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Algal bloom vs pollution - how can you tell the difference?

At this time of year, you might see a creamy brown scum on seas, rivers and lakes caused by algae. It often smells of rotting eggs or vegetables and it can easily be confused with sewage. Here’s how to tell the difference.

There are a few key differences between Algal bloom and a pollution.

Algal blooms - Different coloured foams or algal blooms may be seen between April and August. Algal blooms can form long lines of foam, running along the coastline.

Sewage - Stormwater overflows from a pipe or outfall tend to cause a grey discolouration. The discharges can attract seabirds and may flatten the waves due to any fat or oil present.

In summer, the warmer weather and lighter days encourage algae growth. Algae can look like frog spawn and different types of algae have different colours. It can form in the sea and inland in freshwater too.

A common, non-toxic algae called Phaeocystis, forms large blooms along our coastline, creating a creamy brown foam as it breaks down. Another marine algae, Noctiluca, gives the sea a red, orange or brown discoloration.

You may see blue-green algae - known as cyanobacteria - in calm, nutrient rich water such as ponds or lakes. The water temperature, wind conditions, reduced water flow and the level of nutrients in the water can all affect the growth of this type of freshwater algae. Heavy rainfall can also encourage the growth of blue-green algae, as it washes more nutrients into the water.

There are many types of algal blooms and it’s best to avoid them as some can be toxic, making people or animals sick. Dogs, farm animals and wildlife or plants can all be affected. This includes aquatic plants and fish, which suffer from having less oxygen in the water when algae is present and blocks out the sunlight. You can find more information about different types of algal blooms on the Environment Agency’s website.

Algal blooms are natural, but when they start to break down, how they look and smell means they can be mistaken for sewage. To get more information about your local bathing water, you can sign up for email notifications using our Beachbuoy website, which gives near real time updates on storm release activity.

If you’re not sure if it’s an algal bloom and you think it may be pollution caused by sewage, please let us know as quickly as possible by reporting a pollution. 

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