Assessing phosphorus distribution and bioavailability in Lake Decatur, IL

Dijkstra ML, Corcoran MJ, Sloan JJ, and Lutz BL. 2020. Assessing phosphorus distribution and bioavailability in Lake Decatur, IL. Lake Reserv Manage. 36:376–390. In freshwater systems phosphorus is generally considered to be the limiting nutrient for algal growth, but not all phosphorus species are equally bioavailable to algae. Furthermore, the ratio of bioavailable to nonavailable phosphorus, impacted by in-lake processes and tributary loadings, is known to vary. Limited information, however, is available regarding this variation. Here we present the results from our investigation into spatial patterns in phosphorus concentration and bioavailability for 12 sites on Lake Decatur, a eutrophic 6-basin reservoir in central Illinois. We found that soluble phosphorus was distributed fairly homogeneously among basins, but the particulate fraction varied significantly (representing ∼11% and ∼89%, of phosphorus present in the system, respectively). Bioavailable phosphorus was also not evenly distributed throughout the reservoir. The highest concentration of bioavailable phosphorus (0.063 mg/L) was found in a basin that receives return flow of supernatant water from the dredged sediments dewatering pond. The reservoir’s outflow basin, which has more tributaries and coastline than the other 5 basins, had a similar concentration (0.061 mg/L). Bioavailable phosphorus concentrations in the other basins had a mean of 0.024 mg/L, making these less susceptible to algal blooms and subsequent water quality degradation. Current monitoring and reduction efforts regarding nitrogen (to remediate blooms in the Gulf of Mexico) need to (more fully) include phosphorus. Site-specific kinetics, presented in this work, will support model (re)development to improve the selection and prioritization of effective strategies to remove/control the limiting nutrient in Lake Decatur.