Benthic animal response after liming of three south Norwegian rivers

Studies of benthic animal communities in three limed Norwegian rivers showed different progress in the recovery response of sensitive animals. In River Vikedal highly sensitive species established populations in the limed part of the river after a few months. The response was slower in River Audna. Longer distances to parental populations was probably the main reason for this. The benthic community of the main River Ogna was unstable, while continuous liming of a highly acidified tributary did not result in improvement of the fauna. In River Vikedal, the bivoltine mayfly Baetis rhodani was more numerous in the autumn samples indicating critical water quality during spring. An adjustment of the pH of the limed water resulted in an increase of this species. The relative abundance of sensitive animals increased downstreams from the point of liming. This may partly be explained by clogging of powdered limestone in the areas closest to the lime doser. A better and more stable water chemistry in the downstream reaches is probably also important. Contrary, the suspended lime did not influence the abundance of filter-feeders.