Euro-limpacs Deliverables


Report on DOC and acidification in Finnish lakes

This deliverable takes the form of a manuscript accepted for publication in Science of the Total Environment, (Increasing trends of total organic carbon cocentrations in small forest lakes in Finland from 1987 to 2003, by Vuorenmaa, Forsius and Mannio). The article presents recent TOC trend data for small forest lakes monitored by the Finnish Environment Institute (SYKE) and reviews the possible causes of rising TOC.

There is evidence that TOC concentrations have increased in small forest lakes in Finland. Increasing trends in TOC were detected in both clear water and humic lakes with minimal direct human impacts in their catchments. The proportion of peatland area in the catchment was not correlated with the TOC changes. There are however indications that largest annual increase of TOC has occurred in lakes with the largest average concentrations.

DOC/TOC leaching is an important flux of carbon in boreal catchments and DOC is important in terms of the transport pathways of nutrients, contaminants and energy to surface waters. It is therefore important to understand and quantify the processes determining the observed changes in DOC/TOC concentrations and fluxes and potential future scenarios in relation to climate change and air pollution. Elevated organic matter concentrations cause problems for water treatment.

Decreasing S deposition and associated mineral acid input and improved acid−base status in the soil appears to be a driver controlling the mobilisation of organic acids and TOC. Our results show that larger decreases in lake SO4 concentrations and larger increases in ANC have been accompanied by larger increases in TOC concentrations.

There was little evidence to support the hypothesis that long−term increases in lake TOC concentrations have been driven by long−term trends in runoff. But there was some indication that large seasonal and inter annual fluctuations in runoff could have affected the TOC trends.

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