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.
Wu et al. (2025) studied this question.