Phytoplankton is the main primary producer in pelagic water.The biomass of phytoplankton community represents phytoplankton standing stock in water where they are collected.However,the error of different methods for calculating the community biomass is unclear.Based on a data set of phytoplankton from an enclosure experiment with three treatments covering a series of phytoplankton communities,we compared phytoplankton community biomass and phytoplankton species population biomass by four calculating methods which are widely applied in China,i.e.,standard method,fine-group method,coarse-group method and reference method,and then investigated the correlation between chlorophyll-a concentration and the phytoplankton biomass calculated by the four methods.The results showed that the coarse-group method is high-efficiency to calculate phytoplankton biomass and suggested to apply.The positive correlation between phytoplankton biomass and chlorophyll-a is not always significant and can not be largely promoted by improving calculation of phytoplankton biomass.We refer rare species to the species with a population less than 5% of community in biomass,and checked the effect of removal the rare species on the phytoplankton community biomass, and found that ignoring rare species resulted in a significant reduction of total phytoplankton biomass.Therefore,whether the rare species can be ignored depends on the structure of phytoplankton community.