Nitrogen-based and nitrogen fixation-based support of export production at Station ALOHA: 1989-2001

J. E. Dore, L. M. Tupas, J. R. Brum, D. M. Karl

University of Hawaii, Department of Oceanography, Honolulu, HI 96822

Abstract

Over timescales of months to years, the export of organic nitrogen from the oceanic euphotic zone (principally as sinking particulate nitrogen, PN) is believed to closely balance the input of exogenous combined inorganic nitrogen, i.e., export production is balanced by new production. In the oligotrophic waters of the North Pacific subtropical gyre, there are two significant sources of new nitrogen to the euphotic zone: the upward flux of nitrate from deep water and the fixation of dissolved nitrogen gas by diazotrophic microorganisms in near-surface waters. These N sources have distinct stable isotopic signatures (δ¹⁵N ~ 6.5 for the nitrate flux, δ¹⁵N ~ 0 for nitrogen fixation), therefore the δ¹⁵N of exported PN is constrained between these two extremes. We utilize total PN and δ¹⁵N measurements of sinking particles (captured at 150 m in free-drifting sediment traps) and a simple isotopic mass balance model to deconvolute the relative and absolute contributions of the nitrate flux and nitrogen fixation to the gravitational export of PN at Station ALOHA (22 45 N, 158 W). We find that the sinking flux of PN and its isotopic composition have both varied widely over month-to-month, seasonal, and interannual timescales between 1989-2001. On a seasonal basis, nitrogen fixation correlates inversely with mixed-layer depth, reaching a maximum in Jun-Aug while nitrate-supported export correlates inversely with sea surface temperature, reaching a maximum in Feb-Mar. These patterns are consistent with summertime increases in diazotroph biomass as indicated by phycoerythrin concentrations. On an annual basis the relative contribution of nitrogen fixation to N export has varied from a low of 36% in 1993 to a high of 69% in 1999, with an overall flux-weighted mean contribution of 48%. This fraction demonstrated a significant increasing trend over the twelve-year period of observation. While total PN export seems to correlate well with the Southern Oscillation Index, the nitrate-based and nitrogen fixation-based components appear to respond to tropical climate forcing in different ways.