Sources of new nitrogen in the Vietnamese upwelling region of the South China Sea

Abstract

In the South China Sea, the southwest monsoon between June and September induces upwelling off the southern central Vietnamese coast. During field campaigns in July 2003 and 2004 we evaluated the importance of nitrate and nitrogen fixation as sources of new nitrogen for phytoplankton primary productivity, both in the actual upwelling zone and in the oligotrophic area farther offshore. Complementary to rate measurements of primary productivity, nitrate uptake, and nitrogen fixation, we determined vertical nitrate fluxes by Ekman upwelling (upwelling zone) and turbulent diffusion (offshore waters). Because of the influence of El Niño, upwelling was weaker in July 2003, with an average primary productivity of 28 ± 18 mmol C m−2 d−1, compared to 103 ± 25 mmol C m−2 d−1 in July 2004. Calculated upwelling nitrate fluxes of 17 ± 2 mmol N m−2 d−1 in July 2004 are consistent with N demands of primary productivity, if Redfield stoichiometry is assumed. In July 2003 upwelling fluxes of 14 ± 2 mmol N m−2 d−1 exceeded N demands, indicating that new production was not fully realized. Diffusive nitrate fluxes in offshore waters were determined in July 2003 and are of the order of 2.3 ± 0.6 mmol N m−2 d−1, based on the maximal diffusivities of 1 cm2 s−1 in the nitracline. Nitrogen fixation was a significant N source in offshore waters, but at lower rates in July 2003 (22–119 μmol N m−2 day−1) than in July 2004 (37–375 μmol N m−2 day−1), equaling 1%–5% and probably 2%–25% of the diffusive nitrate fluxes, respectively.