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

Abstract

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.

Keywords: Carbon dioxide; Doliochospermum; Heat waves; N(2) fixation; Nitrogen; cyanobacteria.