Severe algal blooms in freshwater lakes are spreading across the globe

A global survey of freshwater lakes has shown algal blooms that could threaten water supplies are becoming more widespread and intense.

Satellite data from the past 30 years has shown that the onset of the climate emergency is having serious ramifications for the world’s freshwater lakes. In a study published to Nature, researchers from the Carnegie Institution for Science analysed global data on algal blooms to show they are now more widespread and intense than ever before.

In the US alone, some of the most recent outbreaks have resulted in states of emergency being declared in Florida in 2016 and 2018. The potential to cause harm is either because of the intensity of growth of the algal blooms, or because they include populations of toxin-producing phytoplankton.

The researchers said that until this recent data was collected, it was unclear whether the problem was getting significantly worse on a global scale. Similarly, the extent of humans’ contribution to these blooms – through agriculture, urban development and carbon emissions – was uncertain.

The data was collected from NASA’s Landsat 5 near-Earth satellite, which monitored the planet’s surface between 1984 and 2013 at a resolution of 30 metres. This revealed the long-term trends of summer algal blooms in 71 large lakes across 33 countries in six continents.

Lago de Cahora Bassa in Mozambique with prominent algal bloom. Image: NASA Earth Observatory/Jesse Allen, using MODIS data from the Land Atmosphere Near-real-time Capability for EOS (LANCE) and Landsat data from the US Geological Survey.

Need for water management strategies

“We found that the peak intensity of summertime algal blooms increased in more than two-thirds of lakes but decreased in a statistically significant way in only six of the lakes,” said researcher Anna Michalak.

“This means that algal blooms really are getting more widespread and more intense, and it’s not just that we are paying more attention to them now than we were decades ago.”

While the data showed a clear increase in intense algal blooms, the reasons behind their increase was not universal, varying from lake to lake. No consistent patterns emerged in the data when considering factors such as fertiliser use, rainfall or temperature, the researchers added.

However, what was clear was that among the lakes that had improved in the span of 30 years, those which had experienced the least warming were able to sustain improvements in bloom conditions. This finding suggests the climate emergency is already hampering lake recovery in certain areas.

“This finding illustrates how important it is to identify the factors that make some lakes more susceptible to climate change,” Michalak said. “We need to develop water management strategies that better reflect the ways that local hydrological conditions are affected by a changing climate.”