| Summary: | Subarctic ecosystems represent a vulnerable environment for their biota and studying their habitats provides a better understanding of ongoing and accelerated environmental change. Literature suggests that habitat and environmental variables such as water chemistry jointly influence benthic algal species richness and assemblage composition. However, the combined effects of habitat and water chemistry on benthic algal communities are poorly understood especially in subarctic freshwaters. This study compared benthic diatom assemblages on rocks and on sediment and examined if environmental variables affect diatoms similarly in these habitats. A total of 20 subarctic ponds were sampled for diatoms on sediment and rock in summer 2022 in northern Fennoscandia. Differences in species richness and Pielou evenness between habitats were analysed by a paired Wilcoxon signed-rank test. The most significant environmental variables influencing diatom species richness were determined by generalized linear models. Differences in diatom assemblage compositions were analysed using non-metric multidimensional scaling, analysis of similarities, and Jaccard similarity index. Finally, to study how environmental variables shape diatom compositions on rock and sediment, a redundancy analysis was performed. According to our results species richness and evenness were significantly higher on sediment than on rock, and water pH was the major determinant for diatom richness in both habitats. Diatom assemblage composition did not differ between rocks and sediment, and composition was shaped by the same environmental variables such as pond surface area and water pH in both habitats. Our results support the prevailing conception that sediment habitat contains higher diatom species richness than rocks, while assemblage composition does not differ between these habitats. This study highlights the role of water pH on benthic diatoms regardless of their habitat. Given that global warming is expected to affect water pH levels, such a change will ...
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