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Summary
The “SNSF-project” in Norway during 1972–80 investigated the effects of acidification on benthic invertebrates.
Determinations of chironomid communities were done in five lakes. Of these four are oligotrophic clearwater lakes (pH 4.5, 4.8, 5.6, 6.2) and one is a humic lake (pH 4.4). Lowest mean abundance of chironomids were found, independent of lake water pH, in lakes situated in regions with the longest history of acidification.
Typical species and species associations of chironomids in acid humic, acid clearwater and less acid clearwater lakes are given. The number of species decrease with decreasing pH, lowest in the humic acid lake.
The trophic level in the lakes were measured both by chlorophyll a content and chironomid associations. The chironomids in the clearwater lakes, situated in the most acidified area (the lakes with pH 4.5 and 5.6), display a lower trophic level than indicated by the chlorophyll a. This difference does not appear in the lakes lying in areas with lower acidification (the lakes with pH 4.8 and 6.2). It is suggested that the production rate per unit chlorophyll a is reduced in the most acidified area.
The humic acid lake is exposed to high acidification, but the chironomid association shows a higher trophic level than indicated by the chlorophyll a. This discrepancy may be caused by both the oxygen deficiency and the humic substances serving as nutrients.