Global patterns of marine nitrogen fixation and denitrification

Abstract

A new quasi‐conservative tracer N*, defined as a linear combination of nitrate and phosphate, is proposed to investigate the distribution of nitrogen fixation and denitrification in the world oceans. Spatial patterns of N* are determined in the different ocean basins using data from the Geochemical Ocean Sections Study (GEOSECS) cruises (1972–1978) and from eight additional cruises in the Atlantic Ocean. N* is low (< −3 µmol kg−1) in the Arabian Sea and in the eastern tropical North and South Pacific. This distribution is consistent with direct observations of water column denitrification in these oxygen minimum zones. Low N* concentrations in the Bering Sea and near the continental shelves of the east and west coasts of North America also indicate a sink of N* due to benthic denitrification. High concentrations of N* (>2.0 µmol kg−1) indicative of prevailing nitrogen fixation are found in the thermocline of the tropical and subtropical North Atlantic and in the Mediterranean. This suggests that on a global scale these basins are acting as sources of fixed nitrogen, while the Indian Ocean and parts of the Pacific Ocean are acting as sinks. Nitrogen fixation is estimated in the North Atlantic Ocean (10°N–50°N) using the N* distribution along isopycnal surfaces and information about the water age. We calculate a fixation rate of 28 Tg N yr−1 which is about 3 times larger than the most recent global estimate. Our result is in line, however, with some recent suggestions that pelagic nitrogen fixation may be seriously underestimated. The implied flux of 0.072 mol N m−2 yr−1 is sufficient to meet all the nitrogen requirement of the estimated net community production in the mixed layer during summer at the Bermuda Atlantic Time‐series Study (BATS) site in the northwestern Sargasso Sea. Extrapolation of our North Atlantic estimate to the global ocean suggests that the present‐day budget of nitrogen in the ocean may be in approximate balance.