Satellite views of the seasonal and interannual variability of phytoplankton blooms in the eastern China seas over the past 14 yr (1998–2011)

[摘要] The eastern China seas are some of the largest marginal seas in the world,where high primary productivity and phytoplankton blooms are often observed.However, little is known about their systematic variation of phytoplanktonblooms on large spatial and long temporal scales due to the difficulty ofmonitoring bloom events by field measurement. In this study, we investigatedthe seasonal and interannual variability and long-term changes inphytoplankton blooms in the eastern China seas using a 14 yr (1998–2011) timeseries of satellite ocean colour data. To ensure a proper satellite dataset tofigure out the bloom events, we validated and corrected the satellite-derivedchlorophyll concentration (chl a) using extensive in situ datasets from twolarge cruises. The correlation coefficients between the satellite retrievaldata and the in situ chl a on the logarithmic scale were 0.85 and 0.72 forthe SeaWiFS and Aqua/MODIS data, respectively. Although satellites generallyoverestimate the chl a, especially in highly turbid waters, both the insitu and satellite data show that the overestimation of satellite-derivedchl a has an upper limit value (10 μg L⁻¹), which can beused as a threshold for the identification of phytoplankton blooms to avoidthe false blooms resulting from turbid waters. Taking10 μg L⁻¹ as the threshold, we present the spatial-temporalvariability of phytoplankton blooms in the eastern China seas over the past14 yr. Most blooms occur in the Changjiang Estuary and along the coasts ofZhejiang, with a maximal frequency of 20% (about 73 days per year). Thecoasts of the northern Yellow Sea and the Bohai Sea also have high-frequencyblooms (up to 20%). The blooms show significant seasonal variation, withmost occurring in spring (April–June) and summer (July–September). Thestudy also revealed a doubling in bloom intensity in the Yellow Sea and BohaiSea during the past 14 yr. The nutrient supply in the eastern China seasmight be a major controlling factor in bloom variation. The time series insitu nutrient datasets show that both the nitrate and phosphateconcentrations increased more than twofold between 1998 and 2005 in theYellow Sea. This might be the reason for the doubling of the bloom intensityindex in the Yellow Sea and Bohai Sea. In contrast, there has been nosignificant long-term increase or decrease in the Changjiang Estuary, whichmight be regulated by the Changjiang River discharge. These results offer afoundation for the study of the influence of phytoplankton blooms on thecarbon flux estimation and biogeochemical processes in the eastern Chinaseas.

[学科分类] 地球化学与岩石