The coupling of temporal and spatial variations of chlorophyll a concentration and the East Asian monsoons in the southern Taiwan Strait

Abstract

The impact of monsoon on the temporal and spatial variations of phytoplankton standing stock (Chl a) in the southern Taiwan Strait was studied based on long-term satellite data (1997–2008) and field observations (during 1987–1988 and 2006–2007). During the NorthEast (NE) monsoon, the high Chl a was induced by vertical mixing, the Zhejiang–Fujian Coastal Current and upwelling. However, for most of the area, the vertical mixing was the dominant process that enhanced phytoplankton growth. During the SouthWest (SW) monsoon, two low temperature and high Chl a areas were observed: one near the Dongshan Island and the other in the southeast edge of the Taiwan Bank. Both of them were identified as the upwelling areas a half century ago. The Dongshan upwelling is mainly caused by the cold waters, which is derived from the famous “East Guangdong upwelling” induced by the SW monsoon. And the latter upwelling is mainly induced by a shallowing of the topography and the Kuroshio intrusion. In the last decades, most of the studies suggested that the Chl a during the SW monsoon is higher than that during the NE monsoon, due to the upwelling and cold temperature, respectively. However, some recent studies made contrary conclusion. Using the satellite seasonal climatological Chl a data (SeaWiFS and MODIS), we acquired more extensive and long-term records and found a significant difference in the temporal variation patterns of Chl a concentrations in the three sub-areas of southern Taiwan Strait. In the coastal area, two Chl a peaks were observed, which were due to upwelling during the SW monsoon and Zhejiang–Fujian Coastal Current during the NE monsoon. On the Taiwan Bank, the high Chl a occurred throughout the year due to the topographical upwelling all year round. For the shelf break area, a higher Chl a was observed during the NE monsoon, which was induced by vertical mixing. For the whole study area, a significant coupling occurred between Chl a and wind speed ( r 2=0.23, p<0.05) during the NE monsoon, while it was very weak during the SW monsoon. On the other hand, the mean surface Chl a was significantly higher in the NE monsoon (0.69 mg m −3) than in the SW monsoon (0.53 mg m −3). All these results demonstrated that the temporal and spatial variations of phytoplankton standing stock were affected by the East Asia monsoon system in the southern Taiwan Strait.