Skip to main content

The charity making life better by water

Blue green algae – what you need to know

When the weather gets warmer, blue green algae appears on our canals, rivers and reservoirs. This can be harmful to both people and animals. If you spend time near water, it’s important to understand some basic facts to keep yourself and your family safe.

It's really important to keep dogs and children away from blue green algae

What is it?

Firstly, blue green algae (BGA) is not actually algae, but it looks like it. In reality it's many tiny bacteria that can't be seen with the naked eye.

When the bacteria group together, they form large clumps, which look very much like floating algae.

What does it look like?

Blue green algae

BGA can take many forms. It's most recognisable as a shimmering blue-green layer on the surface of the water. At its peak, it can create a much more obvious scum.

It can also appear as green streaks on the surface and as tiny green specks within the water. Sometimes it makes the water look a different colour.

Blue green algae

Warm weather helps BGA to grow, so you're most likely to see it during the summer months.

It usually occurs in still waters where there isn't much flow, such as reservoirs and quieter canals. You might also see it when there hasn't been any rain for a while.

Why is it a problem?

When BGA grows excessively and reaches the stage of producing scum on the water's surface, it can sometimes produce toxins. This causes a problem if animals drink the water. There have been some cases of dogs and cattle dying or developing long-term health problems because they've ingested BGA.

These toxins will affect humans too if we accidentally consume them after putting our hands in the water or swimming. They can cause skin and stomach problems, as well as more serious illnesses. Children are more at risk than adults.

Large amounts of BGA can also use up a lot of the oxygen in the water overnight. This can drastically reduce the amount of oxygen available for fish and other aquatic creatures, which sometimes kills them.

What can I do?

If you think you've seen BGA in the water, please fill in our online contact form and choose the option to ‘Report an issue or a problem with one of our canals or rivers'.

If you see a warning sign about BGA in your area, take it seriously. People and dogs should avoid entering the water altogether. Dog owners can get more information from the Blue Cross.

If you're a boater or you take part in water sports, always wash your hands with soap after coming into contact with the water.

Stay safe by washing your hands after taking part in water sports

What do the Trust do about it?

When our local teams receive a report of BGA, their first job is to put up signs in the area to warn people about it and to advise local water users, such as angling and canoe clubs. We also inform other organisations, such as the Environment Agency and local councils.

A risk assessment is carried out, which looks at things like where the water discharges to, whether we've had any previous environmental problems in the area, and whether there are any sports or leisure activities on the water.

The outcome of this risk assessment helps us to create a monitoring regime, which varies from weekly visual inspections to taking daily oxygen readings if it's a site we're particularly worried about.

Unfortunately there is no way of knowing which clumps of BGA are going to produce toxins and which aren't. On sites that are prone to BGA blooms, we work with the Environment Agency and others to try to identify ways to reduce this in the future.

Kingfisher in flight with small fish in its beak

Support our work

We need your support to keep canals and rivers alive. Donate today to make a difference

Family nature guide 2019

Download your free nature guide

Identify footprints and read fascinating facts about the creatures who make their homes along our canals and rivers

Last Edited: 06 July 2020

photo of a location on the canals
newsletter logo

Stay connected

Sign up to our monthly newsletter and be the first to hear about campaigns, upcoming events and fundraising inspiration