Causes for the increasing trends in DOC and colour

Summary

The concentration of dissolved natural organic matter (DOM) and water colour of lakes and streams in Europe and North America, which have been suffering from acid rain, have showed an unprecedented increase during the last two decades. Several hypotheses to explain this increase have been put forward. This presentation argues that the strong decrease in aluminium concentrations, due to reduced acid deposition, has acted as a strong governing factor for the increased amount of aquatic DOM and specially colour in surface waters in Norway. Aluminium is a powerful precipitator and flocculent of DOM, particularly its more coloured hydrophobic fraction. This conceptual hypothesis is empirically substantiated by a multivariate statistical analysis of monitoring data on surface water chemistry in water courses that have been to varying degree exposed to acid rain in Norway and the UK. Inorganic labile aluminium (Al_i ) and total organic carbon (TOC) show strong opposite scores along the first two principal components (PC) in principal component analysis (PCA) at the 4 most acidified of the 10 monitored water courses in Norway. In addition strong negative correlations were found along either the first or second PC at 5 of the other sites. The same relationships were found in sites in the UK where there has been a significant reduction in Al_i concentrations. Hydrology (discharge) and temperature (daily deviation from norm) were not found to have any significant explanatory value in the Norwegian high order water courses at a long term scale.