Primary, new and export production in the NW Pacific subarctic gyre during the vertigo K2 experiments

This paper presents results on tracer experiments using 13C and 15N to estimate uptake rates of dissolved inorganic carbon (DIC) and nitrogen (DIN). Experiments were carried out at station K2 (47°N, 161°E) in the NW Pacific subarctic gyre during July-August 2005. Our goal was to investigate relationships between new and export production. New production was inferred from the tracer experiments using the f ratio concept (0-50 m); while export production was assessed with neutrally buoyant sediment traps (NBSTs) and the e ratio concept (at 150 m). During trap deployments, K2 was characterized both by changes in primary production (523-404 mg C m−2 d−1), new production (119-67 mg C m−2 d−1), export production (68-24 mg C m−2 d−1) and phytoplankton composition (high to low proportion of diatoms). The data indicate that 17-23% of primary production is exportable to deeper layers (f ratio) but only 6-13% collected as a sinking particle flux at 150 m (e ratio). Accordingly, >80% of the carbon fixed by phytoplankton would be mineralized in the upper 50 m (1-f), while <11% would be within 50-150 m (f-e). DIN uptake flux amounted to 0.5 mM m−2 h−1, which was equivalent to about 95% particulate nitrogen (PN) remineralized and/or grazed within the upper 150 m. Most of the shallow PN remineralization occurred just above the depth of the deep chlorophyll maximum (DCM), where a net ammonium production was measured. Below the DCM, while nitrate uptake rates became negligible because of light limitation, ammonium uptake did continue to be significant. The uptake of ammonium by heterotrophic bacteria was estimated to be 14-17% of the DIN assimilation. Less clear are the consequences of this uptake on the phytoplankton community and biogeochemical processes, e.g. new production. It was suggested that competition for ammonium could select for small cells and may force large diatoms to use nitrate. This implies that under Fe stress as observed here, ammonium uptake is preferred and new production progressively suppressed despite the surplus of nitrate.