In situ nutrient enrichment experiment in the Bohai and Yellow Sea

Abstract

Nutrient concentrations and N:P ratios have changed significantly in the past 40 years in the Bohai and Yellow Sea. How do these changes influence or contribute to the growth of phytoplankton? Nutrient enrichment experiments were conducted in 1998 and 1999 to shed light on which was the first nutrient to limit algal growth and uptake rates of nitrogen and phosphorus. Significant variance analysis, together with nutrient concentration and ratio, demonstrated that phosphorus was the first nutrient to limit the growth of phytoplankton in the Laizhou Bay (South Bohai); nitrogen was the first, whilst phosphate might be the potential, nutrient to limit the growth of phytoplankton in the West Yellow Sea; the Central Yellow Sea was oligotrophic and any one of nitrogen, phosphorus and silicon would limit the growth of phytoplankton; and silicon was confirmed not to limit the growth of phytoplankton, although the silicate concentration decreased acutely, in the Laizhou Bay. Moreover, the ratio of nitrogen to phosphorus in phytoplankton uptake was smaller than that in seawater, which suggested that phosphorus was preferentially used before nitrogen by the phytoplankton. The preference of phosphorus over nitrogen indicates a further limitation of phosphorus in the Laizhou Bay, and increases the high possibility that phosphorus, rather than nitrogen, is the first nutrient to limit the growth of phytoplankton in the West Yellow Sea. Half-saturation constants (Ks) of dissolved inorganic nitrogen (DIN) and phosphate in the Bohai and Yellow Sea were 1.80 μM and 0.13 μM, respectively. Compared with other sea areas, the Ks value of DIN in the Bohai and Yellow Sea was located at the high end of the spectrum.