If you've ever arrived at a beach, lake or river and spotted strange foam, discoloured water or an unpleasant smell, you're not alone.
One of the most common questions wild swimmers ask is:
"Is that sewage?"
The answer isn't always straightforward.
While pollution incidents do happen, many reports of suspected sewage are actually caused by naturally occurring algal blooms. Knowing the difference can help you make more informed decisions before entering the water.
What Is an Algal Bloom?
Algae are a natural part of aquatic ecosystems and play an important role in supporting wildlife.
When conditions are favourable — typically warmer temperatures, longer daylight hours and plenty of nutrients in the water — algae can multiply rapidly, creating what is known as an algal bloom.
Some blooms are harmless, while others can produce toxins that pose risks to humans, dogs and wildlife.
Why Algae Is Often Mistaken for Sewage
When large algal blooms begin to break down, they can produce:
- Unpleasant odours
- Surface foam
- Water discolouration
- Floating scum
These signs can look surprisingly similar to pollution or sewage contamination.
In coastal waters, harmless marine algae can create:
- Creamy brown foam
- Frothy patches along beaches
- Long streaks of foam running parallel to the shoreline
- A smell similar to rotting vegetables or rotten eggs
While unpleasant, these signs are often the result of natural biological processes rather than sewage pollution.
Common Coastal Algae
Phaeocystis
Phaeocystis is a naturally occurring, non-toxic marine algae found around UK coastlines.
During spring and summer, it can form large blooms that resemble frogspawn or oil droplets in the water. As the bloom breaks down, it often creates thick foam along beaches and shorelines.
Although it can look alarming, it is generally considered low risk to human health.
Noctiluca ("Sea Sparkle")
Another common marine algae is Noctiluca.
It can create:
- Red, orange or brown patches in the water
- Surface scum
- Bioluminescence at night, often called "sea sparkle"
Seeing glowing water may look magical, but it is simply the result of a natural chemical reaction within the organism.
Blue-Green Algae: The One You Should Take Seriously
While many marine algae are harmless, blue-green algae (cyanobacteria) found in lakes, reservoirs and slow-moving freshwater can be much more concerning.
Blue-green algae can produce toxins that may affect:
- Swimmers
- Paddleboarders/Surfers
- Children playing near the shoreline
- Dogs
- Livestock
Blooms may appear as:
- Bright green paint-like streaks
- Green, blue-green or brown discolouration
- Floating scum
- Clumps resembling green sawdust
- Jelly-like material gathering along shorelines
During calm weather, blooms often accumulate at the edges of lakes where wind pushes them towards the bank.
Risks to People and Pets
Contact with toxic blue-green algae may cause:
- Skin irritation
- Eye irritation
- Vomiting
- Diarrhoea
- Flu-like symptoms
Dogs are particularly vulnerable because they may drink contaminated water or lick algae from their fur after swimming.
In severe cases, exposure can be fatal for animals.
If you see blue-green algae, keep yourself, children and pets away from the water.
Algae or Sewage? Key Signs to Look For
It May Be Algae If:
✔ The foam is cream, brown or naturally coloured
✔ The bloom appears during warmer months (typically April to August)
✔ Foam forms long lines parallel to the shoreline
✔ There are no visible pipes or discharge points nearby
✔ The water has a green, red, orange or brown tint
It May Be Pollution If:
✔ Water appears grey or cloudy
✔ Discolouration is strongest near an outfall or pipe
✔ There are obvious sanitary items or debris present
✔ Fats or oily residues are visible on the surface
✔ The issue appears immediately after heavy rainfall
Can You Tell If Algae Is Toxic Just By Looking?
Unfortunately not. Toxic and non-toxic blooms can look very similar.
That's why it's always safest to:
- Avoid contact with visible blooms or scum
- Check local water quality information
- Follow any warning signs displayed at the site
- Keep dogs away from affected areas
How SwimSense Can Help
SwimSense is not a laboratory test and does not directly identify bacteria such as E. coli.
However, it can help provide additional insight into water conditions by measuring a range of parameters that may indicate changes in water quality.
Our SwimSense Field Test Kit also includes blue-green algae toxins testing, helping swimmers build a broader picture of the water before entering.
Think of it as another tool to support informed decision-making alongside official guidance, local knowledge and visual observations.
Know Before You Go
Whether you're swimming in the sea, a river or a lake:
- Check local water quality information.
- Avoid visible algal blooms and scum.
- Take extra care after heavy rainfall.
- Keep dogs away from suspected blue-green algae.
If in doubt, stay out.
The water can look beautiful from the bank, but understanding what's happening beneath the surface is one of the best ways to stay safe and enjoy your swim with confidence.
Know your swim. Respect the water.
