Watershed inputs of suspended sediment drive patterns of total phosphorus in Chequamegon Bay, Lake Superior

Abstract

Lake Superior is considered the least anthropogenically stressed and has the lowest offshore phosphorus (P) concentrations of the five Laurentian Great Lakes. However, nearshore habitats in Lake Superior are showing evidence of nutrient related stress. We examined drivers of total P dynamics in Chequamegon Bay, a shallow embayment in southwestern Lake Superior located in a region of the Laurentian Great Lakes that is primarily forested with low human development. Over a nine-year period (2014–2022) we measured total and soluble reactive phosphorus (TP and SRP, respectively), total suspended solids (TSS), and chlorophyll-a (Chl-a) at 12 locations distributed across Chequamegon Bay. Path analysis revealed that TP in this region of Lake Superior is largely sediment bound and driven by watershed inputs of suspended sediment. SRP and Chl-a make up only a small portion of TP. TP and TSS were highly correlated, with a stronger correlation at the most nearshore locations and following extreme precipitation events in 2016 and 2018. TP and Chl-a had a weak positive correlation at low TP concentrations, and lack of correlation at high TP concentrations. This suggests that despite high TP inputs from runoff events, Chl-a response was minimal, likely due to low light availability and limited bioavailability of sediment-bound P. Understanding conditions where episodic inputs of TP could contribute to the reactive P pool and how hydrodynamics affect biogeochemical processes and algal response to nutrient inputs are critical to understanding how an expected increase in extreme events will influence nearshore water quality in large lakes.