A novel model for cyanobacteria bloom formation: the critical role of anoxia and ferrous iron

Summary 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. 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 (Fe2+) that regulates the ability of cyanobacteria to compete with its eukaryotic competitors; (ii) Fe2+ diffusing from anoxic sediments is a major Fe source for cyanobacteria, which acquire it by migrating downwards into Fe2+‐rich anoxic waters from oxygenated waters; and (iii) subsequent cyanobacterial siderophore production provides a supply of Fe3+ for reduction at cyanobacteria cell membranes that leads to very low Fe3+ concentrations in the mixing zone. When light and temperature are physiologically suitable for cyanobacteria growth, bloom onset is regulated by the onset of internal Fe2+ loading which in turn is controlled by anoxia, reducible Fe content of surface sediments and sulphate reduction rate. 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.