Blooms of algae that strangle freshwater ecosystems are occurring more often in lakes across most of the world, according to the first study to map their incidence globally.
An algal bloom is the rapid build-up of algae in a body of water after excess nutrients, such as nitrogen or phosphorous, pollute the ecosystem 鈥� often as a result of fertiliser use on farms. The bloom can harm other organisms, including fish and insects, by blocking out light, depleting oxygen and in some cases producing toxins.
Most algal blooms on freshwater lakes form a greenish blanket on the water鈥檚 surface. at Southern University of Science and Technology in China and his colleagues developed an algorithm that could identify green algal blooms by their colour in satellite images. The algorithm didn鈥檛 pick up some algal blooms that aren鈥檛 green, but these are rare.
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The team analysed 2.91 million images from NASA鈥檚 Landsat programme that were taken between 1982 and 2019. In total, they looked at 248,243 freshwater lakes, covering 57.1 per cent of global freshwater lake area. Lakes at high latitudes 鈥� which make up over 30 per cent of the global freshwater lake area 鈥� weren鈥檛 included due to difficulty in interpreting the data when they freeze over, says Feng.
From the 1980s to the 2000s, algal blooms were detected around 3.6 per cent of the time, but this increased to 5.2 per cent during the 2010s. The only region that didn鈥檛 see this increase was Oceania.
鈥淭he most significant increase we found was in Asia and South Africa,鈥� says Feng. 鈥淭his is because fertiliser use increased substantially [in those regions] in the past decade.鈥�
Global warming may be another factor, he says, as warmer climates can lead to more algal bloom outbreaks.
These events have various effects on people, both direct and indirect. 鈥淲e can use our model to see how [algal blooms] will affect human health, the economy and tourism, for example,鈥� says Feng.
He hopes the database can be used to predict future algal blooms as well as inform efforts to prevent and mitigate them.
Nature Geoscience
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