Heavy rain, fish and bacteria: What stops blue-green alga

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01.08.2024
Heavy rain, fish and bacteria: What stops blue-green algae?

Brain City Berlin

Off to the lake on hot days? That’s not always a good idea. If greenish or bluish-green streaks or layers of algae are floating on the water surface, the water may be contaminated with blue-green algae. The cyanobacteria produce toxins that can be dangerous to humans and animals. A research team from the “Global Lake Ecological Observatory Network” (GLEON) has summarised in an overview study which factors can curb blue-green algae. Researchers from the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB) in Berlin were also involved.

Whether Wannsee, Krumme Lanke or Müggelsee – everything in Berlin’s bathing waters is still within the normal green range. However, as in every summer, this can change quickly. Persistently high temperatures, abundant light and a high concentration of nutrients such as phosphorus and nitrogen cause cyanobacteria – colloquially known as “blue-green algae” – to multiply explosively. If green streaks with blue-green algae blooms are floating in the water, caution is advised. This is because blue-green algae produce toxic substances that can cause vomiting, diarrhoea and breathing difficulties if swallowed. In extreme cases, the bacteria can even damage the nervous system.

Fighting blue-green algae is not so easy. There are several thousand species of cyanobacteria, which differ greatly in their appearance and characteristics. And up to now, research has primarily focussed on the conditions under which blue-green algae proliferate to an extreme degree. A research team from the Global Lake Ecological Observatory Network (GLEON), in which the Berlin Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) is also involved, did it the other way round: In a review study, they compiled which factors can stop the algae blooms. “Knowing which environmental factors and interactions reduce the respective blue-green algae bloom in the natural ecosystem is crucial in order to be able to contain algal blooms more specifically,” says Dr. Stella A. Berger, research group leader in the Plankton and Microbial Ecology Department at the IGB.

Factors that can curb blue-green algae blooms

Mixing and heavy rain: Mixing the water can inhibit the proliferation of cyanobacteria species, as these mainly accumulate on the surface of the water. Wind can also disperse and break up mass accumulations. The gas vesicles of some floating cyanobacteria also collapse due to the hydrostatic pressure of heavy rain.

Chemical substances of other organisms: Aquatic plants, heterotrophic bacteria that feed with the help of other living creatures, fungi and other organisms in the water can also inhibit blue-green algae blooms by producing chemical signals or messenger substances. These so-called allelochemicals damage the cells of various cyanobacteria.

Natural “pathogens”: Parasites, viruses and bacteria can directly attack cyanobacteria and restrict their reproduction. Cyanobacterial phages are an example. These virus-like particles infect a wide range of host species and can stop algal blooms within a few days.

Predators: Larger creatures also keep blue-green algae in check. Zooplankton, for example, literally graze on the algae layers. In the same ecosystem, water fleas can adapt to the toxins of cyanobacteria and use them as a food source. Mussels filter the algal blooms and thus limit their proliferation. The invasive zebra mussels in Europe and North America, such as the “quagga mussel” are particularly effective. Studies show that they can achieve an algae reduction of up to 58 per cent. Some fish species also feed on cyanobacteria.

Food competitors: Some species of algae rob cyanobacteria of the light and nutrients they need to survive, and they also eat smaller species. These competitors are “mixotrophic algae”. This means that they use photosynthesis to generate energy and feed on cells to obtain carbon compounds for their growth.

Studies have also shown that artificial mixing in deeper lakes can be an effective strategy against blue-green algae. In shallow, non-stratified waters, on the other hand, mixing can have a counterproductive effect, as phosphorus released from the sediments stimulates the growth of cyanobacteria. Chemical methods such as introducing copper sulphate and hydrogen peroxide into the water also harm other living creatures. The same applies to biological control by predators, which is difficult to control.

What can be deduced from the study: Many of the mechanisms mentioned above can, in principle, be used to reduce algal blooms. However, the measures are costly and the chances of success are uncertain. Which method is the right one depends on the water, the type of cyanobacteria and the environmental conditions. However, another study has shown that different approaches, individually or in combination, can eliminate up to 100 per cent of the biomass of algal blooms.

There are conflicting study results on the extent to which climate change promotes the growth of cyanobacteria, for example through rising temperatures and higher nutrient pollution of water bodies. For instance, higher temperatures can also lead to high infection rates in cyanobacteria. And that in turn inhibits their spreading. Dr Stella A. Berger calls this juxtaposition of growth and loss processes in research into the development of blue-green algae blooms the “net effect of climate change”.

Anyone who is unsure whether or not to enter unfamiliar waters these days can take the “sight test”: If your feet are still visible in knee-deep water, there is usually no danger of blue-green algae.

Our tip: An up-to-date overview of the water quality of Berlin’s bathing lakes can be found at badestellen.berlin.de (German only). The application was developed as part of Berliner Wasserbetriebe’s “River Hygiene” project (German only). (vdo)