THE ROLE OF BENTHIC IRON DURING CYANOBACTERIAL HARMFUL ALGAL BLOOMS (CYANOHABS) OCCURRENCES

Cyanobacteria predominate in freshwater harmful algal blooms (cyanoHABs), and species such as Microcystis produce a toxins, such as microcystins, which affects the liver. Such cyanotoxins pose a risk to the safety of Iowa’s drinking and recreational waters. In recent years, research has focused on understanding the environmental drivers of cyanoHAB formation and toxin production.

Cyanobacteria generally have a higher Fe requirement than eukaryotic algae, suggesting that Fe availability may drive cyanobacteria proliferation. Atmospheric temperature increase and high nitrogen:phosphorus [N:P] ratios have been linked with cyanoHAB occurrences, but the role of bioavailable iron [Fe] remains unclear. The redox reaction between Fe(II)-Fe(III) is controlled by oxygen availability, thus, hypoxic bottom waters create a favorable condition for release of benthic dissolved Fe(II) into the water column. Historical records show cyanoHAB occurrences in East and West Okoboji lakes (northwestern IA) and five sites were monitored in Summer 2017. Dissolved oxygen and total dissolved Fe concentration profiles were acquired from each site, and surface water N & P species were quantified. Major phytoplankton groups from surface water were distinguished and quantified via in situ chlorophyll a using a Pulse-Amplitude-Modulated (PAM) fluorometer. Porewater Fe(II) in sediment cores were profiled using voltammetric microelectrodes.

Shallow sites (6.7 m) contained higher nutrients (N, P) and had more abundant cyanobacteria than deeper sites (max. depth 41m). Brown (e.g. diatoms & dinoflagellates) and green algae were dominant in May and June, but transitioned to cyanobacteria dominance as water temperatures and P increased in July. Low N:P ratios were observed during this transition. Total dissolved Fe in the water column ranged between 12-16μmol L^-1 in May and June then dropped as low as 6μmol L^-1 in July. Hypoxic bottom conditions were periodically present in shallow sites (<5 m), although Fe(II) was detected in only a few sediment cores. Our findings present questions whether benthic Fe(II) fluxes are enhanced by hypoxic conditions. We will present microcystin measurements that can inform the question of how iron availability might regulate toxin release.