A persistent heatwave in Finland is fueling dense blue-green algae blooms in the Baltic Sea’s Gulf of Finland and Archipelago Sea, with freshwater sightings remaining stable.

Sustained high temperatures have led to algal blooms at several sites in the Haparanda Archipelago, and local authorities are advising people to avoid bathing.

A blue-green algae bloom detected at Lower Thetis Lake’s main beach prompted a Capital Regional District health advisory on July 20, 2025, warning humans and pets to avoid the water.

An algal bloom was detected at the municipal Lulsundet beach in Luleå on July 15, prompting warning signs and questions from residents about potential health and safety risks.

The Kachemak Bay National Estuarine Research Reserve detected Pseudo-nitzschia at bloom levels in Kachemak Bay starting July 4. This diatom can produce the toxin domoic acid, associated with amnesic shellfish poisoning, though toxin production is not yet confirmed. Observed bird deaths and marine mammal strandings have spurred collection of mussel samples for lab testing.