Toxic Algae at Thames Bathing Site: Swimmers Warned to Stay 2026

Toxic algae has been detected at the newly designated Thames bathing site in Ham, south-west London, prompting urgent warnings for swimmers to stay out of the water. This rare occurrence of blue-green algae in a moving river is linked to extreme heat, low flow, and nutrient pollution from sewage waste.

What Is Toxic Blue-Green Algae?

Blue-green algae, or cyanobacteria, are microscopic organisms that thrive in warm, nutrient-rich water. While they occur naturally, certain species produce toxins harmful to humans and animals. Contact can cause skin rashes, eye irritation, and gastrointestinal illness if ingested. In severe cases, liver and neurological damage may occur.

The algae blooms are more common in still water like ponds and lakes, but the current conditions on the Thames—record temperatures and reduced flow—have created a perfect breeding ground. The Environment Agency has confirmed the presence of the algae and advises against any contact with the water.

Why Did the Algae Appear at Ham Bathing Site?

Several factors converged to trigger this unusual bloom. The prolonged heatwave has warmed the river, while low rainfall has reduced flow, concentrating pollutants. Nutrient pollution from sewage effluent, particularly from Thames Water's Hogsmill sewage treatment plant three miles upstream, provides the phosphorus and nitrogen that fuel algal growth.

Marlene Lawrence of the Teddington Bluetits swimming group, which campaigned for the bathing designation, said: "This is so unusual to get blue-green algae in a moving body of water. It is the first time in nearly six years of swimming here that this has happened." She added that climate change makes such events more likely.

Health Risks and Safety Advice

Swimmers and river users are urged to avoid the water entirely. Dogs should also be kept on leads and away from the riverbank. Symptoms of exposure include vomiting, diarrhea, and skin blistering. If you come into contact, rinse thoroughly with clean water and seek medical advice if unwell.

The Environment Agency is monitoring the situation and will update the public. The bathing site, which officially opened in May, is now under an advisory notice. Local authorities have placed warning signs along the river.

Thames Water's Recycling Scheme and Controversy

The algae discovery intensifies opposition to Thames Water's £250 million water recycling project. The plan would abstract 75 million litres of water daily from the Thames at Teddington during low flow and replace it with treated sewage from Mogden sewage works via a new tunnel. The scheme is designated a strategic infrastructure project, allowing it to bypass local planning objections.

Campaigners argue that the treated sewage would raise water temperature, alter salinity, and harm fish and biodiversity. The Environment Agency previously rejected a similar proposal for these reasons. The current bloom underscores the risks of adding more nutrients to an already stressed river.

Comparison of Water Quality at Different Bathing Sites

Key Takeaways

• Blue-green algae is present at Ham bathing site; avoid water contact.
• Heat, low flow, and sewage nutrients caused the bloom.
• Thames Water's recycling scheme may worsen water quality.
• Climate change increases frequency of such events.
• Monitor Environment Agency updates for safety.

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