Regional Increases in Dissolved Natural Organic Matter linked to decrease in Acid Rain through lower Aluminium levels

Sammendrag

The concentration of dissolved natural organic matter (DOM) have showed an unprecedented increasing trend and the water has become browner during the last two decades in many of lakes and streams in Europe and North America that have been suffering acid rain. Several hypotheses to explain this increase in DOC have been put forward. This paper argues that the strong decrease in aluminum concentrations, due to reduced acid deposition, has acted as a strong underlying driver for increased amount of aquatic DOM and color in surface waters in Norway. Aluminum is a powerful precipitator and flocculent of humic matter and increased Al likely served to suppress the solubility of especially the hydrophobic fraction of DOM. This hypothesis is empirically substantiated by a multivariate statistical analysis of monitoring data on surface water chemistry of water courses that have been to different degree exposed to acid rain in Norway and the UK. Inorganic labile aluminum (Ali) and total organic carbon (TOC) show strong opposite scores along the first two principal components (PC) in principal component analysis (PCA) of 4 of the 10 monitored sites in Norway. Strong negative correlations were in addition found along either the first or second PC at 5 of the remaining sites. The same relationships were found in sites in the UK where there has been a significant reduction in Ali concentrations. Hydrology and temperature were not found to have any significant explanatory value in the Norwegian rivers.