Acidification of a Small Watershed in Southern Norway and Some Characteristics of Acidic Aquatic Environments

Continuous monitoring of a small watershed in southern Norway shows that pH has decreased significantly by almost half a unit (from 5.9 to 5.4), conductivity increased from 23 to 29 μS/cm within the period from 1965–1975. Changes are also detected in sediment analyses where Daphnia and Chaoborus are only found deeper than 3–4 cm. Forest clear‐cutting activities have probably accelerated the acidification processes of the already highly vulnerable freshwater localities. The acidic lakes show a solute content below 0.4 meq/1, and the bicarbonate buffer system may no longer work efficiently. Spring thaw and autumn rain episodes are accompanied by strong alternations in the chemical environment of the lakes; by considerable drop in pH and increasing conductivity and sulphate content. With increasing acidification the planktonic community changes from a fish predation governed system (brown trout and Eurasian perch as dominant species) to an invertebrate predation governed one with dominant predators Heterocope and pelagic corixids. Pelagic zooplankton are primarily composed of littoral species: Bosmina longispina, Diaphanosoma brachyurum, Eudiaptomus gracilis and Heterocope saliens. The littoral plant and invertebrate communities are described and discussed.