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Algae Blooms

Algal Blooms in inland lakes, ponds and rivers


Algae occur naturally in inland waters such as rivers, streams and lakes. When conditions are ideal for growth, an algal ‘bloom’ may occur. During a bloom, the water becomes less clear and may look green, blue-green or greenish-brown. Scums can form during calm weather when several bloom forming species rise to the surface. This can look like paint, mousse or small clumps.
Many waters are susceptible to problems with blue-green algae, typically between June and November. Once algal numbers are high, the bloom is likely to persist throughout the season, declining only at the onset of winter weather conditions.
Cyanobacteria or ‘blue-green algae’, a type of blooming algae, can produce toxins. These toxins can kill wild animals, livestock and pets. They can also harm people, producing rashes after skin contact and illnesses if swallowed.
Algal blooms block sunlight from reaching other plants in the water. They also use up oxygen in the water at night which can suffocate fish and other creatures. Oxygen is also used up when the bloom decays.

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You can’t tell if an algal bloom in the sea, a lake or river is toxic just by looking at it, so it’s safest to assume it is.

Keep pets and children away from the water and avoid skin contact with the water or algae. Where the suspected bloom is on council property please contact the council direct to notify us.

If it is on property owned by someone other than the council you can report the bloom to the Environment Agency on their 24 hour hotline detailed below.

Environment incident hotline:  0800 80 70 60 

The Environment Agency have published further advice.

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You must consider how your water is used when assessing risk to people and animals from exposure to blue-green algae.

Landowners are responsible for controlling discharges of water off-site, warning users, putting up signs and restricting access. They are also responsible for deciding when restrictions can be removed.
Signage warning of the bloom could take the following form:

“HIGH CONCENTRATIONS OF BLUE-GREEN ALGAE HAVE BEEN FOUND IN THIS WATER.
SWALLOWING THE WATER OR ALGAL SCUM CAN CAUSE STOMACH UPSETS OR MORE SERIOUS HEALTH EFFECTS.
CONTACT WITH THE WATER OR WITH ALGAL SCUM CAN CAUSE SKIN PROBLEMS.
IT IS A SENSIBLE PRECAUTION FOR YOU, YOUR CHILDREN AND YOUR ANIMALS TO AVOID CONTACT WITH THE SCUM AND THE WATER CLOSE TO IT.”

The Environment Agency will notify the local council of incidents of Blue Green Algal blooms and action can be taken where a land owner fails to implement action.

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Blue-green algal blooms and scums are natural features of some waters. Increasing shade and reducing nutrients in the water can control algae. Your local Environment Agency Officer can advise you on prevention, control and long-term management.

There are a variety of companies claiming methods of control including ultra sound, chemical and biological. If the body of water has become silted and there is a large build-up of decomposed organic matter, this may have contributed to an excess of the nutrients that favour the algae.

If a landowner proposes to try to treat the algae you are advised to seek expert advice from the Agency first. It is not recommended to employ any technique without first consulting the Environment Agency and getting their approval in writing, as there are legal sanctions to protect wildlife and prevention pollution of watercourses that they administer.

If you need further advice please contact the Environment Agency on 0800 80 70 60. 

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Last updated: 12 September 2018 | Last reviewed: 12 September 2018