Nutrient Inputs to the Laurentian Great Lakes by Source and Watershed Estimated Using SPARROW Watershed Models1

Abstract

Nutrient input to the Laurentian Great Lakes continues to cause problems with eutrophication. To reduce the extent and severity of these problems, target nutrient loads were established and Total Maximum Daily Loads are being developed for many tributaries. Without detailed loading information it is difficult to determine if the targets are being met and how to prioritize rehabilitation efforts. To help address these issues, SPAtially Referenced Regressions On Watershed attributes (SPARROW) models were developed for estimating loads and sources of phosphorus (P) and nitrogen (N) from the United States (U.S.) portion of the Great Lakes, Upper Mississippi, Ohio, and Red River Basins. Results indicated that recent U.S. loadings to Lakes Michigan and Ontario are similar to those in the 1980s, whereas loadings to Lakes Superior, Huron, and Erie decreased. Highest loads were from tributaries with the largest watersheds, whereas highest yields were from areas with intense agriculture and large point sources of nutrients. Tributaries were ranked based on their relative loads and yields to each lake. Input from agricultural areas was a significant source of nutrients, contributing ∼33-44% of the P and ∼33-58% of the N, except for areas around Superior with little agriculture. Point sources were also significant, contributing ∼14-44% of the P and 13-34% of the N. Watersheds around Lake Erie contributed nutrients at the highest rate (similar to intensively farmed areas in the Midwest) because they have the largest nutrient inputs and highest delivery ratio.

Nitrogen cycling in Sandusky Bay, Lake Erie: oscillations between strong and weak export and implications for harmful algal blooms

“…Elevated nutrient concentrations were associated with high discharge events from the Sandusky River, demonstrating a strong watershed influence on Sandusky Bay. Indeed, the Sandusky River watershed comprises an area 30 times larger than the bay and delivers large nonpoint loads of N and P to its receiving waters (Robertson and Saad, 2011). Between the two study years, discharge from the Sandusky River varied substantially (tenfold higher in 2015), exhibiting large interannual variability in hydraulic residence time and nutrient concentrations.…”

“…Because the Lake Erie catchment is highly agricultural, the majority of bioavailable N in the lake is derived from farming practices and delivered to the lake via river inflows (Robertson and Saad, 2011;Stow et al, 2015). Once in the lake, dissolved inorganic N (DIN) and dissolved organic N (DON) are subject to consumption by competing biological processes.…”

A spatially explicit suspended-sediment load model for western Oregon

“…Such models have estimated nitrogen and phosphorus transport at the national level (Smith and others, 1997;Elliott and others, 2005), within large hydrologic regions (Alexander and others, 2008;Brown and others, 2011;Garcia and others, 2011;Hoos and others, 2009;Moore and others, 2011;Rebich and others, 2011;Robertson and Saad, 2011;, and at the watershed scale (Ator and others, 2011;Johnson and others, 2013). However, there have only been two applications for suspended sediment-one for the Chesapeake Bay watershed (Brakebill, and others, 2010) and one for the continental United States (Schwarz, 2008).…”

Design and methods of the Midwest Stream Quality Assessment (MSQA), 2013

“…For sites in permeable areas with strong surface connections to groundwater, shallow groundwater also has some of the highest nitrate concentrations nationally (Nolan and Hitt, 2006). Some of the largest loadings of nutrients to the Gulf of Mexico and Lake Erie (not shown) (Robertson and others, 2009;Robertson and Saad, 2011) are derived from the Corn Belt region. Agricultural management practices such as the large amount of tile drains in the Midwest region (Zucker and Brown, 1998) affect the transport of agricultural chemicals.…”