Nutrient dynamics and N-anomaly at the SEATS station

The distributions of nitrate+nitrite, or (N+N), and soluble reactive phosphate, or SRP, at the SouthEast Asian Time-series Study (SEATS) station in the northern South China Sea (SCS) at 18°N and 116°E were determined in 19 cruises between September 1999 and October 2003. In the deep water below 1500 m, the concentrations of (N+N) and SRP stayed constant with depth and time at 38.4±0.3 and 2.84±0.03 μM, respectively. The variability could be accounted for by the analytical uncertainties in the measurements alone. In contrast, variations in the upper water were significantly larger and far exceeded the analytical uncertainties. In the mixed layer, unlike what has been widely reported in other low-latitude waters, there was a distinct seasonal pattern. During most of the year, the concentrations were low and hovered around the detection limits. In the winter, readily detectable concentrations were found as a result of enhanced vertical mixing. In the deep water below 1500 m, the average (N+N)/SRP molar ratio, 13.6±0.2, was noticeably lower than the Redfield ratio of 16. The corresponding N-anomaly, N*, was −4.0±0.6 μM. In the upper nutricline, high (N+N)/SRP and N*, even exceeding 16 and 2 μM, were found. These elevated values are suggestive of the remineralization of nitrogen-rich organic matter in the upper nutricline. Temporally, while there were considerable inter-annual variations, higher (N+N)/SRP and N* were found mostly between September and April during the period of higher depositional fluxes of atmospheric dust. In the Tropical Water, which was situated within the upper nutricline at 150 m, the average N* were 1.9 and −0.3 μM, and the average (N+N)/SRP were 15.0 and 12.6 in September–April and May–August, respectively. These seasonal patterns suggest that the nitrogen-rich organic matter could have originated from nitrogen fixation, which was enhanced by the availability of iron during seasons of high dust-depositions. A box model calculation indicates that, in the absence of nitrogen fixation, the deficit in combined nitrogen would lead to a rapid escalation in the concentration of SRP in the mixed layer in contradiction to the field observations. If nitrogen fixation is allowed, the deficit in combined nitrogen in the mixed layer could be satisfied and a steady state in (N+N) and SRP in the mixed layer maintained by supporting 3–13% of the net primary production by nitrogen fixation.