In this article

1. What you are looking at
2. The bank matters as well as the swim
3. If your dog may have been exposed
4. Check today's advice before you leave
5. Report it to the right place
6. Keep the enjoyable part of the day

The lake was lovely last week. Today there is a strange green streak by the bank, and your dog is already leaning towards the water. This is the moment to change the plan, even if swimming here is usually the highlight of the walk.

Blue-green algae can produce toxins that seriously harm dogs. If you think your dog has been exposed, call your vet immediately, prevent further licking of the coat and rinse with clean water promptly¹.

For future outings, the task is more manageable than becoming an algae expert: recognise reasons to stay away, check local advice and have another way to enjoy the day. A visual check helps identify concern; it cannot certify water as safe².

What you are looking at

“Blue-green algae” is the familiar name for cyanobacteria. Blooms can make inland water look green, blue-green or brownish; surface scum may resemble paint, mousse or clumps. Some blooms contain toxins and some do not. The Environment Agency advises keeping people and pets away from a suspected bloom because appearance cannot establish toxicity².

Natural Resources Wales explains that cyanobacteria can be suspended in water or attached to rocks and other surfaces. Their growth depends on conditions including nutrients, light, temperature and water movement. Scum can form during calm conditions and disperse when wind or waves increase. An attractive stretch of open water is only part of the picture⁵.

Do not make “green” the only word on your mental checklist. NRW describes scums ranging from blue-green to brownish and sometimes reddish-brown, with possible musty or earthy smells. Other algae can look similar. If you are unsure what you have found, choosing a different route is more useful than leaning in for a closer inspection⁵.

The practical decision does not require naming the organism. You can say, “Something about this water concerns me, so we are staying out.” That is a complete walking plan, and it leaves room to report the concern without first proving it yourself.

The bank matters as well as the swim

BVA warns that wind can push toxic blooms towards the edges of a lake or pond. Dogs can be exposed while drinking there or licking material from their coat afterwards. A dog does not have to complete a long swim to encounter the problem. Keeping away includes avoiding a paddle and shoreline investigation³.

A 2022 scientific case report investigated the death of a Flat-coated Retriever after a visit to Wimbleball Lake in Somerset. The dog had been seen licking or nibbling stranded material near dead fish. Paralysis developed within minutes, and the dog died about 45 minutes after exposure. Two laboratory methods confirmed anatoxins in clinical samples⁶.

The researchers considered shoreline material the likely source, but no fish, algae or water samples were collected at the time to confirm that source. Six other dogs in the group remained well. That does not establish that the water was safe: it was one incident with different possible exposures, not a controlled swimming test⁶.

This is why “the other dogs are fine” cannot settle what happened to yours. Describe the individual dog's contact: drinking, paddling, swimming, licking the coat or picking up material at the edge. You are helping the vet understand the event, not trying to prove that everyone nearby had the same experience.

If your dog may have been exposed

PDSA lists vomiting, drooling, twitching, seizures, breathing trouble and collapse among possible signs. Onset varies, and an apparently well dog still needs advice after suspected exposure. Rinsing the coat with clean water and preventing licking reduce further ingestion; these first-aid steps do not replace veterinary assessment¹.

After suspected contact

1. Move awayPrevent more contact with the water or stranded material³.
2. PhoneTell your vet what happened and where¹.
3. RinseUse clean water promptly and stop your dog licking the coat¹.
4. Follow directionsArrange immediate veterinary assessment as advised¹.

There is no known antidote for these toxins, BVA explains, so prompt professional treatment is important. The response depends on the dog and the exposure. Avoid searching for something to give at home while delaying the call. The name of the water body and a clear timeline are useful details to have ready³.

Check today's advice before you leave

The UK Centre for Ecology & Hydrology's Bloomin’ Algae app collects public reports with locations, dates and photographs for verification. Its maps show submitted records; records may be unverified or impossible to verify from the available photographs. An empty patch on a reporting map means no displayed report, not a negative toxin test⁴.

Check the site operator's notices as well as reports, and read signs at the water's edge. Northern Ireland's DAERA guidance says to keep pets out when a bloom is suspected, including where no sign has yet appeared. It provides a confirmed-location viewer and describes how scientists assess reports and notify operators⁷.

Blooms can occur outside the warmest weeks: DAERA describes them as possible throughout the year, although more common from May to September. Its advice is to stay out if the appearance of a water body concerns you. The date on the calendar does not overrule what you find beside the bank⁷.

Build the alternative into the outing before leaving home. Choose a route that also works without the swim, so changing course does not feel like cancelling the whole day. A little flexibility is especially useful when a keen water-loving dog is part of the planning committee.

Report it to the right place

Where you are	A reporting route
England	Report to the Environment Agency2.
Wales	Report to Natural Resources Wales5.
Scotland	Contact the local authority's environmental health team; SEPA is the environment regulator4.
Northern Ireland	Use Bloomin’ Algae or the NIEA incident-reporting route7.

UKCEH asks for a location, date and photograph to help verify a report. Its guidance identifies the local authority and relevant environment regulator as routes for enquiries or incidents. Keep your dog safely away while recording the location; you do not need to collect a sample to make a public report⁴.

In England, the Environment Agency's guidance describes the water owner or operator's role in warnings and access restrictions. Reporting is useful even when you cannot establish whether a bloom is toxic. Let the responsible organisation decide what investigation or signs are needed².

Keep the enjoyable part of the day

BVA recommends keeping a dog on the lead beside suspected or confirmed blooms, carrying on with clean water and shade, and using alternatives such as a paddling pool to cool off at home. You can protect the day's fun without giving access to a doubtful lake³.

Make the next step something straightforward: take the dry path, find a quiet place for a drink and decide whether to continue or head home. The lake can wait for another day. Your dog still has you, the outing and plenty of interesting things to investigate away from the water.

Sources and further reading7 sources

The small numbers in the article link to the source behind each fact. Sources open on their own websites.

1. Blue Green Algae Poisoning in DogsPDSA
2. Algal blooms: advice for the public and landownersEnvironment Agency
3. Stay away from blue-green algae and keep your dogs safeBritish Veterinary Association
4. Bloomin’ AlgaeUK Centre for Ecology & Hydrology
5. Blue-green algaeNatural Resources Wales
6. Confirmation Using Triple Quadrupole and High-Resolution Mass Spectrometry of a Fatal Canine Neurotoxicosis…Toxins
7. Blue-Green Algae Frequently Asked QuestionsDepartment of Agriculture, Environment and Rural Affairs