DENITRIFICATION AND N2 FIXATION IN THE PACIFIC OCEAN

Reference
Deutsch, C, N. Gruber, R. M. Key, J.L. Sarmiento and A. Ganachaud, Denitrification and N2 fixation in the Pacific Ocean Global Biogeochemical Cycles, 15(2),483-506, 2001.

Abstract
We establish the fixed nitrogen budget of the Pacific Ocean based on nutrient fields from the recently completed WOCE program. The budget for fixed nitrogen in the basin 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 TgN/yr is calculated for the Eastern Tropical Pacific using N* Gruber and Sarmiento [1997], in conjunction with water mass age tracers. Based on rates in the literature, we estimate sedimentary denitrification to contribute an additional loss of 15+/-3 TgN/yr. We then calculate the total nitrogen divergence due to the large scale mass transport through the basin, which is composed of flow through a southern zonal transect at 32\deg S, and through the Indonesian and Bering straits. When atmospheric deposition and riverine fluxes are added, we calculate a net divergence of nitrogen into the basin of -4+/-12 TgN/yr. This forms a Pacific ``box'' in which 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 TgN/yr 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. It also gives a mean areal fixation rate within the range of direct and geochemical rate estimates from a single location near Hawaii by 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.

see related article by Gruber and Sarmiento [1997]