Baltic Sea browning at an accelerating rate, surprising researchers

The Baltic Sea is turning brown at an unexpectedly rapid pace, even in open waters, according to new research by the Finnish Environment Institute (Syke).

Satellite data shows browning rates of 5–6 percent annually in the Bothnian Bay, while measurements from the research vessel Aranda indicate a 2–3 percent yearly increase in other open-sea areas. The findings, though not yet published, have already caught scientists off guard.

“There’s still a need for further study to reconcile the differences in data, but the rate of browning is surprising in any case,” said Jukka Seppälä, Syke’s group manager.

The discoloration stems from dissolved organic matter—humus—carried into the sea by rivers. While coastal browning has long been observed, particularly near major river mouths, the phenomenon is now spreading to offshore waters.

“Coastal zones can no longer retain the carbon flowing in from rivers,” Seppälä explained.

Humus darkens the water, impeding photosynthesis in plants and algae. It also releases carbon dioxide into the atmosphere as microbes break it down, worsening oxygen depletion. Seppälä noted that these emissions are not accounted for in national carbon footprint assessments.

Previous studies by Syke and the Finnish Meteorological Institute have already identified the Baltic as a potential source of CO₂.

In shallow archipelago waters, humus has long posed challenges, particularly for perennial algae like bladderwrack, said Jari Hänninen, director of the Archipelago Research Institute. The northern Bothnian Sea, with its peatlands and peat production, sees the highest concentrations. Organic matter also fuels oxygen depletion, further stressing fish reproduction.

While browning and darkening are distinct—browning refers to color change, darkening to reduced clarity from suspended solids—both trends have been observed in recent decades. Unlike suspended particles, which eventually settle, humus has lasting effects.

Despite the rise in humus, Syke’s research suggests a positive shift offshore: sediment loads appear to be stabilizing, improving water transparency.