Marine algae blooms or pollution? How to tell the difference this summer

As we head into the warmer months, Natural Resources Wales (NRW) is reminding the public that marine algal blooms are a natural occurrence around the Welsh coastline, and are especially common between April and August.

Algal blooms are rapid increases in algae in the sea, lakes, and rivers often triggered by warmer temperatures and increased sunlight.

They include seaweeds and tiny plants suspended in the water and can look like green flakes, greenish bundles or brownish dots.

On the coast, these blooms become more noticeable during the spring and summer months, when the weather is warmer and there’s more sun.

It can sometimes be difficult to distinguish the difference between marine algal blooms and sewage pollution, but the following signs can help you know what to look out for:

• Algal blooms usually occur between April and August.

• Long lines of algal foam can often be seen off headlands lying parallel to the coast.

• If conditions are rough, thick mats of foam may be created on the shoreline by wave action.

• Stormwater overflows tend to cause a grey discolouration of the water and often have an origin such as a pipe or outfall, where the discolouration is strongest.

• Sewage discharges can contain fats and oils causing waves to flatten around it and sometimes attract seabird flocks.

Rowland Sharp, Senior Marine Advisor for Natural Resources Wales said:

At this time of year, we often receive reports of suspected sewage pollution along the coast which can turn out to be the breakdown of algal blooms. 

We understand that seeing unusual foams or discolouration in the water can be concerning, especially during the warmer months.

It can be quite difficult to distinguish between algal blooms and sewage pollution but we’re hoping that by sharing the signs of what to look out for, it will be easier to know when something’s a natural occurrence, and when it might need reporting."

While many algal blooms are harmless, some can produce toxins, so we advise the public to avoid touching any bloom material in the sea, lakes, or rivers.

If you’re uncertain about what you’re seeing or suspect it may be pollution, please do report this to our Incident Communication Centre on 0300 065 30000 or by using our online report it form: Natural Resources Wales / Report an incident 

You can also find more detailed information about the different types of algal blooms here: Natural Resources Wales / Marine algal blooms