N fixation impacted by carbon fixation via dissolved organic carbon in the changing Daya Bay, South China Sea.

We present the first concurrent measurements of N fixation rates (N uptake), primary production (C uptake), dissolved organic carbon (DOC) concentrations, and diazotrophic community composition derived from nitrogenase (nifH) abundance in the subtropical Daya Bay (DB) of the coastal northern South China Sea (NSCS) from 2015 to 2017. N fixation rates ranged from n.d. - 4.51 nmol N L h. Such values were generally higher than those reported in the neighbouring NSCS open waters and several well-studied oligotrophic waters, thereby suggesting that N-replete conditions do not prevent N fixation in coastal waters. N fixation rates were positively and significantly correlated with the primary production and the concentration of DOC in DB in the spring and summer. Combined with other lines of evidence, we suggest that N fixation may be facilitated by non-diazotrophic phytoplankton via a probable regulation of the quantity and quality (bioavailability) of DOC in DB. Since DB represents a suitable site that has experienced dramatic human-induced changes in environmental conditions, our results likely provide insights in understanding how N fixation and relevant biogeochemical processes may respond to intensified global anthropogenic forcing in similar coastal settings.