Sweden's lakes and coastal waters are a defining part of a summer trip with a dog, but one official hazard deserves far more attention than it usually gets from visitors: cyanobacteria (blue-green algae) blooms, which produce toxins and occur mainly in midsummer, roughly July and August, with occasional limited blooms into autumn. According to Sweden's county administrative boards (Länsstyrelsen), there is no way to tell by looking whether a bloom is toxic, so official guidance is to treat every visible bloom as though it is.

Länsstyrelsen's own guidance is explicit that pets cannot sense whether the water is toxic, and can ingest a lethal dose. Dogs, together with grazing livestock such as sheep, cows and horses, are specifically named as a high-risk group. A dog doesn't need to drink the water directly to be poisoned — licking toxin off its own fur and paws after swimming is enough, which is why rinsing and drying a dog promptly after any lake swim in bloom season is genuinely protective, not just tidiness.

Early signs of poisoning are vomiting and diarrhoea, and Länsstyrelsen's guidance is direct: a pet showing these symptoms after being near or in a bloom needs a vet immediately, since the toxins involved can act quickly. If a lake, bay or stretch of coastline shows a visible green or blue-green film or discolouration, especially in July or August, the safest move is to keep your dog away from the water and the shoreline entirely, not just avoid letting it drink.