Unprecedented November Algal Bloom in Mälaren Highlights Climate Change Impact

An unusual algal bloom continues to plague Mälaren in November 2025, with experts attributing the phenomenon to rising water temperatures driven by climate change. Ingrid Hägermark, head of the Mälaren Water Conservation Association, explained that since 1959, the lake's water temperature has risen steadily, with a striking 3.8-degree increase recorded in May 2024 compared to the 1964-1993 average. Currently, many parts of Mälaren report temperatures above 7°C, sustaining the sustained bloom.

The most affected zones are the shallow western Västerås and southern Uppsala sections, where green algal cover completely blankets the surface—an uncommon sight for this late in the year. This year's elevated summer temperatures, reaching 19°C in Stockholm's water intake areas (normally 14–18°C), also created challenges for drinking water production. About 2.4 million people depend on Mälaren for their water supply, alongside numerous industries needing clean water.

Hägermark emphasized that even a one-degree temperature increase drastically impacts the aquatic ecosystem and highlighted that the mixing of warmer water layers in autumn might now be influenced by climate change. She warned that unless nutrient runoff from agriculture and waste management is curtailed along with greenhouse gas emissions, the frequency and severity of these algal blooms will likely escalate. The current bloom alters the water's color to a turquoise hue and emits a foul odor, signaling ecological distress.

These developments underline the pressing need for environmental action to protect Mälaren's water quality and ensure sustainable drinking water for millions.