Impacts of coastal aquaculture pond distribution and drainage variations on offshore water quality in China

Coastal aquaculture ponds (CAPs) play a key role in ensuring global food security. However, their rapid expansion and seasonal drainage have triggered severe offshore water quality deterioration, posing substantial challenges to coastal sustainability in China. Therefore, accurately characterizing CAPs drainage dynamics and analyzing their impacts on offshore water quality are practically significant. In this study, we conducted the first national-scale assessment of CAPs drainage dynamics from 2017 to 2022 by utilizing time-series Harmonized Landsat and Sentinel-2 imagery combined with Sentinel-1 SAR data. We further quantitatively assessed the impacts of CAPs distribution and drainage periods on offshore water quality characterized by six parameters. CAPs drainage period exhibited temporal variations across China’s coastline. Interannual analysis of drainage patterns (2017–2022) exhibited significant delays, with an average of 12 and 29 days at the start and end of drainage periods, respectively. These temporal shifts were accompanied by a 19-day mean increase in drainage duration. Moreover, regions with degraded offshore water quality exhibited a high density of CAPs distribution. CAPs area and Chlorophyll-a concentration exhibited a significant positive correlation, which gradually increased as offshore distance decreased. Moreover, offshore water quality parameters exhibited notable peaks during the drainage periods, with the peak timing synchronized with interannual drainage dynamics, while water quality showed minimal fluctuations near the non-drained aquaculture ponds. The research findings could benefit coastal ecosystem governance and the sustainable development of marine fisheries, thereby promoting marine fishery enhancement and safeguarding marine ecological security.