Blue-Green Algae Blooms Also Like It Cold – And Not Just Warm

Mass developments of cyanobacteria, so-called blue-green algae blooms, repeatedly impair the quality of bodies of water and drinking water resources worldwide.

Cyanobacteria are considered warmth-loving, and massive algal blooms are mainly reported in summer, when monitoring is particularly close. An international research network with the participation of the IGB has now shown that blue-green algae blooms can also occur at colder temperatures - even under ice. If the algal blooms go unnoticed, there are risks for drinking water production. The study was published in the journal Limnology and Oceanography Letters.

Cyanobacteria – also known as blue-green algae – can form toxins, remove oxygen from the water and aquatic plants take the light for photosynthesis. Blue-green algae blooms endanger aquatic ecosystems with their living beings as well as drinking water resources and bathing waters.

These algal blooms are particularly important in the context of climate change, because warmer water temperatures above 25 degrees Celsius promote the growth of cyanobacteria - that was the scientific tenor up to now. A research team from the international network for scientific lake monitoring (Global Lake Ecosystem Observatory Network - GLEON) has now shown that cyanobacteria blooms can also develop in relatively cold water temperatures below 15 degrees Celsius in the lake - even under a layer of ice. To do this, the researchers used existing scientific data, including data collected by citizen scientists via an app or with the help of microscope sets.

dr Kaitlin Reinl, leader of the study and research coordinator at the US Lake Superior National Estuarine Research Reserve, says of the importance of the study: “Cyanobacteria blooms are complex and very challenging. The conventional wisdom is that they like it warm, and in many cases that's true. However, there is growing evidence that these blooms can also occur in cold weather. This is also surprising for us researchers because it contradicts the conventional wisdom. Our study underscores the need to understand cyanobacteria blooms in cold waters and reminds us that biological processes in lakes can be very active even in cool temperatures."

Whether cold or warm waters - the same types of water are affected by the same cyanobacteria species
Blue-green algae blooms occur primarily in nutrient-rich, "over-fertilized" waters - whether in cold or warm environments. It was also shown that it is often the same species of cyanobacteria that form mass developments in both warm and cold water. However, these have certain characteristics: They have developed a broad temperature tolerance and special survival strategies for cold temperatures - such as cold shock and antifreeze proteins or a cold-resistant cell wall made of unsaturated fatty acids. “Some filamentous cyanobacteria species are very common in winter even during ice-covered periods because of their ability to adapt to low light levels and low temperatures. By surviving longer periods with less light,

Activation from stagnant stages can also be a trigger for cyanobacteria blooms. Some species form dormant stages that lie at the bottom of the water and wake up as soon as the environmental conditions change - even at colder temperatures. For example, it is known about Microcystis aeruginosa that bacteria living on the ground activate the permanent stages of these cyanobacteria.

Three types of cold-water cyanobacteria blooms
The authors classified three types of cold-water cyanobacteria blooms based on how they form. First, surface blooms that initiate and persist in cold water temperatures. Second: cyanobacteria blooms, which develop in the so-called thermocline, where two layers of water with a strong temperature gradient lie on top of each other and which are brought to the surface by physical processes. And third: algal blooms, which start with warmer temperatures and continue with cold temperatures.

Strong climate change storms could boost algal blooms in cold temperatures
The second type of cyanobacteria bloom, which forms in the thermocline, usually goes undetected at first because it occurs in the deeper water layers. However, storms can cause these algae to rise to the surface. "We were able to observe this a few years ago in Lake Stechlin. A strong storm caused the water column to churn, masses of cyanobacteria came to the surface and worsened the water quality of the lake for several weeks. As strong storms increase in climate change, such events will certainly occur more frequently," explains IGB researcher and co-author Dr. Mina Bizic. This phenomenon has also been described for Lake Zurich or small glacial lakes and reservoirs.

Unnoticed algal blooms in deeper water layers can pollute drinking water
But even if the cyanobacteria blooms remain in the deeper layers of the lake, they can be dangerous for the ecosystem and health. Drinking water extraction points are often far below the water surface and unnoticed cyanobacteria blooms can then pollute the drinking water obtained with toxins.

“The study shows that the paradigm “cyanobacteria blooms like it warm” needs to be broken. For water body monitoring and management, this means that sampling should be extended to the cold season and that one has to keep an eye on the algal blooms that are not obvious in the deeper layers when drinking water is obtained,” summarizes IGB researcher and co-author Prof. Dr. Hans Peter Grossart.