Long term brownification process at the Lammi LTER area in 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. The research question at the Lammi LTER area (Southern Boreal Aquatic and Terrestrial Long-Term Ecological Research Area) was brownification of the lake Pääjärvi. We studied both past trends of organic carbon loading from catchments and water colour in the lake based on observations since early 1990’s. We also made simulations of loading for future climate by the physical Persist and INCA models. DOC concentration in the lake was simulated by the physical MyLake model. Simulated DOC concentration was transformed to water colour and light climate of the lake by empirical equations to study the influence on macrophytes (as an indicator of the ecosystem state). In future growing depths might decrease from 2 m to 1.2 m corresponding to observed shift from reference lakes to impacted lakes. 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.