The latest research into cyanobacteria - also known as blue-green algae blooms - shows that they not only occur in warm weather, but like cold temperatures too.

Cyanobacteria pose a threat to aquatic ecosystems and living organisms, as well as drinking and bathing waters around the world. They can produce toxins and deprive water of oxygen, and aquatic plants of the light they need for photosynthesis.

These blooms have been the focus of attention, especially in the context of climate change, because warmer water temperatures above 25oC favour their growth. Research from the Global Lake Ecosystem Observatory Network (GLEON) has now shown that cyanobacterial blooms can occur in lakes at relatively low water temperatures too - less than 15oC, even under a layer of ice.

The researchers used existing scientific data, including that collected by citizen scientists via an app or with the help of microscope sets.

Dr Kaitlin Reinl, principal investigator of the study and research coordinator at the US Lake Superior National Estuarine Research Reserve, says of the study's significance, "Cyanobacterial blooms are complex and challenging. The conventional wisdom about them 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 conditions. This is surprising even to us researchers because it goes against the conventional wisdom.

"Our study underscores the need to understand cyanobacterial blooms in cold waters and reminds us that biological processes in lakes can be very active even in cool temperatures."

The study was published in the journal Limnology & Oceanography Letters where the authors describe how they classified three types of cyanobacterial blooms in cold water, depending on how they occur:

• Surface blooms initiated and persisting at cold water temperatures
• Blooms that occur in the thermocline, where two layers of water with a steep temperature gradient overlie each other and are brought to the surface by physical processes
• Algal blooms that begin at warmer temperatures and persist at colder temperatures

Cyanobacterial blooms are most common in nutrient-rich, over-fertilised waters and it is certain species of cyanobacteria, that have the capacity to form mass blooms in both warm and cold water. They show broad temperature tolerance and have developed special survival strategies for cold temperatures – such as cold shock and freeze protection proteins, or a cold-resistant cell wall made of unsaturated fatty acids.

“Some filamentous cyanobacteria species are highly abundant in winter even during ice-covered periods, due to their ability to adapt to low-light conditions and low temperatures." explained researcher Dr Stella Berger from the Leibniz Institute of Freshwater Ecology & Inland Fisheries (IGB) in Germany, a co-author of the study. "By surviving extended periods of reduced light, these taxa have a competitive advantage when light returns in spring.”

Activation from resting stages can also be a trigger for cyanobacterial blooms. In fact, some species form dormant stages that lie at the bottom of the water and awaken as soon as environmental conditions change - even at colder temperatures. For example, it is known that Microcystis aeruginosa, a bottom-dwelling bacteria, activate the resting stages of these cyanobacteria.

The blooms which occur in the thermocline go largely undetected because due to depth, but storms can bring these to the surface.

"We were able to observe this in Lake Stechlin a few years ago," said Dr Mina Bizic, another IGB researcher and co-author of the study. "A strong storm caused the water column to circulate, cyanobacteria came to the surface in masses and deteriorated the water quality of the lake for several weeks.

"As strong storms increase due to climate change, such events will certainly occur more frequently,"

Even if the cyanobacterial blooms remain in the deeper layers of the lake, they can be dangerous for the ecosystem and health. Drinking water intakes are often located far below the water surface, and unnoticed cyanobacterial blooms can then contaminate the extracted drinking water with toxins.

"The study shows that the ‘cyanobacterial blooms like it warm’ paradigm needs to be broken. For water monitoring and management, this means that sampling should be extended to the cold seasons, and that we need to keep an eye on algal blooms in the deeper water layers that can be harmful for drinking water," co-author and fellow IGB researcher Professor Hans-Peter Grossart concludes.