Aluminium – the link between increase in water colour and DOC and reduction in Acid rain

Summary

The amount and properties of dissolved natural organic matter (DNOM) are known to exhibit large temporal and spatial variations. The temporal fluctuations in the colour and DOC in dystrophic rivers are mainly due to shifts in predominant hydrological pathways through the soil. These changes are governed by fluctuations in field wetness and rainfall intensity (1, 2), as well as temperature. However, this effect can not solely account for the unprecedented increases we have experienced during the last 10-20 years in southeast Norway and southern Sweden (3), south western Finland, northern great Britain (4, 5, 6), in the foothills of Central Europe (7) and in North-eastern US (8). The increase in colour has been greater than DOC content (see 1, 2), indicating that not only the quantity but also the quality of DNOM is changing. It is apparent that the present increase in colour and DOC is a consequence of more then just more frequent extreme rainfall events. The underlying governing factors are most likely related to environmental aspects where we are experiencing regional changes: e.g. climate, land-use and anthropogenic atmospheric deposition. The challenge lies in unravelling these multivariable effects.