Abstract: Lagoon ecosystem provides important ecological services, including aquaculture, tourism, and leisure. From October to December in 2016-2018, we monitored phytoplankton communities and nutrient levels in an artificial lagoon in northern Hangzhou Bay, to explore the dynamics of phytoplankton community structure in the lagoon and the main kinds of nutrients driving such changes. The results showed that phytoplankton in the lagoon was mainly composed of Bacillariophyta, Chlorophyta, and Cyanophyta. There were many dominant genera. Cyclotella meneghiniana and Melosira varians of Bacillariophyta were the most dominant species. Diversity indices such as Shannon index, Margalef index and diversity threshold increased year by year. Water body was at a state of moderate eutrophication, with biodiversity being increased. Water quality remained in Class I and Class II, water environment was improved, and the ecological risk was low. The results of redundancy analysis showed that silicate, water temperature, inorganic phosphorus, ratio of nitrogen to phosphorus, and salinity were the main factors affecting phytoplankton community structure during the autumnwinter interchange. Controlling inorganic phosphorus input is an important strategy to prevent harmful algal blooms.

Key words: Hangzhou Bay, phytoplankton, redundancy analysis, water quality.