QUALITY MONITORING OF SHENZHEN’S COASTAL WATERS BY SATELLITE AND ITS SPATIOTEMPORAL VARIATION

In recent years, coastal eutrophication and the resulting red tide occurrences have emerged as pressing environmental concerns, posing significant challenges to the sustainable development of Shenzhen, a key city in the China Great Bay Area. Addressing this issue effectively hinges upon a comprehensive understanding of the spatiotemporal dynamics of water quality and red tide outbreaks in Shenzhen's coastal regions. This study utilized the data from the Moderate Resolution Imaging Spectroradiometer(MODIS) to establish models for remote sensing inversion of water quality parameters and automated extraction of red tide occurrences. It has unveiled, for the first time, the spatiotemporal patterns linking key water quality parameters in Shenzhen's coastal waters with red tide outbreaks and analyzed the primary driving factors. Our findings revealed an average annual affected area of red tide occurrence spanning approximately 300.50 km² within Shenzhen's jurisdictional waters, occurring on average 2.38 times annually. Between 2015 and 2022, the annual growth rate of red tide outbreak coverage was approximately 6.7%. Suspended sediment and chlorophyll-a concentrations in the Pearl River Estuary and Shenzhen Bay surpassed those in Dapeng Bay and Daya Bay, exhibiting an increasing trend. Despite the high eutrophication level in the eastern part of the Pearl River Estuary, the frequency of red tide outbreaks remained relatively low, primarily influenced by estuarine suspended sediments. In contrast, regions with superior water quality, such as Dapeng Bay and Daya Bay, experienced higher red tide frequencies that exhibited an annual increasing trend, potentially linking to the variation in nitrogen-phosphorus ratios. The study underscored the imperative need to target nitrogen-phosphorus ratios in the Nan'ao, Kuichong, and Baguang areas and undertook a comprehensive assessment of non-point pollution sources in their terrestrial zones. This information can serve as baseline information crucial for continuously advancing environmental governance, restoration efforts, and management evaluations concerning Shenzhen and its surrounding maritime areas.