Satellite views of seasonal and inter-annual variability of phytoplankton blooms in the eastern China seas over the past 14 yr (1998–2011)
Abstract. The eastern China seas are one of the largest marginal seas in the world, where high primary productivity and phytoplankton blooms are often observed. However, to date, little is known about the spatial and temporal variability of phytoplankton blooms in these areas due to the difficulty of the monitoring of bloom events by field measurement. In this study, 14-yr time series of satellite ocean color data from the Sea-Viewing Wide Field-of-view Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua satellite have been used to investigate the seasonal and inter-annual variability and long-term changes of phytoplankton blooms in the eastern China seas. We validated and calibrated the satellite-derive chlorophyll concentration in the eastern China seas based on extensive data sets from two large cruises. Overestimation of satellite-derive chlorophyll concentration caused by high water turbidity was found to be less than 10 μg L−1. This level can be used as a safe threshold for the identification of a phytoplankton bloom in a marginal sea with turbid waters. Annually, blooms mostly occur in the Changjiang Estuary and along the coasts of Zhejiang. The coasts of the northern Yellow Sea and Bohai Sea also have high-frequency blooms. The blooms have significant seasonal variation, with most of the blooms occurring in the spring (April–June) and summer (July–September). This study revealed a doubling in bloom intensity in the Yellow Sea and Bohai Sea during the past 14 yr (1998–2011), yet surprisingly, there has been no decadal increase or decrease of bloom intensity in despite of significant inter-annual variation in the Changjiang Estuary. The time series in situ datasets show that both the nitrate and phosphate concentrations increase more than twofold from 1998 to 2005. This might be the reason for the doubling of bloom intensity in the Yellow Sea and Bohai Sea. In addition, the ENSO and PDO can affect the inter-annual variation of bloom intensity in the eastern China seas.