Intensification of harmful cyanobacterial blooms in a eutrophic, temperate lake caused by nitrogen, temperature, and CO₂

Warmer temperatures can significantly increase the intensity of cyanobacterial harmful algal blooms (CHABs) in eutrophic freshwater ecosystems. However, few studies have examined the effects of CO₂ enrichment in tandem with elevated temperature and/or nutrients on cyanobacterial taxa in freshwater ecosystems. Here, we observed changes in the biomass of cyanobacteria, nutrients, pH, and carbonate chemistry over a two-year period in a shallow, eutrophic freshwater lake and performed experiments to examine the effects and co-effects of CO₂, temperature, and nutrient enrichment on cyanobacterial and N₂-fixing (diazotrophic) communities assessed via high throughput sequencing of the 16S rRNA and nifH genes, respectively. During both years, there were significant CHABs (50-500 μg cyanobacterial chlorophyll-a L^-1) and lake CO₂ levels were undersaturated (≤300 μatm pCO₂). NH₄⁺ significantly increased the net growth rates of cyanobacteria as well as the biomass of the diazotrophic cyanobacterial order Nostocales under elevated and ambient CO₂ conditions. In a fall experiment, the N₂ fixation rates of Nostocales were significantly higher when populations were enriched with CO₂ and P, relative to CO₂-enriched populations that were not amended with P. During a summer experiment, N₂ fixation rates increased significantly under N and CO₂ - enriched conditions relative to N-enriched and ambient CO₂ conditions. Nostocales dominated the diazotrophic communities of both experiments, achieving the highest relative abundance under CO₂-enriched conditions when N was added in the first experiment and when CO₂ and temperature were elevated in the second experiment, when N₂ fixation rates also increased significantly. Collectively, this study indicates that N promotes cyanobacterial blooms including those formed by Dolichospermum and that the biomass and N₂ fixation rates of diazotrophic cyanobacterial taxa may benefit from enhanced CO₂ levels in eutrophic lakes.