Eutrophication histories of three contrasting lakes in a naturally nutrient-rich boreal watercourse

Abstract

The Iisalmi Route watercourse in central-eastern Finland contains a number of highly eutrophic lakes. Despite substantial impacts from anthropogenic activity, the region appears to be naturally eutrophic because of its geological setting. A passive continental ice sheet deposited fine-grained basal till that contributes to the naturally eutrophic status of the lakes. Detailed paleolimnological studies in the Iisalmi Route are few. Therefore, we studied the eutrophication histories of three contrasting Iisalmi Route lakes: (1) small headwater Lake Saarisjärvi, (2) larger, but very shallow headwater Lake Näläntöjärvi, and (3) large central basin, Lake Porovesi. We compared the suitability of three diatom-total phosphorus transfer functions for application to fossil samples. A local transfer function, specifically targeted to the Iisalmi Route, was the most suitable for the headwater lakes. In contrast, two regional transfer functions, which have larger geographical coverage, performed equally well for the large central basin. All three lakes proved to have been naturally eutrophic for thousands of years. Highest natural variability occurred in the longest core, from shallow headwater Lake Näläntöjärvi. This variability reflects the dynamic development of the basin and its catchment after deglaciation. Diatom assemblage changes in cores from Lakes Saarisjärvi and Porovesi suggest cultural eutrophication, most likely after the region was colonized in the sixteenth century, followed by salinization in more recent times. Our study deepens understanding of naturally eutrophic boreal lakes that are located on fine-grained tills and their sensitivity to natural and anthropogenic forcing. Our results also highlight the importance of selecting appropriate transfer functions and realistic restoration targets when dealing with naturally eutrophic lakes.