Algae are a natural feature of many lakes across Ireland. Under certain conditions, particularly during warm weather, algae can grow rapidly and form what is known as an algal bloom. These blooms may cause the water to appear green, blue-green, turquoise, cloudy, or discoloured, and can sometimes form a visible scum on the water's surface.

While not all algal blooms are harmful, some blue-green algae (cyanobacteria) can produce toxins that pose risks to both human and animal health. Contact with affected water may cause skin irritation, eye irritation, vomiting, diarrhoea, or other symptoms.

Why Do Algal Blooms Occur?

Algal blooms develop when environmental conditions are favourable, including warm temperatures, sunlight, and high nutrient levels in the water. Excess nutrients, particularly phosphorus and nitrogen, can enter waterways through:

• Agricultural runoff.
• Fertiliser application.
• Farmyard runoff.
• Municipal wastewater discharges.
• Industrial discharges.

How to Recognise an Algal Bloom

Blue-green algae blooms may appear as:

• Green, blue-green, or turquoise scums on the water surface.
• Thick mats of algae along shorelines.
• Discoloured or cloudy water.
• Water with a strong earthy or musty odour.
• Bright green patches resembling paint floating on the water.

Advice for the Public

To protect yourself and others, please follow this advice when algal blooms are present:

Avoid Contact

Do not swim, paddle, or take part in water-based activities where algal blooms are visible or suspected. If you come into contact with affected water, wash thoroughly as soon as possible and wash any clothing or equipment that may have been exposed.  

Protect Pets and Livestock

Do not allow pets, livestock, or other animals to enter or drink from affected waters. Algal toxins can be extremely harmful and may be fatal, particularly to dogs. Dog owners are advised to bring fresh drinking water during visits to lakes and waterways.

Follow Warning Signs

Always observe and follow any warning notices or safety signage erected by Roscommon County Council, Waterways Ireland, or other relevant authorities.

Boating and Water Access

Boat launching facilities, jetties, and slipways may remain open during an algal bloom. However, algae can make surfaces slippery, so extra care should be taken when accessing the water.

Drinking Water

Do not use water from affected lakes or waterways for drinking, food preparation, or washing. Boiling or filtering the water may not remove harmful algae toxins

The Environmental Protection Agency (EPA) has produced a fact sheet, including photographs, to help identify lake phytoplankton and recognise potentially harmful algal blooms in lake environments.

This resource provides useful information for members of the public , lake users and community groups who wish to better understand and identify algae that may impact water quality.

View the EPA factsheet at: Phytoplankton Factsheet

If you observe a suspected algal bloom in a lake, river, or other waterbody in County Roscommon, please contact the Environment Department of Roscommon County Council to report the sighting.

Email: environment@roscommoncoco.ie

When reporting, please provide:

• The location of the bloom.
• The date and time observed.
• Photographs where possible.
• Any concerns regarding public or animal health.