Blue-green algae: what you need to know

Blue-green algae, also known as cyanobacteria, is a naturally occurring type of bacteria found in lakes, reservoirs and rivers. Despite its name, it’s not actually an algae.  

It's common across the UK, especially during warm, dry and sunny weather. Sometimes it can grow quickly and form a bloom, which can make the water appear green, blue-green or greenish-brown. It can also create patches on the surface that look like paint, foam or floating clumps.  

Most of the time blue-green algae exists at low levels and causes no problems. However, some blooms can produce toxins that may cause mild health problems in people and animals. Contact with affected water can cause symptoms such as skin irritation, while swallowing contaminated water may cause an upset stomach, vomiting or diarrhoea.

It’s important to note that not every bloom is toxic, and it’s impossible to tell whether a bloom is harmful simply by looking at it.  

The health of our participants is our top priority, so we closely monitor water quality and carry out regular testing in the weeks leading up to the event.  
 
The Royal Parks (TRP) also conduct regular water testing in several locations in the Serpentine lake throughout the summer. 
 
Both London Marathon Events and TRP follow established water quality protocols, including World Health Organization (WHO) guidance on recreational water use, to help assess and manage any potential risks.

Swim Serpentine has never been cancelled due to blue-green algae.  

We know that blue-green algae occurs naturally in many freshwater environment and so LME undertakes regular blue-green algae monitoring every year as part of our normal event planning.  

We will continue to monitor the water quality in the weeks leading up to the event.

If you’re training in open water in the run up to Swim Serpentine, the following simple steps will help you to reduce the risk of illness: 

• Avoid swimming in water where there are visible blue-green algae warnings or obvious surface scums 
• Try not to swallow lake or river water while swimming 
• Avoid touching your face during a swim 
• Shower or rinse thoroughly with clean water as soon as possible after leaving the water 
• Wash swimming kit, wetsuits and footwear after use 
• Do not swim if you have any open wounds or if you feel unwell in any way 
• If you feel unwell following a swim, seek medical advice and mention your open water exposure