assess owing to the small database available; however, there are hints that it might be more important than previously thought. Atmospheric N depositions are important on a regional scale during the intermonsoon in the central Arabian Sea; however, they play only a minor role for the overall N cycling. Emissions of N_2O and ammonia, deep-sea N burial, and N inputs by rivers and marginal seas (i.e., Persian Gulf and Red Sea) are of minor importance. We found that the magnitude of the sedimentary denitrification at the shelf might be ∼17% of the total denitrification in the Arabian Sea, indicating that the shelf sediments might be of considerably greater importance for the N cycling in the Arabian Sea than previously thought. Sedimentary and pelagic denitrification together demand ∼6% of the estimated particulate organic nitrogen export flux from the photic zone. The main northward transport of N into the Arabian Sea occurs in the intermediate layers, indicating that the N cycle of the Arabian Sea might be sensitive to variations of the intermediate water circulation of the Indian Ocean.

Additional Information

© 2000 American Geophysical Union. (Received September 16, 1999; revised February 7, 2000; accepted February 9, 2000.) We acknowledge the help of the officers and crews of the R/V Meteor and R/V Sonne. Thanks are due to the chief scientists during the German JGOFS ship campaigns in 1995 and 1997: F. Pollehne (M32/3), B. Zeitzschel (M32/5, SO120), and W. Balzer (SO117). Special thanks are due to G. Schebeske, T.S. Rhee, U. Schüller, A. Deeken, N. Deiling, and the team of the R/V Sonne scientific-technical service for their invaluable technical support. We are indebted to many colleagues for their generosity in sharing data. We note in particular P. Schafer, S. Seitzinger, E. Maier-Reimer, and S.W.A. Naqvi for providing data and inspiring discussions. Thanks are due to A.J. Kettle for extracting data from the WOA94. Moreover, we thanks S. Rapsomanikis for advice and help during the early stages of the JGOFS project and C. Strametz for help with the manuscript. We gratefully acknowledge. Me Dileep Kumar and two anonymous reviewers for their constructive criticism of the manuscript. The investigations were financially supported by the German Bundesministerium für Bildung, Wissenschaft Forchung und Technologie through grants 03F0137A, 03F0183G0, 3F0241C and by the Max Planck Society.

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