The distribution and diversity of Chironomidae (Insecta: Diptera) in western Finnish Lapland, with special emphasis on shallow lakes

Aim Predictions of aquatic ecosystem change with global warming require basic data that accurately reflect the environmental conditions underlying species distributions. However, in remote arctic areas such baseline data are scarce. We assess the influence of environmental variables on chironomid distribution and taxon richness in shallow, isothermal lakes in a poorly studied arctic region. We pay particular attention to community variation along the treeline ecotonal zone where many environmental variables change abruptly in a relatively small area. Location Lake transect in Finnish Lapland spanning from boreal coniferous forest to arctic tundra. Methods Chironomid assemblages were determined from surface-sediment samples of 50 shallow (< 10 m) natural lakes. Abundance and taxon richness data were related to 24 limnological variables using canonical ordination techniques (DCA, CCA, RDA). A Monte Carlo permutation procedure was used to assess the explanatory power of single variables. Between-vegetation zone differences of richness were tested for statistical significance using one-way ANOVA. Results In total, 7771 chironomid head capsules were identified, consisting of 13 species, 10 species groups, four subgenera, 41 genera, four genus groups, five types and three with uncertain taxonomic affiliation. A hump-shaped relationship between taxon richness and elevation was noted along the study transect with a peak in taxon richness occurring in mountain birch woodland lakes at middle elevations, decreasing then towards both warmer and colder ends of the elevation/temperature gradient. Of the individual parameters, sediment organic content, total organic carbon, pH, and lake-specific air temperature accounted for the greatest amount of variation in the chironomid data. Main conclusions Maximum taxon richness occurred at mid-elevations where aquatic algae also reached their maximum diversity. This area coincides with an ecotonal transitional zone, which seems more likely to account for the peak in species richness. Our study demonstrates that the factors most strongly affecting chironomids in Finnish Lapland (i.e. temperature, and ecosystem features) are those that with great probability will also change as a result of future climate change. This will likely have an effect on the distribution of chironomids in subarctic and arctic areas.