We conducted one mesocosm experiment on board in ocean basin in South China Sea.We studied the effect of different forms of nitrogen and iron on phytoplankton communities in May 2005.The effects of nitrogen and iron on phytoplankton communities,DMS and primary production in one of six experiments were very obvious.In M5 (nitrogen + iron) the maximum value of chlorophyll a,DMS and primary productivity were 27.23μg l -1,5.047ng ml -1,121.63mgC m -3 h -1 respectively.The maximum value of chlorophyll a,DMS and primary productivity in control bag were 4.24μg l-1,2.05ng ml-1,31.05mgC m-3h-1 respectively.The concentration of DMS in ocean area may enhance with the increase of nutrient and iron.So this change may have significant impacts on the climate and environmental change.The increase of nutrient and iron from the atmosphere may stimulate the biomass and primary production in ocean area.The absorption ability of three nitrogen nutrients was different from the experiment: the fastest is ammoniate,the lowest is urea.The dominant species were at first mainly picoophytoplankton (2~20μm) (from chlorophyll a) and then became microphytoplankton in the end (>20μm) in M5,but the succession of dominant species was not obvious on other bags.