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ERDC seeks applications for: Sampling phosphorous efflux from Lake Huron to learn how concentrations change in the downstream direction and ultimately impact Lake Erie Recent analyses (Burniston et al. 2018, Scavia in review, Scavia et al. 2019, 2020, 2022) have shown that the phosphorus (P) load from Lake Huron to the St. Clair River has been underestimated by at least a factor of three. Earlier underestimates were based on assuming the load is determined by offshore concentrations, dismissing river measurements because they were presumed to be biased by sediment-bound P that is not available to algae (IJC 1977). However, Scavia et al. (2020) showed that P to be biologically available. The newer load estimates are based on relatively frequent direct measurements in Canadian waters (e.g., Burniston et al. 2018) and estimates based on daily turbidity measurements in US and Canadian waters, and turbidity-P regressions (Scavia et al. 2022). Scavia et al. (2019, 2020) suggested the higher loads are driven by episodic fluxes of wind-driven resuspended sediment from Lake Huron’s nearshore. Burniston et al. (2018) also reported that P concentrations at the bottom of the river near Port Lambton were substantially higher that those measured at the top near Point Edward. Based on Port Huron/ Point Edward and Port Lambton/Algonac average loads, Scavia et al. (2019, 2020, 2022) suggest these higher loads at the bottom could reflect the episodic events that were missed in samples at the top of the river were mixed during transport and captured at the bottom (Figure 1). Additional sampling targeting more resuspension events are needed to confirm this. Initial efforts also revealed that loads calculated with P concentrations at Port Lambton were higher than those based on concentrations measured across the river at Algonac (Figure 2) even though they are roughly 0.5 km across the river from each other. An important difference between sampling at Algonac and Port Lambton is sampling depth. Algonac samples are taken from 0.5 – 1.0 m below the surface (J. Varricchione, Michigan EGLE, personal communication), whereas Port Lambton samples are from roughly 1 m off the bottom (Burniston et al. 2018). It is possible that the Port Lambton samples encounter resuspended river sediment or bed load. Dissolved substances (e.g., Chloride) move conservatively from Point Edward to Port Lambton, but the fraction of P in particulate form does not (Figure 3). The particulate fraction is much higher at Port Lambton. These differences are also reflected in suspended solids and organic carbon. Additional sampling is required to confirm this. Program Description/Objective: The US Army Corps of Engineers will collect whole water samples from the St. Clair River and deliver them to a laboratory for nutrient analysis. The sampling plan will be designed with the grant recipient and laboratory. The results of the lab work will be delivered to grant recipient, spatial analyses will be performed to understand the fate and transport of phosphorous. Relationships will be established between samples and an existing sensor network maintained by the federal governments of the United States and Canada. These analyses will be fit into a larger body of nutrient work for the St. Clair-Detroit River system including: (Burniston, et. Al., 2009), (Burniston, et. Al., 2018), (Scavia, et. Al., 2014), (Scavia, et. Al., 2019), (Scavia, et. Al., 2022), (Scavia, 2023), (Totten and Duris, 2019).