Nutrient limits phytoplankton growth in nutrient enrichment experiments with seawater from the Indian Ocean
印度洋海水营养盐添加模拟实验中浮游植物生长的营养盐限制作用

Nutrient enrichment experiments were carried out in the Indian Ocean during the Chinese First Around-world Researching Cruise, with N, Fe, N+Fe and N+Fe+P being added in the collected surface sea water separately. Varying some parameters in the experimental water, we studied the factors that limited phytoplankton growth, such as the concentration of nutriment, chlorophyll a, temperature, and etc. The addition of nitrogen resulted in phytoplankton blooming, but the addition of iron did not stimulate the growth of phytoplankton. The combined effect of nitrogen and phosphorus to phytoplankton growth was much stronger than that of nitrogen alone. Furthermore, the utilization of nitrogen proceeded that of phosphorus; and the available phosphorus could be utilized by phytoplankton after nitrogen was exhausted. Variation of the ratios of nitrogen to phosphorus in the experimental water did not match that of chlorophyll a concentration and phytoplankton growing rate (R). The correlation between them was weak. So it was considered that the ratio of nitrogen to phosphorus cannot determine the growth of phytoplankton. In addition, water temperature played an important but not a limiting role in the phytoplankton growth in these experiments , as was shown by the correlation between the temperature of experimental water and chlorophyll a concentration or the value of R.