Natural and human activities driving the spatiotemporal variability of water clarity in lakes across Eastern China

Abstract

• The water clarity presents a pattern of “high in the south and low in the north”. • The month of highest Z sd is postponed from June in the south to August in the north. • Natural and human activities contributed equally to Z sd in Eastern lakes. Water clarity, indexed by Secchi disk depth ( Z sd ), comprehensively reflects water quality and can be monitored by satellite. Z sd may vary across lakes globally, driven by natural and human activities (climate, land use/land cover, lake morphological characteristics, etc.). Based on Sentinel-3 Ocean and Land Color Instrument (OLCI) imagery, we explored the driving factors of Z sd spatiotemporal patterns in lakes across Eastern China (including the Northeast Plain and Mountain Lake (NPML) Zone and the Eastern Plain Lake (EPL) Zone). Spatially, the responses of driving factors were assessed using feature importance and partial dependence in the random forest model. The results showed that the impacts of natural factors on the spatial variations of Z sd (accounting for 52.00 % of contributions) are comparable to that of human activities (accounting for 48.00 % of contributions). The dynamic sediment ratio (DSR = the square root of the surface area divided by the average depth) showed the highest impact (accounting for 19.64 % of contributions). Thus, small lakes have relatively small DSR indices, making them less susceptible to wind- and wave-induced sediment resuspension, leading to higher Z sd than large lakes. Seasonally, the variations could be linked with the East Asian monsoon climate, characterized by hot and rainy summers. Moreover, the month with the highest Z sd was postponed from June-July in EPL lakes (south) to August-September in NPML lakes (north), consistent with the movement of the rainfall belt of the East Asian monsoon. This study could facilitate the knowledge of water quality in Eastern China as well as other developed areas and provide support for the sustainable development and management of lakes.