Chlorophyll-a concentration inversion and distribution with GOCI images in the Changjiang Estuary

Chlorophyll-a concentration, as a direct indicator of phytoplankton abundance, is important in the study of marine ecological environment and ecosystem's substance circulation and energy exchanges. This paper investigates the distribution of chlorophyll-a over time and space in the Changjiang Estuary through combining in-situ observation during scientific cruises and Geostationary Ocean Color Imager (GOCI) satellite images. The field measured spectral reflectance curves show significant differences from the near-estuarine region to the offshore region, including features such as peak and trough locations, slope variations, and spectral symmetry. Several popular inversion algorithms of chlorophyll-a concentration are optimized and compared using the training dataset, in which the generalized algal bloom index algorithm (GABI) gets a better accuracy and stability. Monthly average distribution maps derived from GOCI images show that chlorophyll-a concentration in the Changjiang Estuary and adjacent areas is always low (≤3 mg/m3 ) in winter, and slightly higher near the estuarine. In spring and summer, the chlorophyll-a concentration rises gradually due to the increasing water temperature, especially in areas east of the maximum turbidity zone. Moreover, chlorophyll-a concentration is relatively higher in the sea adjacent to the southeast of Zhoushan Islands in autumn.