Oslofjord sees unprecedented third algae bloom in four months

The Oslofjord has experienced three major algae blooms in just four months—a phenomenon researchers say is highly unusual—with the latest turning coastal waters brown along southern Norway, Dagbladet reports.

Senior researcher Anette Engesmo of the Norwegian Institute for Water Research (NIVA) described the frequency as extreme. “We’ve seen a shift since 2023, with blooms far more intense than before,” she said. Normally, such events occur only three times a year, but this year’s third bloom emerged by mid-April.

The latest outbreak, caused by an unidentified haptofytt algae, stretches from Sandefjordsfjorden to Drøbak and beyond. While its toxicity remains unconfirmed, Engesmo stressed it poses no risk to humans. “It’s safe to swim,” she said, though the blooms may harm fish and worsen the fjord’s already low oxygen levels as decomposing biomass consumes oxygen.

Earlier this year, a record-breaking bloom of Pseudochattonella—dubbed “vortepølsa” (wart sausage) for its appearance—covered the fjord for weeks before collapsing under storm conditions in early April. “That was an extreme bloom. I’ve never seen anything like it,” Engesmo noted, comparing current nutrient levels to pre-1970s pollution before wastewater treatment plants were introduced.

NIVA attributes the surge to excess nutrients from agriculture, wastewater, and coastal aquaculture flowing into the fjord. Engesmo warned that while individual swimmers can do little, the repeated blooms signal deeper environmental strain.

Tags: algae bloom, oslofjord, environmental research