A revised nitrogen budget for the Arabian Sea

Despite its importance for the global oceanic nitrogen (N) cycle, considera ble 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 pres ent estimates of various N sources and sinks such as atmospheric dry and we t depositions of N aerosols, pelagic denitrification, nitrous oxide (N2O) e missions, 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 Ara bian Sea was reassessed. It is dominated by the N loss due to denitrificati on, 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 im portant than previously thought. Atmospheric N depositions are important on a regional scale during the intermonsoon in the central Arabian Sea; howev er, they play only a minor role for the overall N cycling. Emissions of N2O 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 similar to 17% of the total denitrification in the Arabian Sea, indicating that the shelf sediments might be of considerably greater importance for the N cycl ing in the Arabian Sea than previously thought. Sedimentary and pelagic den itrification together demand similar to6% of the estimated particulate orga nic nitrogen export flux from the photic zone. The main northward transport of N into the Arabian Sea occurs in the intermediate layers, indicating th at the N cycle of the Arabian Sea might be sensitive to variations of the i ntermediate water circulation of the Indian Ocean.