| Summary: | The structure of littoral oligochaete communities in relation to pH was studied in five small lakes in South-western Karelia. Among the lakes, three were affected by acidification and characterized by low pH (mean pH = 4.2–5.9). Two lakes were neutral (mean pH = 6.9 and pH = 7.1, respectively). The oligochaete communities of the acidified lakes were poorer compared to the neutral ones. Taxa richness, total biomass of the oligochaetes, their relative density and relative biomass in macroinvertebrate communities were lower in the strongly acidified lakes. Changes of major taxon proportions in the total density and biomass of the oligochaetes were recorded with lowering of pH. For the analyses, the oligochaetes were separated into three functional feeding groups, as gatherers (S) that selectively ingest mainly on the sediment surface and other substrates, gatherers (T) that selectively ingest mainly in the sediments, and predators. Total density of gatherers (T) as well as their relative density in the oligochaete assemblage and macroinvertebrate communities were lower in the acidified lakes.
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