Iron availability allows sustained cyanobacterial blooms: a dual-lake case study

Lakes dominated by cyanobacterial harmful algal blooms (CyanoHABs) are geographically widespread. These blooms are associated with fish kills and toxin production and have negative impacts on human health. There is compelling evidence that, in addition to nitrogen and phosphorus, iron (Fe) can also regulate CyanoHABs. Low Fe concentration (tens of ng L−1) has been documented to limit CyanoHABs in freshwaters, whereas high Fe concentration (hundreds of μg L−1) can promote CyanoHABs. However, the extent to which Fe influences CyanoHABs in lakes that lack an extensive anoxic zone and where the Fe concentration ranges from tens to <400 μg L−1 is unclear. 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 utilised a multiwavelength 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. Multiwavelength fluorescence data show cyanobacteria peaked (>50% cyanobacteria Chl-a) during a DFe minimum. The DFe is 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 CyanoHABs. This study suggests an underlying mechanistic role of Fe on CyanoHABs; the intermediate range of Fe concentrations investigated here may be sufficient to sustain CyanoHABs.