Combined effects of nitrogen to phosphorus and nitrate to ammonia ratios on cyanobacterial metabolite concentrations in eutrophic Midwestern USA reservoirs

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 (NO3) and reduced (NH3) nitrogen, the NO3:NH3 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 NO3:NH3 ratio. Both N-2- and non-N-2-fixing cyanobacteria seemed to produce secondary metabolites and had higher concentrations per unit biovolume when NO3:NH3 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 NO3:NH3 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 NH3 (or both), causing decreases in TN:TP and NO3:NH3 ratios, respectively, may stimulate cyanobacteria having the metabolic ability to produce geosmin, MIB, or microcystins. Future studies should address how the NO3:NH3 ratio affects phytoplankton community structure and occurrence and production of cyanobacterial secondary metabolites.