Freshwater cyanobacterial blooms have become ubiquitous, posing major threats to ecological and public health.
Decades of research have focused on understanding drivers of these blooms with a primary focus on eutrophic systems; however, cyanobacterial blooms also occur in oligotrophic systems, but have received far less attention, resulting in a gap in our understanding of cyanobacterial blooms overall.
In this review, we explore evidence of cyanobacterial blooms in oligotrophic freshwater systems and provide explanations for those occurrences.
We show that through their unique physiological adaptations, cyanobacteria are able to thrive under a wide range of environmental conditions, including low‐nutrient waterbodies.
We contend that to fully understand cyanobacterial blooms, and thereby mitigate and manage them, we must expand our inquiries to consider systems along the trophic gradient, and not solely focus on eutrophic systems, thus shifting the high‐nutrient paradigm to a trophic‐gradient paradigm.
Recent studies have shown that the total nitrogen to total phosphorus (TN:TP) ratio and nitrogen oxidation state may have substantial effects on secondary metabolite (e.g., microcystins) production in cyanobacteria. We investigated the relationship between the water column TN:TP ratio and the cyanobacterial secondary metabolites geosmin, 2-methylisoborneol (MIB), and microcystin using multiple years of data from 4 reservoirs located in the Midwestern United States. We also examined the relationship between water column concentrations of chemically oxidized (NO 3 ) and reduced (NH 3 ) nitrogen, the NO 3 :NH 3 ratio, cyanobacterial biovolume, and associated secondary metabolites. We found that the cyanobacterial secondary metabolites geosmin, MIB, and microcystin primarily occurred when the TN:TP ratio was <30:1 (by mass), likely due to higher cyanobacterial biovolumes at lower TN:TP ratios. We also found that relative cyanobacterial biovolume was inversely related to the NO 3 :NH 3 ratio. Both N 2 -and non-N 2 -fixing cyanobacteria seemed to produce secondary metabolites and had higher concentrations per unit biovolume when NO 3 :NH 3 ratios were relatively low. Our data thus are consistent with the hypothesis that lower TN:TP ratios favor cyanobacterial dominance and also suggest that relatively low NO 3 :NH 3 ratios provide conditions that may favor the production of cyanobacterial secondary metabolites. Our data further suggest that increases in the absolute concentrations of TP or NH 3 (or both), causing decreases in TN:TP and NO 3 :NH 3 ratios, respectively, may stimulate cyanobacteria having the metabolic ability to produce geosmin, MIB, or microcystins. Future studies should address how the NO 3 :NH 3 ratio affects phytoplankton community structure and occurrence and production of cyanobacterial secondary metabolites.
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