Two cruises were carried out in the Yangtze River Estuary and its adjacent sea (29 degrees 30' - 32 degrees 00'N, west of 123 degrees E) in July (summer) and November (autumn), 2005. A total of 345 phytoplankton species, including 43 species causing red tide, were identified. Skeletonema costatum was the dominant species. The average cell abundance was lower in July (5.48 x 10(4) cells L(-1)) than in November (2.70 x 10(5) cells L-(-1)), but the average chlorophyll a concentration was higher in July (2.34 mg x m(-3)) than in November (1.32 mg x m(-3)). The average diversity index (H) was higher in July (1.51) than in November (0.86), as was average evenness (J) (0.59 and 0.34, respectively). Spatial distribution of phytoplankton featured distinct regionality, and the seasonal variation was controlled by factors such as water source, monsoon, nutrient, suspended matter, etc., and the diurnal variation mainly correlated with tide and stratification. At the same time, an evaluation of long-term monitoring data (1996 - 2005) showed that phytoplankton community structure had been changed. Long-term unbalance of N/P ratio caused dinoflagellates to increasingly dominate the phytoplankton population, and led to an increasing frequency of red tide caused by dinoflagellates. Controlling nutrient ratios is more important than just controlling terrigenous contamination input in improving sea area environmental status.
