Simulated heat waves promote the growth but suppress the N₂ fixation rates of Dolichospermum spp. and cyanobacterial communities in temperate lakes

The intensity and duration of heat waves are expected to increase this century, occurrences considered favorable for harmful, bloom-forming freshwater cyanobacteria. Dinitrogen (N₂) fixation is a major component of nitrogen cycling in freshwater systems, yet little is known with respect to how diazotrophic (N₂ fixing) cyanobacteria respond to heat waves. Here, we quantified the effects of elevated temperature and fixed N enrichment on the growth and N₂ fixation rates of Dolichospermum isolates from differing latitudes and cyanobacterial communities in four lakes across New York state, USA, during the summer and early fall. In culture experiments, temperatures ≥ 26℃ significantly reduced maximum growth rates of the sub-polar Dolichospermum strains from Finland relative to those grown at 20 − 25℃, while a strain of Dolichospermum isolated from subtropical Australia grew significantly faster at 20 − 30℃ compared to cooler temperatures. All strains, however, exhibited significantly lower N₂ fixation rates at higher temperatures (29 − 30℃) relative to lower temperatures (13-26℃). During ecosystem-based experiments with cyanobacterial communities comprised of Nostocales and other diazotrophic cyanobacteria, short-term (3–4 day) exposures to elevated temperatures mimicking heat waves (+3-6℃; 27-31℃) and/or N (NH₄⁺) enrichment caused a significant increase in cyanobacterial net growth rates but also caused a significant decline in N₂ fixation rates. Collectively, results indicate that while heat waves and fixed N loading can promote cyanobacterial communities, they also suppress diazotrophy, which is generally regarded as a significant component of nitrogen cycling in eutrophic freshwater systems. Such reductions in N₂ fixation would promote or intensify N limitation of primary production in lakes during the summer, an outcome that would be exacerbated under future warming scenarios.