Seasonal and interannual variability in sources of nitrogen supporting export in the oligotrophic subtropical North Pacific Ocean

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 balance the input of exogenous combined inorganic nitrogen (i.e., export production balances new production). In the oligotrophic North Pacific subtropical gyre, there are two significant sources of new nitrogen: the upward flux of nitrate from deep water and the biological fixation of dissolved N2 in near‐surface waters. Because these sources have distinct stable isotopic signatures, we were able to use the total PN and Δ15N measurements of sinking particles and an isotopic mass balance model to deconvolute the relative and absolute contributions of the nitrate flux and nitrogen fixation to the gravitational export of PN. The sinking flux of PN and its isotopic composition both varied widely between 1989 and 2001. Seasonally, N2 fixation correlated inversely with mixed‐layer depth, reaching a maximum in June‐August, whereas nitrate‐supported export correlated inversely with sea surface temperature, reaching a maximum in February‐March. These patterns were consistent with summertime increases in diazotroph biomass and water column N2 fixation rates, as indicated by phycoerythrin pigment concentrations and 15N2 tracer studies. Annually, the relative contribution of N2 fixation to N export varied from 36 to 69% (mean = 48%) and showed a significant increasing trend over the period of observation. Although total PN export correlated with the Southern Oscillation Index, the nitrate‐ and nitrogen fixation‐based components appeared to respond to climate forcing in distinct ways, complicating our interpretation of the mechanisms of climate‐induced changes in particle export.