ESTIMATION OF DIRECT GROUNDWATER DISCHARGE AND NUTRIENT INPUTS TO A LAKE ERIE SHORELINE

Lake Erie is being threatened in recent decades by increases in harmful algal blooms (HABs) partly due to high nutrient loading in the western basin of the lake. To understand the prevalence of HABs many studies have focused on nutrient loading via runoff, but direct groundwater discharge to the coast may also be a significant source of nutrients. In this study we estimated groundwater-borne nutrient flux to the coast of Lake Erie at Cedar Point National Wildlife Refuge using seepage meters and pore water samples in shallow lakebed sediment. The average seepage rate is 3.4 cm d^-1 per unit centimeter of lakebed. The average flux of total N (NO₃^- + NO₂^-) and PO₄^3- per square meter of lakebed is 0.35 and 2.8 mg d^-1 respectively. If these rates are representative of the broader Lake Erie coast, we extrapolate an average volumetric rate of groundwater discharge to Lake Erie of 180 m³s^-1. Total N and PO₄^3- load to the coast is estimated as 5.8 and 45 mg s^-1 respectively. Compared to the Maumee River depositing ~797 g s^-1 of total N and ~31.3 mg s^-1 of PO₄^3-, direct groundwater discharge is not important for delivering N but may be a significant source of PO₄^3- to Lake Erie. More research is still needed on groundwater inputs to better understand water and nutrient budgets for Lake Erie.