A multi proxy analysis of climate impacts on the recent development of subarctic lake Saanajdrvi in Finnish Lapland

Responses to recent climatic changes in the sediment of subarctic Lake Saanajärvi in northwestern Finnish Lapland are studied by comparison of various biological and sedimentological proxies with the 200-year long climate record, specifically reconstructed for the site using a data-set of European-wide meteorological data. The multi-proxy evidence of simultaneously changing diatom, Cladocera, and chrysophyte assemblages along with the increased rates of organic matter accumulation and pigment concentrations suggest that the lake has undergone a distinct typological change starting from the turn of the 20th century. This change, indicating an increase in lake productivity, parallels a pronounced rise in the meteorologically reconstructed mean annual and summer temperatures in the region between ca. 1850 and 1930's. We postulate that, during the Little Ice Age, the lake was not, or was only weakly, thermally stratified during summer, whereas the subsequent increase in air and hence epilimnetic water temperatures resulted in the development of the present summer stratification. The increased thermal stability of the lake created more suitable conditions for the growth of phyto- and zooplankton and changed the overall primary production from benthos to plankton. Mineral magnetic and carbonaceous particle records suggest long-distance pollution, particularly since the 1920's, yet the observed changes in lake biota and productivity can hardly be explained by this very minor background pollution; the 20th century species configurations are typical of neutral waters and do not indicate any response to pollution.