Lakes dominated by cyanobacterial harmful algal blooms (CyanoHABs) are geographically widespread. These blooms are associated with fish kills and toxin production, which have negative impacts on human health. There is compelling evidence that iron (Fe) can also regulate CyanoHABs besides nitrogen and phosphorus. Low iron concentrations (tens of ng L-1) has been documented to limit CyanoHABs in freshwaters whereas high Fe concentrations (hundreds of μg L-1) can promote CyanoHABs. However, it is unclear the extent which Fe influences CyanoHABs in lakes that lack an extensive anoxic zone and where the magnitude of Fe concentration ranges from tens to < 400 μg L-1. The aim of this study was to assess Fe trends, measured as dissolved Fe (DFe), associated with cyanobacteria dynamics in lakes in northwestern Iowa. We utilized a multi-wavelength fluorometer to monitor and classify phytoplankton based on their unique fluorescence spectra from chlorophyll a (Chl-a) and accessory pigments. Here we report a declining DFe trend (drawdown) potentially due to phytoplankton uptake in East Okoboji and its effect on CyanoHABs. Multi-wavelength fluorescence show cyanobacteria peaked (> 50% cyanobacteria Chl-a) during a DFe minimum. The drawdown in DFe inversely correlated with phosphorus, suggesting decoupled Fe and phosphorus cycling. The strong positive correlation between phosphorus and cyanobacteria Chl-a supports a role for phosphorus in triggering the CyanoHAB. This study suggest an underlying mechanistic role of Fe on CyanoHABs: the intermediate range of Fe concentrations investigated here may be sufficient to sustain CyanoHABs.