ACID-RAIN AND NATURAL ORGANIC-MATTER (NOM)

Twentyfive years of research on the effects of acid rain on rivers and lakes has, to a very small extent, documented changes in the nature a nd properties of natural organic matter (NOM). In Western Norway, a le -watershed-artificial-acidification-experiment'' took place in the per iod 1988-1996. The goals of this long-term experiment were to study th e role of NOM in acidification of surface water and the effects of aci d precipitation on the quality and properties of NOM. In the HUMEX pro ject (Humic Lake Acidification Experiment) one half of a lake and the corresponding catchment was artificially acidified with H2SO4 and NH4N O3 over a period of 5 years. The other half of the lake and catchment served as a control. In addition to monitoring of the general chemical composition of the water from the two lake halves, a number of other chemical and biological characteristics were studied. Here, we report the results related to changes in the nature and chemical properties o f NOM. During the first few years of acidification, a significantly lo wer concentration of NOM was recorded in the acidified half of the cat chment, compared with the control. However, statistical analyses of al l data (covering a 2-years pre-treatment period and 5 years of treatme nt) related to the concentration of NOM (TOC, colour, and UV absorbanc e) did not suggest any significant effect on the quantity of NOM. This apparent discrepancy between the initial decrease in the concentratio n of NOM and no effect when the whole 5-years period is considered, ma y be due to the results of two different simultaneous processes. The r esults suggest that there first was a reduction of TOC and colour, as a consequence of the acidification, followed by an increase, perhaps o wing to increased fertilisation (nitrogen) and in addition to a genera l temperature increase during the period. In addition, short-term stud ies of the aquatic surface microlayers, lipophilicity of the NOM, cont ent of organic sulfur, and molecular size indicate differences in the quality of the NOM between the two lake halves, which could affect lig ht absorption.