Spatio-Temporal Distribution of Sea Surface Chlorophyll-A in Coral Reefs of the South China Sea Over the Past Decade Based on Landsat-8 Oli Images

Abstract

The concentration of chlorophyll-a (Chl-a) in seawater can indicate the phytoplankton growth and the water eutrophication, so they are usually used to evaluate the primary productivity and the carbon source/sink of coral reefs. However, the precise delineation of Chl-a concentration in coral reefs remains challenging when utilizing ocean satellites with low spatial resolution. In this study, a remote sensing inversion model for Chl-a was developed in fringing reefs (R2=0.7721, RMSE =0.87) and atolls (R2=0.8237, RMSE =0.06), utilizing the reflectance data from the sensitive band of the Landsat-8 Operational Land Imagers (OLI) with a spatial resolution of 30 m. The aforementioned model was utilized to invert the high-resolution distribution maps of Chl-a concentration in six major coral reef regions of the South China Sea from 2013 to 2022, and subsequently analyze the variations in Chl-a concentration and its influencing factors. The results indicate a gradient in Chl-a concentration among coral reefs, with Daya Bay, Weizhou Island, Luhuitui, Xuwen, Huangyan Island, and Xisha Island exhibiting a respective order. The nearshore region exhibits a higher concentration of Chl-a than the offshore area in each reef. The Chl-a concentration in the South China Sea coral reefs exhibits significant seasonal fluctuations, characterized by higher concentrations during the winter and the spring, and lower concentrations during the summer and the autumn. The concentration of Chl-a in coral reefs exhibits a negatively correlated with precipitation and a positive correlation with average wind speed.

Keywords: coral reef, Chl-a, Landsat-8 OLI, Remote Sensing, Spatio-temporal variations