Nutrient and eutrophication characteristics of the Dongshan Bay,South China

We recorded NO₃-N, NO₂-N, NH₄-N, PO₄-P, SiO₃-Si, salinity, and temperature data at 10 stations in the Dongshan Bay in May, June, July, August, October, and November 2008, analyzing nutrient and eutrophication characteristics. The mean concentration of dissolved inorganic nitrogen (DIN) was 0.30–0.40 mg/dm³; generally, NO₃-N was the main form in most areas. The mean concentrations of PO₄-P and SiO₃-Si were 0.040–0.060 mg/dm³ and 1.00–1.50 mg/dm³, respectively. We proved that the majority of the SiO₃-Si in the Dongshan Bay came from the Zhangjiang River, with some coming from the Bachimen Strait. DIN originated from both the Zhangjiang River and the Bachimen Strait. Most PO₄-P originated from the Bachimen Strait, and some came from the Zhangjiang River. We found that P was an overall limiting factor to the phytoplankton community in most of the Dongshan Bay, and that Si and N were in surplus. However, near the Bachimen Strait Si became a limiting factor, especially for diatoms, while P and N were in comparative surplus. We used a potential eutrophication assessment method to analyze eutrophication, and showed that the most serious eutrophication occurred near the Zhangjiang River estuary and near the Bachimen Strait. In 2008, DIN levels were four times higher than that in 1988; PO₄-P levels were threefold higher, while SiO₃-Si was approximately double. Dissolved nutrients increased between 1988 and 2008. DIN increased at the greatest rate comparing to PO₄-P and SiO₃-Si, thus the N/P and N/Si mol ratios increased. Further studies on the effects of high DIN concentrations on the phytoplankton communities and marine ecosystems of the Dongshan Bay are needed.