Understanding Algal Blooms

Warmer weather and agricultural runoff can bring algal blooms - here's the facts on what to look for

James Lee

Summer is a time to explore the water, with friends, family and animal friends, and 2021 is about getting outside and enjoying the countryside a little further afield than we’ve been used to for a while. But as things heat up, it is important to be aware of where and when you might find algal blooms.

Blue-green algae is a photosynthesising bacteria rather than a true algae, and naturally occurs in freshwater globally. Whilst it occurs in most water bodies it can become an issue in extreme quantities, when it forms a ‘bloom’. An algal bloom is triggered by a high level of nutrients entering a water body, particularly nitrogen and phosphorus, which usually end up there through agricultural runoff. Major problems associated with blue-green algal blooms are the production and release of toxins, the discolouring of water resulting in horrible tasting, smelly water, and accumulations of surface scum. The decay of the algal bloom also results in a loss of dissolved oxygen in the water causing the death of fish and other aquatic biota due to lack of oxygen. 

The danger of blue-green algae for swimmers is that water containing these blooms can be toxic if swallowed, or cause rashes and itchiness to the skin if splashed on the body. These toxins can be neurotoxins (affecting the nervous and respiratory systems), toxins that can irritate or inflame the skin, or hepatotoxins (affecting the liver) that can be fatal (but this is extremely rare!).

Environment Agency

These blooms sound serious, and they are if you do swim in blue-green algae and get ill, but it is also very rare and something to be aware but not scared of. It is most often found during periods of sunny weather, and in the warmest water, so at the margins of lakes. It also needs nutrient sources, so runoff from urban or agricultural areas around the lake – it is therefore less likely to occur in mountainous lakes, and more likely in a water body surrounded by intensively managed farmland (i.e. lots of crops or high levels of livestock) or when there is a lot of human activity around a water body (i.e. Windermere).

Photos of blue-green algae often show huge blooms that are glaringly obvious, which you’d not want to swimming in anyway as they’re so ‘gloopy’. However, it is much more likely that you’ll find small patches of it at the shore. Avoid just stepping over it and getting in – the water at the lake margins is the warmest and least disturbed, so it gathers there, but it is likely the bacteria concentration in the lake as a whole is higher and therefore could be dangerous, and it can also sink to the bottom so just because you can’t see it doesn’t mean it is there But will still pollute the water. 

To avoid illness from blue-green algae consider what the weather has been like over the past few weeks before you decide to swim – if it has been very warm and sunny then it is worth putting blue-green algae on your radar. Then do some research – ask local swim groups if anyone has seen it in any water bodies so far that year, or heard of anyone who has. Also search the news – it is commonly shared online if there has been an outbreak of it. When you get to a swim spot, do a recce along the shore of the lake before you get in to look for any algae or horrible smelling water – if you find some, swim another day or somewhere else, don’t just get in somewhere further along the shore. If you find some whilst swimming or come into contact with it then get out and wash yourself and your kit with warm water and soap as soon as possible. If you swim and get skin rashes, eye irritation, vomiting, diarrhoea, fever or cold like symptoms and muscle and joint pain after swimming go to A and E immediately and mention you have swum in an outdoor water body recently.