This paper studies the response of phytoplankton cell density to environmental factors in the Yueqing bay based on the survey data of the Yueqing bay in May 2004. This study analyses the ecological threshold of phytoplankton density under the salinity environment gradient using the Gaussian model. The results show that the optimum salinity of phytoplankton growth is 26. 12,the ecological threshold of salinity is [24. 36,27. 88 ],the optimum ecological threshold is [25. 24,27. 00],the corresponding salinity ...