Phytoplankton algae are the main primary producers in pelagic water, and their species diversity is a measure of the structure and state of communities.Phytoplankton is sensitive to external conditions such as nutrients, predators and hydrodynamics, because of its small size and high reproductive rate.In phytoplankton succession, both dominant species and rare species can markedly change with ecological factors, even over a short period.Species richness, Shannon's diversity and Simpson's diversity are widely used.However, these diversity indices are usually calculated by using abundance as the variable, while biomass is only rarely used.As abundance and biomass emphasize different aspects of community function, there may be a significant dissimilarity between the diversity calculated from these two variables.In the present study, an enclosure experiment system (nine enclosures, 4 m× 4 m and 6 m deep for each) was conducted, comprising three treatments (control, nutrient enrichment and fish stocking) in Liuxihe Reservoir, Guangdong.Each treatment had three replications.Nutrient enrichment and fish stocking were manipulated to simulate intermediate disturbance relative to the control (without fish stocking and nutrient enrichment).In the nutrient enrichment treatment, KH₂PO₄ was added to a targeted total phosphorus concentration of 0.018 mg/L, while fish was stocked to a fish density of 2 g/m³.Each enclosure was sampled every ten days from May 4 to July 26, 2012.We explored the changes in species composition by checking rare species, dominant species and diversity indices.Shannon's diversity and Simpson's diversity indices were calculated based on both abundance and biomass.The results showed that the phytoplankton community evolved most significantly in the nutrient enrichment treatment.Both nutrient enrichment and fish stocking treatments increased species diversity of the phytoplankton community, as predicted by the intermediate disturbance hypothesis.A significantly negative correlation between diversity indices and phytoplankton biomass only occurred when the dominance of dominant species became high.Shannon's diversity and Simpson's diversity indices displayed a similar pattern during the succession of the phytoplankton community.The diversity indices with the variables of abundance and biomass did not significantly differ in explaining the structure of the phytoplankton community.