2015 was a record year for the occurrence of harmful algal blooms (HABs) on Iowa lakes. HABs are a nuisance and public health issue, producing toxins that affect the safety of people, pets, and livestock, and interfere with the public’s ability to use surface waters for recreation. This is a particularly troubling issue in Iowa, where years of excess nutrient (nitrogen and phosphorus) loading has been linked to the emergence of frequent HABs in Iowa. As HABs are often composed of cyanobacteria rather than eukaryotic algae, a persistent question is how the geochemical conditions in Iowa’s lakes might select for HAB-forming cyanobacteria. Nutrient loading has enhanced primary productivity and the deposition of organic carbon to lake sediments, resulting in anoxic sediments and bottom waters, which facilitate release of soluble iron from the sediments. Iron is an essential micronutrient, which is required in higher amounts by cyanobacteria as compared to eukaryotic algae, and even may be toxic to certain types of algae. The role of iron supplied under anoxic conditions in regulating photosynthetic activity is a topic already under investigation by the PI. This project proposes to track the abundance and availability of iron in the water column and sediments at Iowa’s Great Lakes, which encompass a shallow lake with severe HABs (East Lake Okoboji), and a deep lake with less severe HABs (West Lake Okoboji). Sediment cores and water column samples will be collected from the lakes and analyzed at the Iowa Lakeside Laboratory Regents Resource Center (ILLRRC) near Milford, IA and the PI’s lab at Iowa State University (ISU). The concentrations of iron and phosphorus will be determined in samples collected from different depths within the water column over the course of a summer season. We will profile cores immediately after collection with iron microelectrodes to determine the sedimentary iron flux. Finally, a fluorescence method will be developed to identify and quantify the major phytoplankton groups from the water column in order to test whether their identity and abundance is linked to iron bioavailability. The basic science of this proposal is to determine if iron availability is linked to higher cyanobacterial abundances, as this class of bacteria is invoked in HABs. However, if the availability of sedimentary iron is linked to HABs, the results will inform future monitoring strategies. Our results will help to determine if routine iron measurements would be useful in forecasting HABs. We will also develop protocols for quickly and easy assessing the phytoplankton composition in Iowa’s lakes, which could be used for future monitoring by state agencies, or in collaboration with teams of citizen scientists.