Denitrification and N₂ fixation in the Pacific Ocean

We establish the fixed nitrogen budget of the Pacific Ocean based on nutrient fields from the recently completed World Ocean Circulation Experiment (WOCE). The budget includes denitrification in the water column and sediments, nitrogen fixation, atmospheric and riverine inputs, and nitrogen divergence due to the large-scale circulation. A water column denitrification rate of 48 ± 5 Tg N yr^-1 is calculated for the Eastern Tropical Pacific using N^* [Gruber and Sarmiento, 1997] and water mass age tracers. On the basis of rates in the literature, we estimate sedimentary denitrification to remove an additional 15 ± 3 Tg N yr^-1. We then calculate the total nitrogen divergence due to the large scale circulation through the basin, composed of flows through a zonal transect at 32°S, and through the Indonesian and Bering straits. Adding atmospheric deposition and riverine fluxes results in a net divergence of nitrogen from the basin of -4 ± 12 Tg N yr^-1. Pacific nitrogen fixation can be extracted as a residual component of the total budget, assuming steady state. We find that nitrogen fixation would have to contribute 59 ± 14 Tg N yr^-1 in order to balance the Pacific nitrogen budget. This result is consistent with the tentative global extrapolations of Gruber and Sarmiento [1997], based on nitrogen fixation rates estimated for the North Atlantic. Our estimated mean areal fixation rate is within the range of direct and geochemical rate estimates from a single location near Hawaii [Karl et al., 1997]. Pacific nitrogen fixation occurs primarily in the western part of the subtropical gyres where elevated N* signals are found. These regions are also supplied with significant amounts of iron via atmospheric dust deposition, lending qualitative support to the hypothesis that nitrogen fixation is regulated in part by iron supply.