The Importance of Organic Acidity in Finnish Lakes

Abstract

A lake survey with 987 randomly selected lakes demonstrated that small lakes throughout Finland have high organic matter concentrations (median TOC concentration 12 mg l−1). This characteristic of Finnish lakes is related to catchments contain large proportions of peatlands and acidic organic soils in coniferous forests. Most of the lakes (88%) have TOC concentrations≥5 mgl−1. Humic lakes are on an average more acid than clear water lakes. The median pH in the full data set is 6.3. The proportion of lakes with pH values lower than 5.5 out of the total sample of the lakes is 21%, and 93% of these acidic lakes have TOC concentrations 5≥ mg1−1. The organic anion, estimated by ion balance calculations, is the main anion in the full data set. However, in southern parts of the country where the acidic deposition is highest, the anthropogenic contribution to the acidity is more important than the catchment-derived organic acidity.