Blue Green Algae (also known as cyanobacteria) are naturally occurring organisms found in freshwater lakes. Under certain conditions, they can multiply rapidly and produce toxins that pose health risks to swimmers.
The Royal Parks routinely monitors the Serpentine for the presence of blue-green algae (cyanobacteria), naturally occurring microorganisms found in many freshwater lakes and reservoirs.
Most cyanobacteria are harmless. However, under certain environmental conditions, some species can increase in number and produce toxins that may cause skin irritation, eye irritation or stomach upset, particularly if water is swallowed.
The Serpentine Lake is a closed body of water with no natural flow or freshwater inlet. Fresh water is pumped into the lake via submerged boreholes. Blooms of Blue Green Algae are unpredictable and challenging to manage.
The Royal Parks carries out a programme of water quality monitoring throughout the year, including laboratory analysis and visual inspections of the lake.
The monitoring framework is based on the World Health Organisation’s 2021 Guidelines on Recreational Water Quality and uses four status levels.
Level 1 – Monitoring
Swimming unaffected
Clear water conditions.
Routine monitoring continues.
No restrictions or additional precautions.
Level 2 – Vigilance
Swimming unaffected
Some increase in cyanobacteria levels may be detected.
Additional monitoring may be undertaken.
No restrictions or additional precautions.
Level 3 – Alert Level 1
Swimming continues with additional precautions
This level may be triggered by:
Elevated laboratory measurements of cyanobacteria; or
Obvious green turbidity in the water; or
Thin green surface films or streaks in localised areas.
At this level swimming in the public Lido will be suspended. The Royal Parks has approved a risk assessment and management process from the Club which will allow members to continue swimming activities subject to:
Enhanced member communications.
Daily visual monitoring.
Exposure reduction measures.
Additional guidance for members who may be more vulnerable to adverse effects.
Members are encouraged to make an informed personal decision about swimming and should consider not swimming if they:
Have open wounds or broken skin.
Are immunocompromised.
Have significant underlying health conditions.
Are currently unwell.
Prefer a precautionary approach
Level 4 – Alert Level 2
Swimming prohibited
This level is reached when conditions indicate a significantly increased risk to public health, typically associated with widespread visible surface accumulations (“scums”) of blue-green algae.
At Alert Level 2:
The Royal Parks prohibits swimming.
The Serpentine Swimming Club suspends all swimming activity.
Swimming can only resume when The Royal Parks confirms that conditions have improved.
During Alert Level 1:
Avoid swallowing water where possible.
Shower as soon as practical after swimming.
Remove wet swimwear promptly.
Wash swimwear and kit before reuse.
Seek medical advice if you experience symptoms following swimming and inform the healthcare professional that you are an open-water swimmer.
Any change in status will be communicated through:
Email to members.
The SSC website.
Club WhatsApp groups.
SSC social media channels.
Notices at the Lido.
The Serpentine Swimming Club’s approach is based on informed choice, clear communication and proportionate risk management. Our objective is to enable swimming whenever it is safe and appropriate to do so, while ensuring members have the information they need to make their own decisions.
You can download the detailed World Health Organization (WHO) guidance here: WHO Guidelines on Recreational Water Quality (2021)