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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.

Subsurface chlorophyll maxima layers (SCML) are ubiquitous features of stratified aquatic systems. Availability of the micronutrient iron is known to influence marine SCML, but iron has not been explored in detail as a factor in the development of freshwater SCML. This study investigates the relationship between dissolved iron

The micronutrient iron has been noted to play a crucial role in regulating phytoplankton growth; however, most studies have focused on large lakes with persistent phytoplankton blooms that are known to undergo iron limitation, such as Lake Erie. Iron abundance in boreal lakes is also known to correlate with dissolved organic carbon and increased iron concentrations causing “browning.” To assess the spatial distribution of dissolved Fe (DFe) in lakes throughout Iowa, a landscape once dominated by prairies, DFe was measured in surface waters of 124 lakes distributed across the state over the 2018 summer season. Thirty lakes were selected for 15 weeks of weekly DFe monitoring to assess temporal trends over the summer season. Dissolved Fe concentrations in surface waters ranged from 5 to 1000 μg L‐1. Iowan lakes exhibited temporal trends in DFe, with decreasing concentrations from May to mid‐July and an increase into August. Unsupervised learning method (k‐means) identified three main groups of lakes based on temporal DFe trends. In this study, surface water temperature was associated with DFe trends in some lakes. This study serves as a baseline for DFe in Iowa’s lakes and can provide insights into iron biogeochemical cycling and its role in phytoplankton blooms, which are important to ecosystem and public health.

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