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What Are Blue-Green Algae?

Blue-green algae warning sign

Sometimes when you are near a pond, lake or river you might notice a sign warning of the presence of blue-green algae in the water. The signs usually warn people not to enter or drink the water or let any pets do so either. But what exactly are blue-green algae? And why do we need warning signs? This post will explain.

Blue-green Algae Basics

The term blue-green algae actually refers to a large group of species called cyanobacteria. Most scientific authorities now recognise cyanobacteria and algae as different groups. This is because cyanobacteria and algae have different cell characteristics. However, like their namesake, blue-green algae do photosynthesise. This means they are able to convert sunlight into energy. Another similarity is their aquatic nature. Like many algae, blue-green algae are found suspended in water or attached to surfaces such as rocks and pebbles. They are most common in non-flowing fresh water such as ponds and lakes, although they do also occur in rivers, estuaries and the sea.

Blue-green algae
Blue-green algae favour non-flowing freshwater habitats like ponds and lakes

Much of the time, blue-green algae are in balance with the surrounding ecosystem and invisible to the naked eye. When they are present at normal levels, they are important components of the water body. They release oxygen and carbon dioxide into the water via the process of photosynthesis. In addition, they take up minerals and are a vital part of the food chain. Many aquatic species feed on blue-green algae and they are ultimately an essential part of many freshwater ecosystems.

Blue-green Algal Blooms

Under certain conditions, blue-green algae can grow much faster than normal as part of a process called eutrophication. Eutrophication results from high levels of phosphorous and other nutrients in a water body. This in turn can have a number of causes, including natural processes such as warm temperatures and good levels of sunlight. However, growth is much more rapid when there are human-related nutrient increases. Fertiliser runoff from farmland and gardens or discharges from sewage works are some of the biggest contributors to this. The result of these rapid growth spurts is a visible bloom as the microscopic bacteria clump together. These blooms can have a number of different forms. Some resemble a blue-green scum on top of the water as the organisms rise to the surface. Others produce a foam at the edge of a water body, or green flakes or brown specks within it. They do not, though, have small, round leaves like duckweed or other aquatic plants.

The small, round leaves here show this is duckweed rather than blue-green algae

It is when these blooms occur that the problems can start. Although not all species do so, some, called Harmful Algal Blooms (HABs) produce toxins. These toxins can be fatal to livestock, pets and wild animals if ingested. In some cases, an animal can show symptoms within just 15 minutes of swallowing affected water. Although they rarely kill humans, contact or ingestion can cause extreme skin irritation, stomach upsets, fever and eye problems. Children are more vulnerable than adults. In addition, blue-green algae blooms can greatly affect the health of a water body. As the bloom decays, it reduces water oxygen levels. This can cause mass fish die-offs. Blooms also block sunlight from reaching below the surface. This prevents aquatic plants from photosynthesising, stunting their growth or killing them entirely. These dead plants then release more CO2, which can also kill fish and other organisms. There have been more HABs globally since the 1980s, possibly due to a combination of climate change and higher fertiliser use. Scientists say that these are the two main causes of this summer’s unprecedented levels on Lough Neagh in Northern Ireland, for example. Researchers also believe rising temperatures may lead to more storms and flooding which will then lead to more nutrient runoff.

Dealing With Blue-green Algae

Although not all blooms are toxic, it is impossible to tell just by looking at them. Therefore, if you see a suspected blue-green algal bloom, you should assume it is harmful and report it to the Environment Agency at the 24-hour environment incident hotline below. The UK Centre for Ecology and Hydrology also runs an app which you can log sightings of HABs on. Logging on the app helps to build up a picture of where blooms are at any one time. You can also view other users’ sightings to check for blooms in your area. They have a useful gallery of types of cyanobacteria to help understand what to look for, too.

Blue-green algae can be fatal to livestock

Never enter or drink water you suspect is affected or allow any children or pets to do so either. If you think anyone has had contact with blue-green algae or ingestion has occurred, contact a medical practitioner immediately. Likewise, if your dog has swum in or swallowed affected water, call a vet straight away as symptoms can take hold rapidly. The Blue Cross has more information about the symptoms to look for in dogs.

If you are a landowner with a suspected bloom, you also need to report it to the Environment Agency. However, you are responsible for putting up signs warning of the dangers. You will need to consider who can access the site and what activities take place there when assessing the risks to humans and animals.

Further Reading

Report suspected HABs to the Environment Agency’s 24-hour hotline on 0800 80 70 60