Seasonal changes in phytoplankton were studied in Shenzhen Bay from February to November 2008. A total of 150 species were found, with different species dominating in each season. The density of phytoplankton varied throughout the seasons, with the highest density in summer. The water quality in the bay was found to be eutrophic, with high levels of nutrients. The community composition of phytoplankton showed a trend towards simplification, indicating a fragile ecological system in Shenzhen Bay.