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The coastal area adjacent to the Changjiang River estuary is a high‐risk region for harmful algal blooms. Both Changjiang River discharge and Kuroshio intrusion influence the hydrological conditions and nutrient composition in this area which could affect the onset of diatom and dinoflagellate blooms. However, current understandings on their roles in regulating different microalgal blooms are limited. In this study, changes in stratification, water masses, and the ratio of phosphorus to nitrogen were examined during the diatom and dinoflagellate blooms in 2005 using ship cruise data. The observations showed that both the diatom and dinoflagellate blooms occurred in the continental coastal water. By enhancing the pycnocline and phosphorus stress, the Changjiang diluted water plays an important role in affecting the diatom bloom. The dinoflagellate bloom is partly supported by the supply of phosphate transported by Kuroshio intrusion and upwelling. The results of this study will help elucidate the mechanisms of large‐scale harmful algal blooms in the coastal area adjacent to the Changjiang River estuary. Plain Language Summary: The coastal area adjacent to the Changjiang River estuary is a high‐risk region for harmful algal blooms. Intensive blooms of diatoms and dinoflagellates have been observed every year from spring to early summer. In 2005, field cruises were conducted in the East China Sea to investigate the succession process from the diatom to the dinoflagellate bloom. Based on the field investigation data, the characteristics and variation of stratification, water masses, nutrient composition, and microalgal blooms were analyzed to find out how Changjiang River discharge and Kuroshio intrusion affected the dynamics of different microalgal blooms. Four water masses were identified during the blooms, and both types of microalgal blooms were found to occur in the continental coastal water. The Changjiang River discharge was found to play an important role during the diatom bloom by enhancing the pycnocline and phosphorus stress, while the Kuroshio intrusion supplied phosphate for the development and maintenance of the large‐scale dinoflagellate bloom. The results could help elucidate the mechanisms of harmful algal blooms in the coastal area adjacent to the Changjiang River estuary. Key Points: Blooms of diatoms and dinoflagellates in the East China Sea are subject to the impacts of Changjiang River discharge and Kuroshio intrusionChangjiang River discharge leads to strong pycnocline and phosphorus stress and has significant influences on the diatom bloomsKuroshio intrusion and upwelling supply extra phosphate to sustain the large‐scale dinoflagellate blooms [ABSTRACT FROM AUTHOR]

Abstract: Nutrient input from the Changjiang River (Yangtze River) has been increasing dramatically since the 1960s. At the mouth of the Changjiang River, the nitrate concentration has increased about three-fold in 40 years, from 20.5μmol/L in the 1960s to 59.1μmol/L in the 1980s and to 80.6μmol/L in 1990–2004. Phosphate concentration increased by a factor of 30%, from 0.59μmol/L in the 1980s to 0.77μmol/L in 1990–2004. The increasing nitrate input has arisen mostly from the mid and lower reaches of the Changjiang River, where the river meets one of the most strongly developed agriculture areas in China. Responses of the coastal phytoplankton community to the increasing nutrient inputs are also seen in the available monitoring data. First, a trend of increasing phytoplankton standing stock from 1984 to 2002 appeared in the Changjiang River estuary and adjacent coastal waters, especially in late spring. Secondly, the proportion of diatoms in the whole phytoplankton community showed a decreasing trend from about 85% in 1984 to about 60% in 2000. Finally, red tides/harmful algal blooms increased dramatically in this area in terms of both number and scale. About 30–80 red tide events were recorded each year from 2000 to 2005 in the East China Sea. The scale of some blooms has been in excess of 10,000km2. [Copyright &y& Elsevier]