The influence of impoundments on the transport and fate of nitrogen and phosphorus in two southeastern Michigan river catchments.

Abstract

Improved understanding of the sources, transport, and fate of nitrogen (N) and phosphorus (P) in Midwestern watersheds is critical for improved management of aquatic ecosystems. While much research has been conducted on reach-scale processing and transport of nutrients in streams, relatively little research has been directed toward understanding the impact of dams and their associated impoundments on nutrient dynamics in the affected river systems. Even less research has focused on the interactions between stream reaches and impoundments. To better understand these nutrient dynamics, I investigated two predominately agricultural watersheds in southeastern Michigan, the Huron (2,330 km<super> 2</super>) and Raisin (2,780 km<super>2</super>), both draining into the western end of Lake Erie. Nutrient budgets of the two watersheds documented greater nutrient retention in the more heavily impounded catchment. Previously described budget approaches differed in how the system was defined, in what fluxes were considered to be internal to the system versus inputs or outputs from the system, and in which fluxes were excluded because they were considered insignificant or highly uncertain. I developed an approach for N and P budgeting where the system was defined as the soil and water within the watershed boundary and all significant fluxes that could be accurately quantified were included. Paired sampling sites located above and below impoundments as well as above and below selected stream reaches demonstrated more nutrient removal occurred in impoundments. Impoundment outlets exhibited decreased total P (TP), total dissolved P, soluble reactive P, total N (TN), total dissolved N, and nitrate relative to inflowing water. I inferred that the dominant processes responsible for increased nutrient removal in impoundments were particle settling and denitrification. Finally, simulation modeling using the Soil and Water Assessment Tool indicated that TP and TN export loads approximately doubled when all impoundments were removed from the Huron watershed. Simulated impoundments placed near river mouths or in high source areas were most effective in reducing export loads, and many smaller impoundments were better than a single large impoundment. In addition, simulated impoundments increased interannual nutrient load variability.