Patterns in the distribution, composition and diversity of diatom assemblages in relation to ecoclimatic factors in Arctic Lapland

Aim Relationships between diatom distribution, composition, species richnes s and environmental variables in lakes in Arctic Lapland were explored with particular attention to temperature. Location Transect in northern Fennoscandia (66 degrees 92-69 degrees 27) sp anning boreal coniferous forest to treeless tundra along an altitudinal gra dient of 108-1024 m a.s.l. Methods Surface-sediment diatom assemblages and 22 physical and chemical li mnological variables from 64 lakes were determined. Ordination techniques ( PCA, DCA, CCA) and associated Monte Carlo permutation tests were used to id entify the main environmental parameters controlling the distribution of th e diatom taxa. Temperature optima and tolerances were estimated by weighted averaging (WA) regression. Diversity indices and cluster analysis were use d to discover patterns in biodiversity and composition of diatom assemblage s along the vegetational and altitudinal gradients. Results A total of 370 diatom taxa representing 40 genera were recorded, mo st of which were periphytic species. Lake-water pH, calcium, sediment organ ic content, mean July temperature, and total organic carbon (TOC) were iden tified as the most important environmental variables in explaining the diat om distributions. Lakes located in the middle of the temperature gradient ( i.e. mountain birch woodland in the 'forest-tundra' transition zone) was fo und to have the highest species diversity. The temperatures at around 13 de greesC (approximate to 300 m a.s.l) and at around 9.5 degreesC (approximate to 750 m a.s.l.) were identified as the major biotic boundaries. Main conclusions Our results indicate that distribution, composition and sp ecies diversity of diatoms are markedly regulated by temperature and other climate-related factors in ecotonal waters. Ecological boundary areas are h ighlighted as important areas for maintaining biodiversity. The overall res ults lend support to the concept of 'aquatic ecotone'.