[Microcosm experiments on the influence of different N/P ratios on phytoplankton community growth in the East China Sea].

In the present study, a microcosm experiment was conducted in situ for 30 days, in order to investigate the effects of different N/P ratios (1N: 1P, 4N: 1P, 8N: 1P, 16N: 1P, 32N: 1P, 64N: 1P, 128N: 1P, and 256N: 1P) on phytoplankton community growth in the East China Sea. The results indicated that the species number, cell abundance, Chl-a content, specific growth rate as well as species composition of the phytoplankton community significantly varied with the N/P ratios. After 6 days of culture, the species number, cell abundance, Chl-a content and specific growth rate in the high N/P ratio groups were significantly higher than those in the low N/P ratio groups. After 30 days of culture, the cell abundance in the groups close to the Redfield ratio (8N: 1P, 16N: 1P, and 32N: 1P) was significantly higher than those in the other treatments. On the other hand, the phytoplankton community in all the treatments was observed a definite succession from diatoms to dinoflagellates during the present study. Nonetheless, the N/P ratios strongly affected the duration of dinoflagellate bloom as well as the dominated species: at the beginning of culture (0th day-12th day), the diatoms dominated the phytoplankton community; then the relative contribution of dinoflagellates to the total cell abundance exceeded diatoms in the 4N: 1P, 16N: 1P, and 32N: 1P groups on the 18th day. After that, the relative contribution of dinoflagellates exceeded diatoms ordinally in the other groups from the 24th day to 30th day. At the end of culture, the phytoplankton community was dominated by the dinoflagellates in all the groups except for the 8N: 1P, 16N: 1P, and 32N: 1P treatments.