The nitrogen cycle in the Arabian Sea

Despite their importance for the global oceanic nitrogen (N) cycle, estimates of N fluxes in the Arabian Sea remain in considerable uncertainty. In this report, we summarize current knowledge of important processes, including denitrification, N2 fixation and nitrous oxide emissions. Additionally, we discuss anthropogenic impacts on the N cycle in the region. Existing studies suggest that the Arabian Sea is a significant source of N2O, and a major sink for fixed-N mainly due to enhanced rates of denitrification that occur in suboxic portions of the water column in the Arabian Sea. Sedimentary denitrification is small compared to water column denitrification, and additions of fixed-N via N2 fixation also are small compared to pelagic denitrification. As a consequence, the fixed-N budget of the Arabian Sea is dominated by an advective supply from the south, and by the sink arising from pelagic denitrification. Although relatively small compared to the advective supply, inputs of fixed-N from runoff and from the atmosphere may have significant impacts on surface waters and on the coastal waters of western India, and these inputs are rising because of human activities. Overall, the Arabian Sea's nitrogen cycle is likely to respond sensitively to climate change and, in turn, have an impact on climate via its N2O and denitrification components.