Internal Phosphorus Loading in Lake Erie's Central Basin

Abstract

Internal phosphorus loading from lake sediments is a major contributor to eutrophication in Lake Erie, but our understanding of this process is more uncertain than for riverine inputs of phosphorus (P). This research aims to quantify internal phosphorus loading in Lake Erie’s central basin as a function of location and time of year. Chapter I focuses on the data produced from 56 controlled and replicated sediment core experiments to estimate internal phosphorus flux under oxic and anoxic conditions. Our estimations of anoxic P flux rates are 5.4 – 27.6 mg m-2 day-1. Chapter II focuses on a complementary timeseries experiment performed in the lake. Continuous in situ nutrient analyzers were deployed at two sites within Lake Erie’s central basin hypolimnion from July 25 to October 10, 2019 to provide a timeseries of soluble reactive phosphorus concentrations at a 6-hour frequency. Positive sediment P flux rates during anoxic conditions ranged from 0.8 mg m-2 d-1 to 106 mg m-2 d-1. At one location, displacement of the anoxic hypolimnion with normoxic water caused an immediate cessation of positive phosphorus flux. Following a return of anoxic conditions after 48 hours, positive P flux rates resumed immediately at higher rates ranging from 15 to 138 mg m-2 d-1, which are indicative of rapid, redox-controlled phosphorus desorption. Extrapolations of these flux rates over the timing and spatial dimensions of seasonal hypoxia suggest internal loading rates are comparable to direct annual tributary loads of P to the central basin. Comparison of results from the two different approaches shows that release of phosphorus is negligible while the overlying water is hypoxic and only begins after the overlying water reaches anoxia. Rapid phosphorus flux from the sediment begins within 24 hours of overlying water becoming anoxic. Re-oxygenation of the overlying water immediately stops the flux of P from the sediment, and upon the return of anoxia the flux rates returned to the same levels as before.