Tracking freshwater browning and coastal water darkening from boreal forests to the Arctic Ocean

Abstract

The forest cover of Northern Europe has been steadily expanding during the last 120 years. More terrestrial vegetation and carbon fixation leads to more export to surface waters. This may cause freshwater browning, as more degraded plant-litter ends up as chromophoric (colored) dissolved organic matter. Although most freshwater ultimately drains to coastal waters, the link between freshwater browning and coastal water darkening is poorly understood. Here, we explore this relationship through a combination of centennial records of forest cover and coastal water clarity, contemporary optical measurements in lakes and coastal waters, as well as an ocean drift model. We suggest a link between forest cover in Northern Europe and coastal water clarity in the Baltic, Kattegat, and Skagerrak Sea and show how brown-colored freshwater from Northern European catchments can dictate coastal water clarity across thousands of kilometers, from the Baltic lakes to the Barents Sea.