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1. Algal blooms in inland waters
Algae occur naturally in inland waters such as rivers, streams and lakes. When conditions are ideal for growth, an algal bloom can 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.
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.
2. Algal blooms in the sea
Marine algae include seaweeds and microscopic plants called phytoplankton. A bloom is usually made up of one species and occurs when conditions for growth are ideal, often in the summer.
Some algae can harm marine creatures by producing toxins or clogging the gills of fish.
Blooms of toxic algae are rare in English coastal waters.
Some non-toxic blooms can be mistaken for sewage pollution. One of the most common bloom-forming algae in English coastal waters forms a brown, frothy scum. This is often blown onto the shore where it breaks down into an unpleasant brown slime that smells like sewage. This soon breaks down and disappears.
3. Advice for the public
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. Report the bloom to the Environment Agency.
4. Advice for landowners
You must consider how your water is used when assessing risk to people and animals from exposure to blue-green algae.
You’re responsible for controlling discharges of water off-site, warning users, putting up signs and restricting access. You’re also responsible for deciding when restrictions can be removed. The local council has these responsibilities for beaches and water they own.
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.
5. Report an algal bloom
Call the Environment Agency to report algal blooms:
Environment incident hotline