Quantification of the reasons for the bird lake brownification in Finland

Browning of surface waters due to increased terrestrial loading of dissolved organic carbon is observed across the Northern Hemisphere. Brownification is often explained by changes in large scale anthropogenic pressures (acidification, climate and land use). We quantified the effect of environmental changes on observed brownification of an important bird lake Kukkia in Central Finland. Water bird densities have decreased there during last decades, probably due to brownification of the lake. We studied past trends of organic carbon loading from catchments based on observations since 1990’s. We created scenarios for atmospheric deposition, climate and land use change to simulate their quantitative effect on brownification of the lake by process-based models (PERSiST for hydrology, INCA-C for carbon loading and MyLake for carbon processes in the lake). Increase in forest cut area appeared to be the primary reason for brownification of the lake. Decrease in acidic deposition has resulted in a lower leaching of dissolved organic carbon, but the effect is small. Runoff and total organic carbon leaching from terrestrial areas to the lake is smaller than it would have been without observed increasing trend in temperature by two degrees.