TOC concentrations in Norwegian lakes: The effect of sea-salts and anthropogenic acid components

Concentrations of organic matter have increased in several lakes on the northern hemisphere since the mid 1970s. This is an important feature, since organic matter is an essential constituent for a range of biological, chemical and physical processes in water bodies. There have been several explanations for this increase, including climate change issues. There has however been no uniform increase o f TOC concentrations in lakes (similar situation in several countries on the northern hemisphere; i.e. EvANS et al. 2006, RouLET et al. 2006, SKJELKVÂLE et al. 2001). Changes in ionic strength in precipitation is mainly been controlled by natural sea-salt episodes and anthropogenic acid rain components, and the ionic strength has declined significantly in precipitation over the past decades, mainly due to the decline in sulphate emission from anthropogenic sources. Since the solubility of organic matter is reduced with increased ionic strength (e.g. TIPPING & HURLEY 1988), we have looked at the effect of changes in ionic strength in precipitation, and hence also in catchment surface waters, in Norwegian lakes. We have also looked at the importance of catchment buffer capacity, since pH might control the solubility o f soi l organic matter (i. e. DE WIT et al. 200 l). Our hypothesis has been that lakes with low buffer capacity, and in where sulphate has been a major inorganic constituent, have been the ones with the most pronounced percentage increase in TOC concentrations. Other types of lakes, either with different chemical composition or with some buffer capacity, were thought to have been less affected by the decline in anthropogenic acid components in precipitation.