SUMMARY1. A novel conceptual model linking anoxia, phosphorus (P), nitrogen (N), iron (Fe) and sulphate to the formation of noxious filamentous and colonial cyanobacteria blooms is presented that reconciles seemingly contradictory ideas about the roles of P, N and Fe in bloom formation. 2. The model has several critical concepts: (i) P regulates biomass and productivity in fresh waters until excessive loading renders a system N-limited or light-limited, but it is the availability of ferrous ions (Fe 2+ ) that regulates the ability of cyanobacteria to compete with its eukaryotic competitors; (ii) Fe 2+ diffusing from anoxic sediments is a major Fe source for cyanobacteria, which acquire it by migrating downwards into Fe 2+-rich anoxic waters from oxygenated waters; and (iii) subsequent cyanobacterial siderophore production provides a supply of Fe 3+ for reduction at cyanobacteria cell membranes that leads to very low Fe 3+ concentrations in the mixing zone.3. When light and temperature are physiologically suitable for cyanobacteria growth, bloom onset is regulated by the onset of internal Fe 2+ loading which in turn is controlled by anoxia, reducible Fe content of surface sediments and sulphate reduction rate.4. This conceptual model provides the basis for improving the success of approaches to eutrophication management because of its far-reaching explanatory power over the wide range of conditions where noxious cyanobacteria blooms have been observed.

Previous work suggests that a high rate of internal ferrous iron (Fe2+) loading from anoxic sediments into overlying waters favours cyanobacteria dominance (>50% of the phytoplankton biomass) over eukaryotic algae. This Cyanobacteria–Ferrous conceptual model was assessed along the Georgian Bay coastline of Lake Huron, Ontario, in one meso-eutrophic and three oligotrophic embayments that experience natural hypolimnetic anoxia. Cyanobacteria dominated all embayments in the relatively warmer summer of 2012 but not in the much cooler summer of 2014, although hypolimnetic anoxia and internal Fe2+ loading were observed in both summers in all embayments. A cyanobacteria bloom large enough to turn the lake visibly green was observed only in warmer 2012 in the meso-eutrophic embayment. Results show that warm summer temperatures and internal Fe2+ loading are necessary preconditions for cyanobacteria dominance, while high nutrient levels are needed to form large blooms. There were no consistent patterns between dominance and total and dissolved phosphorus (P), total nitrogen, ammonium, and nitrate. Internal P loading was not a necessary precondition for dominance. While P removal programs will decrease phytoplankton biomass in eutrophic waters, oxidized surficial sediments must be maintained throughout an aquatic system to prevent cyanobacteria dominance.