Microcystins (MC) are a class of cyanotoxins produced by many cyanobacteria taxa. Although toxic to metazoans, the evolution of microcystin pre-dates the appearance of metazoans, and so MC did not originate as a toxin to potential metazoan grazers. One hypothesized functional role of microcystin is the management and acquisition of metals, several of which form complexes with MC intracellularly. Metals are often used to build enzymes within the cell that allow cyanobacteria to use non-preferred nitrogen (N) and phosphorus (P) sources, such as nitrate, urea and organic P. If trace metals are in low supply, primary producers may become limited because of their inability to access these non-preferred N and P forms. Furthermore, if MC are used for metal acquisition and management, we would expect that as demand for these trace metals varies, so will the production of MC. We performed 7 mesocosm experiments in triplicate on naturally occurring phytoplankton communities from two nearshore habitats that experience annual cyanobacterial blooms (Green Bay, Lake Michigan and Maumee Bay, Lake Erie). In these experiments, we provided natural communities with amendments of labile nutrients (NH4+ and/or PO43-) and trace metals (Fe, Zn, Ni and Mo) and measured growth (as chlorophyll a), the relative abundance of MC-producing genes (mcyE gene copies), the relative abundance of MC-producing RNA and the MC concentration. Experiments were performed by James H Larson and Sean W Bailey at the Upper Midwest Environmental Sciences Center (UMESC). Genetic measurements were performed by Erin A. Stelzer (Ohio-Indiana-Kentucky Water Science Center) on samples collected at UMESC. Cyanotoxin measurements were performed by Keith A. Loftin (Kansas Water Science) on samples collected at UMESC.