EFFECTS OF AFFORESTATION ON ACIDITY AND INVERTEBRATES IN DANISH STREAMS AND IMPLICATIONS FOR FRESH-WATER COMMUNITIES IN DENMARK

The relationship between acidity and benthic invertebrate communities was investigated in two small streams in coniferous forests in central Jutland, Denmark, during 1992 and 1993. Stream pH was acid (mean pH 4 .8 and 5.4) and alkalinity fluctuated greatly with mean values around zero. During rain events, water from springfed reaches upstream were m ixed downstream with brown humic acid water with high aluminium conten t, and pH declined from circumneutral near the springs down to pH belo w 4 over a very short distance (0.5-1.5 km). The benthic invertebrate communities in the most acidic downstream reaches of both streams were dominated by filipalpian stoneflies, mainly Leuctra nigra (Olv.). Com munity composition remained very similar throughout the sampling perio d despite variations in pH. In the upper (neutral) reaches invertebrat es intolerant of low pH such as Gammarus pulex L. were found. The conc entrations of chloride and sulphate in the streams were more than doub le the concentrations in a nearby moorland stream indicating a strong impact of acidifying atmospheric deposition on the two forest streams. Results of this study indicate that afforestation with conifers on sa ndy Danish soils with low buffering capacity (about 25% of the total D anish land area) may lead to an increased acidification of surface wat ers and thereby also to an impoverished aquatic fauna.