Modelling of long term brownification process in Southern Finland

Browning of surface waters due to increased terrestrial loading of organic carbon is observed in boreal regions. It is explained by large scale changes in ecosystems, including decrease in sulphur deposition that affects soil organic matter solubility, increase in temperature that stimulates export of dissolved organic carbon (DOC) from organic soils, and increase in precipitation and thus runoff. Land use changes and forestry measures are also observed to be one reason for increased transport of DOC. The effects of brownification extend to ecosystem services like water purification, but also freshwater productivity through limiting light penetration and creating more stable thermal stratification. We studied past trends of organic carbon loading from catchments based on observations since early 1990’s. We made simulations of loading by the physical Persist and INCA models to three small catchments at the Lammi LTER area. We upscaled simulations to the Kokemäenjoki river basin (17 950 km²). Even though river processes did not play a role in small catchments, they had influence on DOC concentration at the whole river basin. Brownification was driven mainly by the change in climate and decay of organic matter in soil, with smaller impact of land use change on organic soil types. Decrease in sulphur deposition had only minor effect on brownification.