Hydrology and Nutrient Flux in the Shiawassee Flats

Abstract

The Saginaw River system is a principal source of nutrient loads to Saginaw Bay, an area of Lake Huron experiencing multiple symptoms of eutrophication , yet the Shiawassee Flats wetland/floodplain region is poorly understood in terms of impacts on hydrology and nutrient fluxes. This study analyzed water, nitrogen, and phosphorus inputs and outputs to the Shiawassee Flats and evaluated the current role that the connected wetlands and floodplain play in water and nutrients to the Saginaw River and the downstream Bay. Discharge measurements and water samples were taken at all of the major inputs to, and output from, the Flats, throughout the spring and summer of 2012-2013, and chemical analyses were conducted for nitrogen and phosphorus. When hydrologic storage was occurring, the output flow was reduced by over 20%; conversely, when combined upstream inputs were very low, at times 80% of the output flow was wetland water. This indicates that the Shiawassee Flats is critical both to flood storage and maintaining base flows. In terms of inorganic nitrogen, the Shiawassee Flats seem to provide a minor decrement in load and a major decrease in concentrations downstream. In terms of total reactive phosphorus, the Flats area contributed to the load in the Saginaw River but reactive phosphorus concentration downstream changed little although there was a trend towards reduction during low flows, particularly in 2013 following large-scale spring flooding. Total phosphorus loads and concentrations increased in the Saginaw River due to contributions (likely organic) by the Flats wetlands.