Coastal Water Clarity in Shenzhen: Assessment of Observations from Sentinel-2

Shenzhen is a crucial city in the Guangdong-Hong Kong-Macao Greater Bay Area (GBA). With high-intensity land development and rapid population growth, the ocean has become an essential space for expansion, leading to significant variations in water quality in the coastal area of Shenzhen. Water clarity (Zsd) is a key indicator for evaluating water quality. We applied the quasi-analytical algorithm (QAA) to Sentinel-2 data and retrieved the Zsd of the coastal area of Shenzhen. By adjusting the red band for distinguishing water types, we avoided underestimating Zsd for clear water. This study pioneered the production of a 10 m Zsd product for the coastal area of Shenzhen from 2016 to 2021. The results showed that the coastal area of Shenzhen exhibited a spatial distribution pattern with low Zsd in the west and high in the east, with Pearl River Estuary (PRE: 0.41-0.67 m) and Shenzhen Bay (SZB: 0.30-0.58 m) being lower than Dapeng Bay (DPB: 2.7-2.9 m) and Daya Bay (DYB: 2.5-2.9 m). We analyzed the seasonal and interannual variations and driving factors of the four areas, where PRE and SZB showed similar variation patterns, while DPB and DYB showed similar variation patterns. PRE and SZB are important estuaries in southern China, significantly affected by anthropogenic activities. DPB and DYB are important marine aquaculture areas, mainly affected by natural factors (wind speed, precipitation, and sea level). The Zsd of the coastal area of Shenzhen, along with the analysis of its results and driving factors, contributes to promoting local water resource protection and providing a reference for formulating relevant governance policies. It also provides a practical method for assessing and monitoring near-shore water quality.