Why does surface chlorophyll-a bloom in Taiwan Shoal all year round?

Abstract

Based on 10 years (January 2012–December 2021) of satellite observations, sea surface chlorophyll-a (Chl-a) was found to be present in the Taiwan Shoal throughout the year. The underlying mechanisms were investigated. The seasonal mechanism was investigated based on satellite remote sensing data, reanalysis data, and Argo buoy data. The relationship between the seasonal variation of Chl-a and sea surface temperature (SST), wind speed, photosynthetically active radiation (PAR) and salinity in Taiwan shoals was calculated. The results revealed that a large amount of Chl-a was present in the study area throughout the year, where the Chl-a concentration reached a maximum of 0.88 mg m⁻³ in July, which was 1.76 times greater than that (0.50 mg m⁻³) in April. In summer, there was increased upwelling and a thinner mixed layer, which pulled the bottom nutrients to the surface and led to the inflow of South China Sea water (SCSW) with Chl-a in the horizontal direction. In winter, the Kuroshio Branch Water (KBW) intrusion and the inflow of Chinese coastal water into the northern part of the Taiwan Strait (TWS) resulted in a zone of high temperature gradient, whereas the strong northeasterly monsoon strengthened the vertical mixing of the study area, which coincided with the zone of high Chl-a values. The combination of nutrient-rich waters, suitable temperatures in spring, and sufficient photosynthetically active radiation (PAR) in autumn provides favorable conditions for the accumulation of surface Chl-a. This study provides valuable insights into the ecological impacts of seasonal variations in Chl-a blooms in the TWS.