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.
SSC follows World Health Organization (WHO) guidance and works closely with The Royal Parks (TRP) to ensure safe swimming practices. Decisions are based on multiple factors - not just BGA cell count - and aim to protect all members, especially those more vulnerable to illness.
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.
Swimming is permitted when conditions are stable and do not present a significant health risk. However, we will issue advisory notices when:
BGA levels are elevated
Results show a rising trend
Weather conditions favour bloom formation (e.g., warm, calm, nutrient-rich)
Advisory note: Members are encouraged to make informed personal choices, particularly those who may be more susceptible to illness (children, older adults, immunocompromised individuals). Early signs such as mild discoloration or unusual odour may indicate changing water quality.
Swimming is suspended when conditions present a clear health risk. This includes:
Presence of visible surface scum and/or detectable odour
BGA levels above 100,000 cells/mL or likely to exceed this threshold based on weather conditions and recent trends
Reports of illness linked to recent swimming
Sudden environmental changes that increase exposure risk
In these cases, WHO recommends immediate public health action to reduce exposure. Swimming will resume once conditions return to safe levels.
Decisions are based on a combination of factors, including:
BGA cell counts
Trends in results
BGA levels above 100,000 cells/mL or likely to exceed this threshold based on weather conditions and recent trends
Visual signs (e.g., scum, discoloration)
Weather and environmental conditions
Local reports of illness or discomfort
This multi-factor approach reflects WHO’s emphasis on early intervention and informed personal choice.
You can download the detailed World Health Organization (WHO) guidance here: WHO Guidelines on Recreational Water Quality (2021)