Biodiversity is driven by both historical processes and environmental conditions. This results in a heterogeneous pattern of biodiversity on the landscape, which often differs between macroorganisms and microorganisms. These differences are thought to arise as a product of dispersal capacity. It is reasoned that the diversity patterns of microorganisms are more heavily driven by environmental factors, because of their small size and ease of dispersal. To better understand patterns of diversity in microorganisms, we analyzed the latitudinal diversity gradient and diversity by ecoregion of North American freshwater diatoms. Diatoms are a very diverse group of single-celled eukaryotic algae ubiquitous in aquatic ecosystems. We used the Neotoma Paleoecology Database to analyze alpha and beta diversity patterns of ~2,000 species of diatoms from the surface sediment samples of ~1,200 lakes. All diversity analyses were done at the species and genus level in R (vegan and betapart packages). We used Shannon entropy to evaluate alpha diversity, and beta diversity was decomposed into turnover and nestedness components. We used five-degree latitude bins to evaluate the latitudinal diversity gradient, and the US Environmental Protection Agency’s level one ecoregions to determine diversity by ecosystem type.
Results/Conclusions
At both the species and genus level, we found the lowest alpha diversity of diatoms at low and high latitudes, and the highest alpha diversity in the mid latitudes. This follows other global trends for freshwater microorganisms but contradicts some known patterns for marine microorganisms. The results for beta diversity were less clear and the patterns differed when analyzed at the species and genus level. For instance, at the species level, nestedness was low at low latitudes and high at high latitudes, but at the genus level this pattern is replaced by relatively flat nestedness across latitudes. When analyzing ecoregions, we found that, at the species level, Taiga was the most diverse and the Great Plains was the least diverse ecoregion for diatoms. However, at the genus level, the Great Plains still had the lowest diversity, but Taiga, Northern Forests, and Eastern Temperate Forests all had similar high levels of diatom diversity. These differences in diatom diversity across ecosystem type suggests that environmental factors influence diatom diversity. Our future work will pair environmental parameters of the lakes in our database to evaluate the environmental and spatial variables underlying the structure of North American diatom communities.