A research team led by Profs. ZHANG Yunlin and SHI Kun from the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences has completed its first global assessment of lake water clarity, uncovering widespread shifts driven by both climate change and human activities.

Published recently in Science Bulletin, the study analyzed 1.4 million satellite images to track the transparency of 170,799 large lakes worldwide between 1995 and 2023. Its findings reveal that North American lakes stand out with higher transparency levels than the global average, boasting a multiyear mean of 2.76 ± 1.14 meters.

Over the nearly three-decade study period, global lake transparency experienced a notable decline of approximately 13%, translating to an average rate of −0.10 meters per decade. This downward trend was particularly pronounced across major continents: 69% of lakes in North America, 70% in Europe, and 56% in Asia showed decreasing transparency. Overall, 51.4% of the analyzed lakes exhibited statistically significant changes in Secchi disk depth (SDD)—a critical factor influencing the functioning and development of lake ecosystems. Among these lakes with significant declines, 65% were located in North America and 28% in Europe.

Attribution analysis conducted as part of the research identified precipitation, wind, the Normalized Difference Vegetation Index (NDVI), algal blooms, and permafrost as the primary drivers of these changes for shallow lakes. Specifically, these factors influenced 19%, 17.2%, 20.3%, 29.7%, and 13.7% of global shallow lakes, respectively. For deep lakes, the dominant drivers were precipitation, NDVI, algal blooms, and permafrost, affecting 23.9%, 25.2%, 30.3%, and 19.6% of global deep lakes, respectively.

Regionally, the study found that in North America and Europe, rising temperatures have accelerated permafrost thaw and algal proliferation, leading to widespread drops in water clarity. In contrast, approximately 44% of lakes in Asia saw increased transparency, a trend primarily linked to watershed "greening"—vegetation recovery that serves as a natural filter, reducing sediment and nutrient runoff.

"Our work is the first to unravel how climate, watershed dynamics, and in-lake processes collectively shape the lake light environment on a global scale," Prof. ZHANG noted. The research team emphasized that these findings provide new insights for guiding ecosystem protection efforts and advancing sustainable lake management strategies amid ongoing global environmental change.