Abstract: To investigate the effects of nitrogen (N) and phosphorus (P) on the growth of phytoplankton, an enrichment incubation experiment was carried out in the waters of Nansha Islands in summer of 2013. Two experimental sites, S1 (9°30′N, 109°30′E) and S2 (4°30′N, 109°30′E), were in the southwest continental shelf and near the James shoal area, respectively. The results showed that the chlorophyll-a increased significantly (P<0.05) after addition of single N or mixed N and P. The chlorophyll-a concentration increased from 0.05 mg·m^-3 to 0.29 and 0.80 mg·m^-3 at S1 site, and from 0.09 mg·m^-3 to 0.79 and 1.02 mg·m^-3 at S2 site after addition of single N and mixed N and P, respectively. However, the enrichment of P did not promote the phytoplankton growth because the chlorophyll-a concentration showed no significant increase by adding single P at both sites. The N concentration was found to be the limitation factor of phytoplankton growth at S1, but both N and P concentrations were at S2. Moreover, the most adaptable N/P ratio for phytoplankton growth should be between 5 and 25. The results of correlation analysis showed there was no significant relationship between the N/P ratio and the concentration or the growth rate of chlorophyll-a, suggesting that the N/P ratio would not influence the phytoplankton growth in the waters of Nansha Islands in summer.

Key words: Michaelis constant, cadmium pollution, uptake rate, rice