Groundwater phosphorus contributions comparable to tributaries in a large, mesotrophic, polymictic lake

Groundwater is increasingly recognized as a significant yet underappreciated source of nutrients and pollutants to lakes. This study examines the role of groundwater seepage in nutrient dynamics during the summer season in Oneida Lake, a 207 km² polymictic lake in central New York, USA. Over nine weeks in 2020, we measured groundwater seepage and collected pore water samples at 10 representative shoreline sites. Concurrently, we collected grab and automated samples from the five main tributaries and obtained streamflow data from U.S. Geological Survey gauging stations. Data was summarized to compare the relative importance of the two sources of phosphorus (P) during the summer months, both at baseflow only and including high flow events as well. Tributaries dominated both external water input and the Soluble Reactive P (SRP) loading to the lake in summer of 2020. However, total P (TP) loads from groundwater was similar to the TP loads from tributaries and up to twice as high in weeks with baseflow conditions in streams. Nearshore groundwater and tributary P loading together represented ∼50 % of the late summer increase of P in the water column, with the remainder assumed to be from internal loading or deep groundwater seepage. Our results show that groundwater seepage can be a large component of the P budget even in large lakes and adds to the handful number of studies that have quantitatively measured and compared nutrient loading from both streams and groundwater into large lakes.