Seasonal Distribution and Mechanism of Chlorophyll-a in Beibu Gulf

This study used moderate-resolution imaging spectroradiometer (MODIS) data and a high-resolution unstructured grid finite volume community ocean model (FVCOM) to investigate the seasonal and regional characteristics of physical and ecological elements in the Beibu Gulf. The findings reveal that, in the Beibu Gulf, strong temperature gradients exist in winter, with sea surface temperatures becoming uniform at around 30 °C in summer. Sea surface salinity is influenced by low-salinity water from the Qiongzhou Strait and rivers, as well as high-salinity water from the open sea, reaching its minimum levels in autumn. Chlorophyll-a (Chl-a) concentrations peak in nearshore areas, particularly in autumn. In the Beibu Gulf, a significantly negative partial correlation exists between sea surface temperature and Chl-a (ρ=−0.5). Furthermore, wind patterns exert a considerable influence on different regions outside the eastern Beibu Gulf (|ρ|≥0.49), particularly showing a negative correlation along the Vietnamese coast. Cooling-induced water instability, coupled with wind-driven upwelling, enhances vertical water movement, thus facilitating the upward transport of nutrients. High Chl-a concentrations in the estuarine area are attributed to the nutrient-rich discharges (ρ=0.55) and the strong estuarine gravity circulation induced by intense density gradients. There are also notably high Chl-a concentrations in the eastern Beibu Gulf and the western coast of Hainan Island despite the presence of adverse environmental conditions. These anomalies can be attributed to wind-induced upwelling and tidal mixing front-induced upwelling, respectively. This study presents a comprehensive analysis of the distribution and mechanisms of Chl-a in the Beibu Gulf, thus highlighting the complex interactions among various factors and emphasizing the importance of local environmental conditions.