Satellite Reveals Widespread Declines in Visual Wavelength of Color in China’s 2500 Lakes over the Past 40 Years
Lakes provide critical resources for humankind, such as drinking water, food, biodiversity, and transportation. The dual effects of climate change (e.g., shifts in temperature and precipitation regimes) and human activities significantly alter lake environments. A lot of studies monitored individual water quality indicators, yet a comprehensive assessment of the prevalence of shifts in lake conditions and relevant drivers needs further investigation.
From the perspective of lake optics, the interaction of optically active constituents in the water with solar radiation makes lakes on the Earth colorful. In general, lake color is related to productivity, water quality, and ecological state; hence, the influences of climate and human activities on lake environments can be reflected in the water color, to some extent. Compared with a single water quality parameter, lake water color can reflect the comprehensive state change of the lake environment. However, a systematic study on the spatial and temporal trends of shifts in lake color across the China is missing, and, more in general, the relevant factors regulating the long-term changes in lake color, are unknown.
In recent, Dr. CAO Zhigang from Prof. MA Ronghua’s group at the Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, along with their collaborators, conducted a comprehensive examination of color in 2550 Chinese lakes from 1984 to 2021 using Landsat missions and revealed the spatial and temporal patterns of shifts in lake color as well as the relations to the climate and human activities.
Their findings were published in Geophysical Research Letters on April 26, 2023.
“In the past 40 years, the blue lakes in western China became bluer, and the green-yellow lakes in eastern China shifted to greener colors,” said Dr. CAO.
Whether blue lakes in the western area or non-blue (green and yellow) lakes in the eastern area, the results find that declines in visual wavelength have been widespread. The decrease in visual wavelength of color is found in 1723 lakes (68%) (mean changing rate: -8 nm/decade). Lakes in the Tibetan Plateau had larger declines in wavelength than lakes in other areas.
“The climate and the human activities varied in different areas of China; as such, the controls of climate and humans on changing patterns of lake colors are heterogeneous across China, ” said Prof. MA.
Most of lakes in western China (e.g., Tibetan Plateau) are blue. The bluer color is mainly related to the higher temperature and rainfall, which also resulted in lake expansion and elevated water clarity there.
The lakes in eastern China (e.g., Yangtze River plain) are often “yellow-green”. With the decrease in wind speed over the past decades, the resuspension of sediments, which usually relates to the lake turbidity, was weakened. Likewise, more forest and grassland around the lake reduced the substances flowing into the lakes. These processes reduced the reduced sediments in the surface water and made the lake color shift to a green direction. In Lake Taihu and Chaohu, where the cyanobacterial blooms frequently break out, the trends of “greener” colors are more significant.
Nanjing Institute of Geography and Limnology