Anthropogenic salinization of naturally dilute boreal lakes in Finland

Recent findings from North America and Europe suggest that anthropogenic salinization of freshwaters is an emergent threat in the Northern Hemisphere. Causes of salinization include irrigation, water abstraction, resource extraction, land clearing and accelerated weathering, road de-icing, and sewage. Our study shows that freshwater salinization is not only restricted to arid and temperate regions, but is also evident in dilute boreal lakes located on the slowly weathering, mainly granitic and gneissic bedrock of the Fennoscandian Shield. We used quantitative paleolimnology to study natural as well as present-day diatom assemblages and diatom-inferred epilimnetic electrical conductivity (a proxy for salinity) of 70 lakes in southern and central-eastern Finland.

According to our results, electrical conductivity increased from natural to present-day conditions in 61 of the 70 lakes, most likely due to agriculture and urbanization. In these 61 lakes, the maximum increase was 11.7 mS m^-1 and average increase was 3.0 mS m^-1, both notable compared to the natural variation between lakes ranging from 2.3 to 7.2 mS m^-1 (average 3.8 mS m^-1). Natural electrical conductivities were higher in lakes located on the clayey coast of Finland than in lakes further inland characterized by catchments rich in sandy till. In conclusion, our study 1) establishes an estimate of natural salinity variation in dilute boreal lakes overlying acidic bedrock, 2) emphasizes that boreal lakes are also threatened by freshwater salinization, and 3) promotes the importance of management actions targeted to prevent freshwater salinization.