The effectiveness of cyanobacteria nitrogen fixation: Review of bench top and pilot scale nitrogen removal studies and implications for nitrogen removal programs

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One of the primary goals of eutrophication management of freshwater systems is to lower the risk of cyanobacterial blooms. This is typically accomplished at the watershed scale by placing controls on phosphorus (P) discharge from point sources. However, several researchers have questioned the predominance of the P management paradigm, arguing that dual nitrogen (N) and P controls would be more effective at preventing cyanobacteria blooms than P controls alone. This hypothesis is predicated in part on the hypothesis that if cyanobacteria are starved of N, which is an essential nutrient, cyanobacteria N2fixation rates will not be high enough to maintain growth rates and biomass yields at or near previous levels. However, several single species cultures of heterocystous cyanobacteria directly examining the effect of removing N show that, when deprived of ammonium and nitrate, N2fixing cyanobacteria compensate biochemically for the high energy cost of fixation when supplied with sufficient nutrients other than N. Biomass and growth rates were only slightly different under N2than when grown under ammonium and nitrate, which is consistent with observations from the long-term experimental fertilization of Lake 227. Collectively, these bench top and pilot scale studies suggest that N control programs will not have a major impact on the magnitude of freshwater cyanobacteria blooms, although cyanobacteria species composition and toxin production might be affected.