Statewide Assessment Reveals Spatiotemporal Variability of Iron in Iowa Lakes

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.