Over the last 40 years, the majority of the world's lakes have changed colour, according to a new study. The research team analysed 32 million satellite observations from over 67,000 lakes. Major changes in the lake ecosystems are thought to be the cause.
Lakes are critical components of Earth's ecosystem. They provide habitats for aquatic and terrestrial species, support biodiversity and help maintain ecological balance. Lakes are also crucial for providing drinking water, supporting agriculture and influencing climate through their effects on temperature, humidity and atmospheric processes.
The colour of lakes is an important indicator of how healthy they are. Different shades reveal the ecological state of lakes and the ongoing physical and biochemical processes.
In a new study published in the scientific journal Water Resources Research, a team of researchers analysed 32 million satellite images of over 67,000 lakes from 1984 onwards.
After cross-checking with climate and population data, the researchers found that only 14 per cent of the lakes studied maintained stable colours over time. The fact that so many lakes have changed colour indicates significant ecosystem disruption caused by changes in water quality, algae concentrations, the flux of dissolved organic matter, and other contributing factors.
"Our results show the strong correlation between changes in lake colours, climate change and human impact. We discovered that 60 per cent of the lakes had undergone significant colour changes," says Zheng Duan, a physical geography researcher at Lund University.
Lakes have different colours depending on where they are located. Blue lakes are primarily located in areas at northern latitudes, while green lakes are more prevalent in densely populated mid-latitude regions, such as southern Europe. Reddish and yellowish lakes are mainly found in the Southern Hemisphere.
In the study, the researchers found that most of the lakes that have shifted toward shorter wavelengths (toward blue) over the last 40 years. A lake with a bluer colour generally indicates clearer water and may reflect a healthier ecological state, although this will vary depending on the natural characteristics of the lake and its surroundings.
However, significant changes in lake colour can signal ecological disturbances, such as increased nutrient loads or other drivers affecting the physical and biochemical properties of lake water. The differences between regions were remarkable.
"Lakes in high-latitude regions, such as North America and northern Europe, showed a more pronounced change in colour compared with those at the Equator and in the Southern Hemisphere. We don't really know why this is the case. This regional variation suggests that climate change and human activity are impacting ecosystems in complex and localised ways," says Zheng Duan.
By mapping lake colours globally, the new study highlights how climate change and human activity are affecting lake ecosystems, which in turn have a major impact on food production, water supply and recreation. Understanding these changes can help communities and policy-makers make informed decisions about water resource management, conservation and environmental protection. Furthermore, the study shows how satellite-based remote sensing can be a scalable and cost-effective tool for monitoring environmental change over time, which is crucial for sustainable development in a rapidly changing world.
"The remote sensing-based approach offers a solution for long-term monitoring of water quality and ecological stability, allowing for early intervention," says Zheng Duan.
