The analysis of phytoplankton blooms off the Yangtze River Estuary in the spring of 2007

The Yangtze River Estuary and the Zhejiang coastal waters have a high incidence of spring phytoplankton bloom and even red tide, but there are rarely phytoplankton blooms in the open waters off the Yangtze River Estuary in the spring considering its relative oligotrophy. From 25 April to 7 May 2007, two strong phytoplankton blooms were observed in the open sea area (the east by 125° E-box C), and the chlorophyll-a concentration, whether measuring through remote sensing or in situ, consistently indicated bloom characteristics. Chlorophyll-a concentration in this area increased significantly, even up to 11 mg·m−3 at individual stations during this period. Average chlorophyll-a measured by remote sensing in region C (125–127°E, 30–32°N) was 2.07 mg·m−3, while the climatic average was only 0.97 mg·m−3 during 2000 to 2008. Analyses demonstrated that suspended sediment concentrations had almost no change, but a slight decrease occurred during the two blooms periods as well as low nitrate and phosphate and higher silicate and salinity in box C. It was suggested that the increased chlorophyll-a concentration was not caused by the intrusion of dilute water from the estuary with high suspended sediment concentrations and nutrients, but probably due to local strong winds on the night of 26 April to the morning of 27 April. A further study with profile data of chlorophyll-a, temperature, salinity and other parameters indicated that strong mixing by the local wind was the main reason for the blooms.