Satellite Observations Reveal Recent Increases in Water Clarity in Chinese Coastal Waters

Coastal ecosystems in China have undergone profound transformations under the combined pressures of climate change and human activity, yet the long-term dynamics of water clarity and its underlying drivers remain poorly resolved. Here, we derive high-quality Secchi depth (Zsd) and optically active constituent (OAC) products─including total suspended matter (TSM), colored dissolved organic matter (CDOM), and chlorophyll-a (Chl-a)─from MODIS observations spanning 2003-2022. Our analysis reveals that 77.4% of Chinese coastal waters experienced recent increases in water clarity, with 22.7% showing continuous increases and 54.7% exhibiting nonlinear transitions marked by recovery following earlier declines. TSM emerged as the dominant control on Zsd variability, accounting for ∼60% of the influence overall and up to 80% in nearshore waters, while CDOM contributed more prominently offshore. Our findings highlight the importance of capturing nonlinear changes and constituent-specific drivers in assessing and managing coastal water quality under accelerating environmental change.