Flowering decreased only in 6 water bodies out of 71 – they are located in those places where average temperatures over the past 30 years have either dropped or increased only slightly.
Long-term observations of dozens of large freshwater lakes around the world indicated that in 68% of them the size of the spots of flowering algae, as well as their brightness and toxicity, increased sharply. The cause of this growth has been global warming, environmentalists write in the journal Nature.
“The peak flowering rate of algae in the summer months increased in 68% of the studied lakes, but at the same time it fell only in six water bodies. This means that we see the growth of flowering not because we began to actively monitor the lakes in recent years, but because of the real increase in the area of algae spots and their intensity, “Anna Mikhalak, one of the authors of the study, commented on the press service of the Carnegie Institute of Science (USA).
As a rule, water begins to bloom due to photosynthetic bacteria, as well as due to brown or blue-green algae that live both in freshwater bodies and in the seas and oceans. When they begin to multiply rapidly, extensive bright spots appear in the water, which fills a huge number of these microorganisms. They pose a threat to human and animal health due to the toxins that algae release into the water, clearing living space from competitors.
Environmentalists and oceanologists suggest that global warming and an increase in average water temperature will accelerate the flowering of algae and make them more dangerous for the inhabitants of lakes, rivers, seas and people. The first manifestations of this phenomenon off the coast of the United States have already led to the mass death of fish, birds and animals. They were poisoned with domoic acid, a hallucinogenic substance that releases large amounts of algae of the Pseudo-nitzschia species.
Michalak and her colleagues found new evidence that algal blooms are increasing around the world due to anthropogenic climate change, studying the images that NASA and the US Geological Survey (USGS) climate probes have received over the past three decades.
Greening effects of warming
In total, scientists obtained images of 71 large lakes located in 33 different countries on all continents of the Earth, except Antarctica, and monitored how their color changed during the summer months.
For such observations, scientists developed a special algorithm that allowed them to find spots of blooming algae, based on how much the color of a lake changed in summer compared to its typical color throughout the year and in other seasons. Environmentalists also compiled statistics on what types of microorganisms were found in each of these lakes and whether they caused bloom in their waters in the past.
The analysis of satellite images showed that the scale and intensity of algal blooms have increased markedly over the past three decades in the vast majority of lakes. This was especially characteristic of those parts of the Earth where average temperatures rose significantly, including for the central regions of Asia, southeast Africa, the US coast, and also the southern regions of Russia.
Interestingly, the flowering situation improved only in the few lakes that were in those corners of the Earth where average temperatures either did not rise in the last 30 years, or rose only to minimal values. Scientists have not found clear links between frequent flowering and the use of fertilizers, the presence of toxic algae in lake waters, and the increase or decrease in rainfall.
All this, according to researchers, suggests that while global warming is the main reason why toxic algae spots appear on the surface of all freshwater bodies of the world more and more often. This must be taken into account when combating such pollution, environmentalists conclude.