How to identify algal bloom

Algae occur naturally in seas, rivers and lakes, but can sometimes be mistaken for sewage or other forms of pollution. Some algal blooms can be harmful to humans and animals.

What is algal bloom?

Algae are microscopic plants that live in water. They’re a natural part of the food chain, providing food for other species. When conditions are right, algae can grow rapidly to form a bloom that can discolour water, change its appearance and create a smell, especially when it breaks down.

Algal blooms can be brown and smelly, which lead people to mistake them for sewage. Algal blooms are becoming more common globally, influenced by rising water temperatures.

Nutrient levels in the water also play a role and can come from sewage treatment works and other sources like agriculture, industry and urban run off. Thanks to improved treatment processes, the level of phosphates released into rivers from sewage treatment works has reduced significantly in recent years and further investment is planned to continue these improvements.

How can you stay safe around algae?

It’s safest to assume all algae blooms are potentially harmful. You should avoid skin contact or activities like swimming that might lead you to swallow affected water. If you come into skin contact, wash immediately with clean water.  It is also essential to keep animals/pets out of the water when algal blooms are present, because these can cause serious harm.

Algae can cause anything from mild skin rashes to sickness and very rarely potentially life-threatening illnesses.

Symptoms include:

• Rashes
• Vomiting
• Stomach pains
• Nausea
• Fever
• Headache

Always follow any advice or signage carefully and seek medical help if you develop symptoms after exposure.

How can you tell the difference between an algal bloom and sewage from a storm overflow?

We’re working hard to reduce reliance on storm overflows, which release dilute wastewater into the sea when the sewage system is overwhelmed. When storm overflows are in operation, you might see:

• A discharge of grey water, which is
• Concentrated in a particular place (near an outfall or pipe).

With an algal bloom, you might find:

• Water is discoloured green, brown, blue/green or a combination of these colours
• A mat of foam or scum on top of water
• A bad smell, like rotten eggs
• Other substances that look like jelly, sawdust or clumps.

Very rarely, a wastewater pipe might leak or a misconnected pipe from a private home might send human waste into waterways. If this happens, you might find:

• Water has visible poo in it and smells of poo
• Grey papery mulch in the water, from toilet paper and wet wipes
• Water looks milky and soapy due to laundry detergent and washing up liquid

If you think you have spotted a misconnection or leaking sewage pipe, let us know by calling 0344 346 2020 or reporting it online.

When is it safe to swim?

The South West region has one third of the nation’s designated bathing waters. In 2024, 95% (149 out of 157) of designated bathing waters in the South West Water region were classified as ‘excellent’ or ‘good’ for bathing water quality by the Department for Environment, Food, and Rural Affairs (DEFRA). You can find information about current bathing water quality using WaterFit Live.

You should always pay attention to signage warning you not to swim because of an algal bloom, especially around freshwater where harmful blue-green ‘algae’ blooms can occur.

Whose job is it to check for harmful algae in reservoirs?

We monitor the water from our reservoirs regularly. Almost all samples will have some algae present as this occurs naturally. Most strains of algae we find are green algae, which is rarely harmful.

We use an agreed trigger level warning system to decide whether an algae bloom is likely to cause concern or be a risk to users. The higher the concentration of algae (number of cells per millilitre of water), the more likely it is to be harmful, especially when blue-green ‘algae’ (which is actually bacterial) is present.

If we find that water samples show algae at concentrations could be harmful, we advise the South West Lakes Trust (SWLT), who manage the areas surrounding our reservoirs. SWLT takes appropriate action to reduce risk, ranging from putting up warning signs in particular areas where blooms may be present to a strict ban on any water use. 

Identifying algal blooms

Blue-green algae (Cyanobacteria)

A toxic bacteria (despite its common name, it’s a bacteria not an algae) that occurs in freshwater, especially lakes. Signs include:

• Cloudy water with a green, blue-green or green-brown appearance
• Blue or green scum
• May look like jelly, foam, clumps or sawdust.

If you think you have spotted a bloom of blue-green algae, let us know.

Marine algae

The most common type of marine algae around Britain is Phaeocystis. Signs include:

• Frogspawn-like colonies that look like oil drops in their early stages
• Creamy-brown foam on water as the bloom breaks down, especially in windy conditions
• Rotten smell like bad eggs

A thick mat of foam along the shoreline, often holding brown particles of sand or silt.

Rotting seaweed

Seaweed can sometimes also be mistaken for sewage, especially in warm weather. Signs include:

• Clumps of rotting matter on rocks along a beach
• When bleached by the sun, the seaweed can look white and stringy (like broken-down toilet paper)
• A bad smell of rotting eggs or vegetables.

Run off

After heavy rain, water washes carries traces of things like soil and clay, pesticides, fertiliser, animal slurry, oil, diesel and petrol from roads and residue from tyres and brakes. Signs include:

• Brown water caused by mud particles
• Oily residue

Is it algae or sewage?