A Fixed Nitrogen Budget for the Pacific Ocean

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 deposition and nitrogen convergence due to the large-scale circulation. A water column denitrification rate of 48 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 of in the literature, we estimate sedimentary denitrification to contribute an additional loss of 12 TgN/yr. We then calculate the total transport convergence of N* into the basin, composed of contributions from a southern zonal transect at 32 S, and the Indonesian and Bering outflows. The resulting transport convergence of 19-28 Tg/yr is supplemented by an additional 12 Tg/yr of nitrogen deposition at the sea surface. 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 must contribute 30-80 TgN/yr in order to balance the Pacific N budget. The impact of this nitrogen input can also be seen directly in the elevated N* signal of the subtropical gyres where iron supply is also quite high. This result is consistent with the tentative global extrapolations of Gruber and Sarmiento [1997], based on nitrogen fixation rates estimated for the North Atlantic thermocline. This supports the emerging view of a marine N cycle that is more dynamic, and approximately in balance.