Observed and predicted future changes of total organic carbon in the lake Päijänne catchment (southern Finland): Implications for water treatment of the Helsinki metropolitan area

Lake Päijänne (1083 km2) is the main drinking-water source for more than one million people of the Helsinki metropolitan area and the lake is important also for fisheries and recreation. We carried out a detailed study on observed and modelled future changes of total organic carbon (TOC) in this large lake catchment (26 459 km2), and assessed the implications for current water treatment processes. Concentrations of TOC/CODMn in the lake and its sub-catchments increased during the period 2000–2014 (CODMn was used as a TOC proxy in the statistical analysis because of long-term data availability). Stepwise multiple regression analysis on the relationships between CODMn, water quality parameters, air temperature and runoff in selected 18 catchments indicated that the explaining variables are site-specific and that the observed recent changes are mainly due to a combination of the simultaneously interacting factors of climate and atmospheric deposition. Linear regression analysis revealed no relationships between CODMn increases and land-use related factors or specific catchment characteristics. A catchment-scale dynamic model system (VEMALA) was used to predict future concentrations and fluxes of TOC, using three different climate change scenarios. TOC concentrations were predicted to decrease by 19% in the Dry scenario and increase by 13% in the Wet scenario by the end of the century. The current treatment processes in the city of Helsinki can likely meet the challenges set by climate change in form of changed raw water TOC and quality. Integrated carbon (C) balance calculations indicated large changes in future C fluxes and lake retention time. These changes would markedly affect several key ecosystem processes such as transport of organically bound nutrients to sea areas and the importance of surface waters in the overall landscape C budgets.