Large-scale variation in phytoplankton community composition of >1000 lakes across the USA

Although environmental impacts on the biodiversity and species composition of lakes have been studied in great detail at local and regional scales, unraveling the big picture of how lake communities respond to environmental variation across large spatial scales has received less attention. We performed a comprehensive analysis to assess how the phytoplankton community composition varies among >1000 lakes across the conterminous United States of America. Our results show that lake-to-lake similarity in species composition was low even at the local scale, and slightly decreased with geographical distance. Analysis of the compositional data by Dirichlet regression revealed that the geographical variation in phytoplankton community composition was best explained by total phosphorus (TP), water temperature, pH, and lake size. High TP concentrations were associated with high relative abundances of cyanobacteria and euglenophytes at the expense of other phytoplankton groups. High lake temperatures stimulated cyanobacteria, dinoflagellates, desmids and euglenophytes, whereas cryptophytes, golden algae and diatoms were relatively more abundant in colder lakes. Low lake pH correlated with high dissolved CO₂ concentrations, which may explain why it benefitted phytoplankton groups with inefficient carbon concentrating mechanisms such as golden algae and euglenophytes. Conversely, the relative abundance of cyanobacteria showed a pronounced increase with lake pH. Large lakes showed higher relative abundances of cyanobacteria and diatoms, whereas small lakes showed higher relative abundances of chlorophytes, desmids and euglenophytes. Biodiversity increased with lake temperature, but decreased at high TP concentrations and pH. The key environmental variables identified by our study (high phosphorus loads, warm temperature, low pH) are associated with anthropogenic pressures such as eutrophication, global warming and rising atmospheric CO₂ concentration. Hence, our results provide a comprehensive illustration of the major impact of these anthropogenic pressures on the biodiversity and taxonomic composition of lake phytoplankton communities.