Measurement of Coastal Water Quality Indicators Using Sentinel-2; An Evaluation Over Hong Kong and the Pearl River Estuary

Abstract

The Pearl River Estuary is a complex case-II waters, the adjacent coastal waters of Hong Kong consist of many diverse aquatic species as well as under adverse threat due to high pollution and sediment loads from the neighboring areas. Frequent algal blooms often referred as red tide, occur in various parts of Hong Kong’s coastal areas. It is always challenging to measure Chlorophyll-a (Chl-a) with accuracy in presence of high concentration of suspended solids (SS) and therefore difficult to detect algal blooms accurately in spatial domains. Land adjacency, shallow water, and small spatial area of bloom are also some factors limiting bloom detection in this region. However, Sentinel-2 MSI with red edge bands and with 10-20 m spatial resolution provides a new opportunity to monitor these sensitive and complex waters. This study presents a new evaluation of using red edge algorithm and C2RCC for estimating Chl-a; and red band algorithm and C2RCC for estimating SS using Sentinel-2 MSI data over the Pearl River Estuary and the coastal waters of Hong Kong. C2RCC-Nets method outperformed with coefficient of determination R2 = 0.71 (RMSE 2.1 µg/l) and R2 = 0.73 (RMSE 2.4 mg/l) for estimating Chl-a and SS respectively. Thin slicks of red tide are detected in highly turbid Pearl River Estuary using MSI data. Results show that Sentinel-2 data can be used to estimate Chl-a and SS with high accuracy and thin red tide slick and red tide blooms can also be detected in presence of high concentration of SS.