Chironomid response to environmental drivers during the Holocene in a shallow treeline lake in northwestern Fennoscandia

Abstract

The significance of different environmental drivers for the chironomid compositional changes during the Holocene was assessed using partial redundancy analysis. These drivers included pollen-inferred mean July air temperature, vegetation cover as pollen percentages, diatom-inferred water pH and total organic carbon (TOC), and sediment properties as loss on iginition (LOI). Variance partitioning showed that on the Holocene timescale, chironomid assemblages were significantly responding to changes in Betula percentages. This is explained by the co-occurring response of chironomids to the Holocene climatic development, which has also been responsible for the development of vegetation in the catchment. However, during the very early Holocene period from 10 500 cal. yr BP to 8300 cal. yr BP chironomids were responding more directly to the changes in water quality, ie, pH. During the latter part of the Holocene, no environmental variable appeared to be significant in explaining the variation in chironomid assemblages. This may be due to the underestimation of the actual water TOC by the diatom-based TOC model. However, for the latter part of the Holocene, from 5000 cal. yr BP to the present, the importance of limnological changes attributed to the development of marginal mires on the midges is evident. Our study shows that factors other than climate may have had more direct influence on the chironomid abundances at certain time periods. Although these factors are probably indirectly connected to climate through catchment processes, more understanding of the catchment-mediated climate effects on lake ecology and chironomids in high latitudes is needed, in addition to careful site selection, in order to better understand the environmental reconstructions.