Can You Swim in Blue-Green Algae? What is it and what you need to know
Jenny UrwinDuring summer, you hear about algae a lot, and it often comes with some pretty serious warnings. But what actually is it? Why does it appear, and what can it do to your body?
In this easy to read blog, we're diving in and getting back to the basics of algae, particularly the blue-green variety that outdoor swimmers need to know about.
Algae: The Simple Definition
Algae are photosynthetic organisms that mostly live in water. They are not technically plants, although many behave in a similar way by using sunlight, water and carbon dioxide to produce energy through photosynthesis.
Algae come in a huge range of forms, from tiny single celled organisms to giant kelp forests.
Some of the key ingredients that encourage algae to grow are:
- Sunlight
- Nutrients, such as nitrogen and phosphorus. These find their way into the water system through various ways, from sewage mismanagement to natural sources like decaying plants and animals.
- Warm water
- Calm or slow-moving water
Looking at that list, you can see why algae become such a talking point during summer.
Long periods of warm, sunny weather can create ideal conditions for algae to multiply. If the water is relatively calm and there are plenty of nutrients available, algae can build up rapidly and create what is known as an algal bloom.
What Are the Main Types of Algae?
There are thousands of species of algae, including green, red and brown algae, as well as microscopic groups such as diatoms. They can be found across freshwater and marine environments and are a normal part of aquatic ecosystems.
But there is one name outdoor swimmers are particularly likely to hear during the warmer months:
Blue-green algae.
Despite the name, blue-green algae are not actually algae. They are a type of photosynthetic bacteria called cyanobacteria.
Some cyanobacteria can produce harmful toxins, which is why swimmers, paddleboarders and dog owners need to pay particular attention to them.
What Does Blue-Green Algae Look Like?
When I have come across suspected blue-green algae in the wild, I have described the water as looking a bit mouldy or scummy.
The problem is that you cannot tell simply by looking at a bloom whether it contains harmful toxins. So if something looks suspicious, this is one occasion when curiosity does not need to get the better of you.
Warning signs that should make you stop and investigate include:
- Water that resembles pea soup or looks unusually green
- Green specks, flecks, foam or surface scum
- Fuzzy-looking green clumps
- A slimy-looking residue
- Dead or distressed fish in or around the water
Do not rely on appearance alone, though. Check for warning signs and local water-quality information before swimming.
And if you are thinking, "I'll just put my hand in and see what it feels like," perhaps don't.
If you accidentally come into contact with suspicious water, wash your hands thoroughly afterwards.
How to Check for Blue-Green Algae Before You Swim
Unfortunately, there is not a home test you can do at the water's edge that will reliably tell you whether a bloom is toxic.
The safest approach is to combine official information with your own observations.
Before leaving home
Check for any current warnings for your swim location.
For designated bathing waters in England, government bathing-water information can help you check current water quality and pollution risk.
For lakes, reservoirs and other inland swim spots, also check the relevant landowner, local authority, National Park, venue or swim group's website and social channels for current blue-green algae warnings.
When you arrive
Check any signs around the water.
Landowners and local authorities may display warnings where blue-green algae have been identified.
Then use your eyes.
Before changing, take a proper look at the water, particularly around the shoreline.
Watch for:
- Unusually green or greenish-brown water
- Green flecks or clumps
- Foam
- Surface scum
- A pea-soup appearance
- Blooms can move and accumulate with changing conditions, so do not assume that because one part of a lake looks clear, another part will be the same.
If something does not look right, do not swim.
And importantly, do not stick your hand in to test whether something is slimy. If you suspect a bloom, avoid contact altogether.
What Are the Symptoms of Blue-Green Algae Exposure?
Contact with harmful blue-green algae can cause symptoms including:
- Skin rashes
- Eye irritation
- Vomiting
- Diarrhoea
- Fever
- Muscle and joint pain
Symptoms can vary depending on how you were exposed, how much you encountered and the particular cyanobacteria or toxins present.
If you become unwell after swimming in water where blue-green algae may have been present, seek medical advice and make sure you tell them you have been open water swimming.
What About Dogs?
This is where blue-green algae becomes particularly serious.
Dogs can be exposed by swimming in contaminated water, drinking it, or licking contaminated water from their fur afterwards.
Some cyanobacterial toxins can cause severe illness and can be fatal.
Symptoms can include:
- Vomiting or diarrhoea
- Excessive drooling
- Muscle tremors, stiffness or rigid limbs
- Weakness, wobbliness or sudden collapse
- Seizures or unconsciousness
- Breathing difficulties
- Yellowing of the skin, eyes or gums
If you think your dog has been exposed to blue-green algae, or they become unwell after being in suspicious water, contact a vet urgently.
It is one of those situations where waiting to see whether they are okay is not worth the risk.
So, Can You Swim When There Is Blue-Green Algae?
Here is the frustrating answer:
No swim today.
If there is a suspected blue-green algal bloom, an official warning is in place, or the water looks suspicious, stay out of it. That means swimming, dipping and other activities that could bring you into contact with the water. Keep dogs away too.
Yes, it is a buzzkill, particularly when you have packed your kit, driven to your favourite spot and mentally prepared yourself for that glorious dip.
Blooms can change as weather and water conditions change, but do not assume that a cold spell, heavy rain or windy day automatically means the problem has disappeared.
Check local warnings and water-quality information before heading back in.
Sometimes being a good outdoor swimmer means knowing when not to swim.
Be patient, stay safe, and there will always be another dip. 🧡
Reporting Blue Green Algae:
Contact the Environment Agency’s Incident Hotline on 0800 807060 or report via UKCEH’s Bloomin Algae App.
Since writing this blog post, I've used the app to report suspected Blue Green Algae. What's great about the app is that someone actually responds to your report and advises whether they think it is Blue Green Algae.
