Blue-green algae: how to recognise it and when to stay out

Of all the things to check at a lake, blue-green algae is the one that should stop you outright. If a lake has it, or you're not sure, don't swim.

What blue-green algae is

Blue-green algae is the common name for cyanobacteria. Natural Resources Wales describes them as occurring naturally in inland waters, estuaries and the sea. They're a natural part of the water. The trouble starts when their numbers grow out of hand and form a bloom.

Blooms form where there's plenty of phosphorus and the other conditions for growth are right. Natural Resources Wales names light, mixing, flow and temperature. Some blooms produce toxins, and those are known as harmful algal blooms.

You can't tell a toxic bloom from a harmless one by looking. The Environment Agency and UKHSA both say so, which is why the advice is to assume any bloom is toxic.

What a bloom looks like

The Environment Agency describes water with a bloom as less clear, and green, blue-green or greenish-brown. Up close, the algae can look like flakes of green sawdust or brownish pinheads. In calm weather, scums can form on the surface and drift to the shore, where they can look like paint, mousse, jelly or small clumps.

Other things to look out for:

• Some blooms smell musty, earthy or grassy, according to Natural Resources Wales.
• Blooms can cause foam along the shoreline, which is sometimes mistaken for sewage.
• NHS Tayside notes that algal crusts at the water's edge can look brown to nearly black.

Blooms also come and go. Scums form in calm weather, and Natural Resources Wales says some disperse quickly when the wind and waves pick up. What you saw last week may not be what's there today.

When blooms happen

If a lake has had blooms before, look closely each time you go. DAERA in Northern Ireland says blooms are most common from May through to September but can occur at any time of year, and that sunlight, still or slow-flowing water and nutrients favour them. NHS Tayside says the algae multiply during the summer months, and that blooms are most likely in water affected by agricultural, domestic or industrial discharges.

SEPA expects the problem to grow. Its senior scientist describes hotter, drier summers as ideal conditions for more blooms.

Small, shallow lakes carry a greater chance of algae. There's more on why in small, shallow lakes.

The risks to you

The Environment Agency and UKHSA say contact with blooms and scums can lead to skin rashes, eye irritation, vomiting, diarrhoea, fever, and muscle and joint pain. Natural Resources Wales adds that these illnesses haven't led to long-term effects or death, but some have been severe. NHS Tayside says symptoms are usually mild but can sometimes be severe, and names swimmers, canoeists and windsurfers among those at risk.

If you've already been in water you're unsure about, Yorkshire Water's advice is to shower with fresh water straight away and get medical attention if you feel unwell. The Swim healthy guidance adds that if you see a doctor, tell them you've been swimming in open water.

The risks to your dog

Dogs are especially at risk. NHS Tayside explains that dogs tend to drink more in hot weather and may eat algal crusts along the shore. The Environment Agency says swallowing scum can be fatal to dogs and livestock, and SEPA describes dog deaths in Scotland as recurring incidents every year.

The PDSA's advice for dog owners:

• Don't let your dog touch, drink or swim in water you're unsure about, and carry water for them so they're not tempted.
• If they've been in, stop them licking their fur, rinse them with clean water as soon as you can, and contact your vet straight away.
• Signs of poisoning can take anything from 15 minutes to a few days to appear.

How to check before you swim

Look for signs. The Environment Agency and UKHSA say warning signs are put up when a toxic bloom is confirmed. DAERA asks people to follow signs put up by local authorities and advice from whoever runs the water.

Don't treat no sign as an all-clear. Scums can form and clear with the weather. NHS Tayside asks anyone who finds water they suspect is affected, with no warning sign, to contact their local environmental health service.

Look yourself. Check the edges and the water you'd walk in through. If you see scum, discolouration or foam you can't explain, give it a miss.

Check recent reports. Wild's spot pages carry recent reports from swimmers who've been in. Act on any report of scum or discoloured water.

Who to report it to

Reporting a bloom means it can be checked. SEPA, for example, analyses samples from suspected blooms in recreational waters. In the UK:

• England: the Environment Agency incident hotline, 0800 80 70 60 (24 hours).
• Scotland: SEPA's Pollution Hotline, 0800 807 060, or its online form.
• Wales: Natural Resources Wales incident hotline, 0300 065 3000 (24 hours).
• Northern Ireland: DAERA, 0800 80 70 60.

You can also report a suspected bloom through the Bloomin' Algae app, which NHS Tayside, SEPA and DAERA all point to.

Algae is one of several things to check at a lake. For the rest, see safe lake swimming: the complete guide. If you swim in rivers too, river water quality and sewage covers the other side of water quality.

Sources

1. Environment Agency, Algal blooms: advice for the public and landowners
2. Environment Agency, Telling the difference between an algal bloom and sewage
3. Environment Agency and UK Health Security Agency, Swim healthy
4. NHS Tayside, Blue-green algae monitoring
5. SEPA, A bloomin' year for algae and cyanobacteria
6. Natural Resources Wales, Blue-green algae
7. Department of Agriculture, Environment and Rural Affairs (Northern Ireland), Blue-Green Algae
8. PDSA, Blue Green Algae Poisoning in Dogs
9. Yorkshire Water, Visitor safety

This guide is general information, not a substitute for local knowledge or training. Conditions change: check them on the day, and if in doubt, don't go in. Images on the blog are illustrations, not pictures of specific spots.