At some point, almost all water bottles within a radius of 80 kilometers were sold out. When a green, slimy layer spread on Lake Erie in the USA in 2014, the drinking water supply in the Toledo region in the US state of Ohio collapsed.
In the fourth largest of the five large lakes in North America, algae and bacteria had spread in large numbers. These poisoned the water with microcystins. These are poisons that can lead to liver damage, diarrhea and skin irritation. The National Guard moved to supply drinking water to the more than 400,000 affected people, and authorities called for a state of emergency.
"Toxic algae bloom"
Such algal blooms are not uncommon in freshwater lakes. According to US researchers, however, they have increased significantly in the past thirty years, as they report in the journal Nature. While not all cases are as drastic as in Ohio, they are usually a burden on the ecosystem.
The thick algae carpet shields aquatic plants from vital sunlight. If the algae die off, the oxygen in the water also becomes scarce, a danger to the fish living in it. It becomes particularly uncomfortable when simultaneously cyanobacteria spread, producing the toxic microcystins.
"Toxic algal blooms affect drinking water supplies, agriculture, and tourism," says Jeff Ho of the Carnegie Institution for Science. It is estimated that algae blooms in freshwater cost the US economy about $ 4 billion a year.
For their analysis, the researchers studied 71 large lakes on six continents around the world. They used data from the NASA satellite "Landsat 5", which was active between 1984 and 2013. The result: In two-thirds of the lakes, the intensity of algal blooms had increased significantly over the period. Only in six lakes had the situation eased in recent years.
"This means that algal blooms are really increasing and intensifying and that we are not just paying more attention to them than decades ago," says study author Anna Michalak. The reasons are manifold and varied from lake to lake.
A big problem is fertilizer, which reaches the lakes from surrounding fields. The contained therein phosphorus and nitrogen compounds such as nitrate promote just in summer the explosive algae growth. Many lakes in Germany are also burdened. In many places, even in groundwater, the applicable limits are exceeded. (Read more here.)
"We have to adapt the water management"
Stricter regulations apply in the region around Lake Erie, after huge algal blooms came in the fifties and sixties. Since then, the situation has improved, but for several years, the flowers are piling up again. (Read more here.)
Temperature also plays a crucial role. According to the researchers, the lakes that were the least warm were just recovering. This suggests that climate change is hampering the recovery of some lakes. "We need to adapt water management to the local hydrological conditions that are changing as a result of climate change," Michalak says.
Algal blooms do not only occur in freshwater lakes. Brown algae are also becoming increasingly widespread in the Atlantic. Last year, the rug ranged from West Africa to the Gulf of Mexico.