Cell size and acid tolerance constrain pond diatom distributions in the subarctic

Niche characteristics and traits affect species distributions. However, their effects on the distribution of microorganisms remain understudied, especially in subarctic regions. Our aim was to determine the effects of species niche characteristics and traits on benthic diatom (Bacillariophyceae) elevational distributions, and to identify the key drivers of trait diversity across climate-sensitive subarctic ponds. We sampled 102 ponds along an elevational gradient of 10 to 1,038 m a.s.l. in subarctic Finland and Norway. We calculated the niche characteristics by Outlying Mean Index analysis using an independent data set collected from 141 lakes in northern Fennoscandia. We ran generalised linear models to identify traits most strongly affecting the distribution of individual species, and used a functional dispersion metric to explore trait diversity at community level. Benthic diatom distributions correlated positively with niche breadth, implying that generalists were more widely distributed than specialists, but negatively with niche position, suggesting that marginal species had narrow distributions. Cell size and acid tolerance were the key traits affecting diatom distributions: small size and strong acid tolerance were associated with a wider distribution. Local abiotic variables were more important than elevation in shaping trait diversity, which increased with pH and decreased with conductivity. Several acidic wetland ponds located at low elevations exhibited functionally clustered communities, reflecting stressful conditions that comprised a filter that selected acid-tolerant species. Our results suggest that benthic diatom distributions across subarctic ponds are jointly driven by species cell size and tolerance towards water chemistry variables, especially acidity. Although elevation summarises the effects of several environmental factors, local abiotic variables were more strongly associated with trait diversity than were elevation effects. Our findings provide new insights into the key mechanisms associated with aquatic microbial distributions in highly sensitive climatic regions.