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Aim Relationships between diatom distribution, composition, species richness and environmental variables in lakes in Arctic Lapland were explored with particular attention to temperature. Location Transect in northern Fennoscandia (66°92–69°27) spanning boreal coniferous forest to treeless tundra along an altitudinal gradient of 108–1024 m a.s.l. Methods Surface‐sediment diatom assemblages and 22 physical and chemical limnological variables from 64 lakes were determined. Ordination techniques (PCA, DCA, CCA) and associated Monte Carlo permutation tests were used to identify the main environmental parameters controlling the distribution of the diatom taxa. Temperature optima and tolerances were estimated by weighted averaging (WA) regression. Diversity indices and cluster analysis were used to discover patterns in biodiversity and composition of diatom assemblages along the vegetational and altitudinal gradients. Results A total of 370 diatom taxa representing 40 genera were recorded, most of which were periphytic species. Lake‐water pH, calcium, sediment organic content, mean July temperature, and total organic carbon (TOC) were identified as the most important environmental variables in explaining the diatom distributions. Lakes located in the middle of the temperature gradient (i.e. mountain birch woodland in the ‘forest–tundra’ transition zone) was found to have the highest species diversity. The temperatures at around 13 °C (≈ 300 m a.s.l) and at around 9.5 °C (≈ 750 m a.s.l.) were identified as the major biotic boundaries. Main conclusions Our results indicate that distribution, composition and species diversity of diatoms are markedly regulated by temperature and other climate‐related factors in ecotonal waters. Ecological boundary areas are highlighted as important areas for maintaining biodiversity. The overall results lend support to the concept of ‘aquatic ecotone’.
Weckström et al. (Mon,) studied this question.