Inter-annual variation in macroinvertebrate communities in a shallow forest lake in eastern Finland during 1990-2001

As a part of the Integrated Monitoring (IM) Programme of Air Pollution Effects on Ecosystems, the macroinvertebrate community of a pristine forest lake was monitored from 1990 up to the present. Lake Iso Hietajarvi is a small and shallow, weakly stratified oligotrophic headwater lake situated in Patvinsuo National Park, eastern Finland. Benthic macroinvertebrates were sampled annually, once in autumn at depths of 3 - 4 m (sublittoral zone) and at 7 in (profundal zone). The water quality of the lake and meteorological characteristics in the area were monitored during the same period. The inter-annual variation in macroinvertebrate abundance and taxonomic composition in the lake unaffected by human activities is described, compared across depth zones and related to environmental variation. The annual variation in density and taxa richness was relatively small, being proportionately greater in the profundal zone. There was a considerable year-to-year variation in the abundance of common and dominant taxa. Most taxa were always few in number and occurred only occasionally - in 1 or 2 years - in both depth zones. The inter-annual community persistence (constancy of taxa occurrences) and stability (consistency of relative abundance of taxa) as measured by the Sorensen distance, were however, relatively high and generally greater in the sublittoral zone. The patterns of variation in density, taxa richness, abundance of dominant taxa and community persistence and stability were generally different in the two zones compared, and few correlations with the measured environmental variables were found. Inter-annual stability and persistence, however, closely traced the course of wintertime North Atlantic Oscillation index, suggesting that the rate of local community change may be linked with large-scale climatic variation in a subtle way. The implications of the results are discussed in the context of biomonitoring.