CAN REDOX CHANGES IN LAKEBED SEDIMENTS TRIGGER ALGAL BLOOMS? A CASE STUDY OF CARLYLE LAKE, ILLINOIS

Algal blooms are triggered by excess inputs of normally limiting nutrients, such as P, and can be responsible for a range of human health and environmental risks. Successful management of nuisance and harmful algal blooms requires an understanding of P dynamics in aquatic systems. Of particular interest are reservoirs in agricultural landscapes that often provide local communities with drinking and irrigation water, flood control, and recreation, but also feature high P inputs. Thus, this study aims to determine the role of lakebed sediments in triggering algal blooms, particularly the role of redox-sensitive metals (e.g., Fe and Mn) in the sorption and release of P. Our study site, Carlyle Lake, is a large, shallow reservoir (area: 105 km²; mean depth: 3.4 m) located in an agricultural basin (~70% farmland) in Illinois. To assess spatial and temporal trends in lakebed geochemistry, sediments were collected using a Petit Ponar sampler five times (June 2016 to May 2017) across 10 sites. Sediment subsamples were: 1) extracted with deionized water and 2.0 M KCl to characterize P in porewater and loosely bound to sediment and 2) heated to 550°C to determine organic matter (OM) content, then digested in 5.0 N HCl to find total P. The extracts and digests were run on a Westco SmartChem® 170 discrete analyzer. Additional subsamples were dried and analyzed for metals known to sorb P (e.g., Fe, Mn, and Ca) using an Olympus Delta portable x-ray fluorimeter. We observed strong positive correlations between total P and both Fe (R² = 0.64) and Mn (R² = 0.74); there was no correlation with Ca (R² = 0.05), which we use as a proxy for the carbonate fraction. There was a weak positive correlation between total P and OM (R² = 0.28). There were no significant changes in total P, Fe, or Mn over the study period (p > 0.28), but extractable P and OM did vary over time (p < 0.05). We observed a strong gradient in P, Fe, and Mn across the lake, with the highest concentrations occurring near the dam wall. This indicates that fine-grained sediments, which are rich in redox sensitive metals with adsorbed P, may move across the lake and accumulate near the dam. These Fe and Mn-rich sediments near the dam have the potential to release loosely bound P during anoxic conditions, increasing P in the water column. This suggests that algal blooms may be more likely to occur near the reservoir dam.