Nitrate Uptake Capacity and Efficiency of Upper Mississippi River Flow-Regulated Backwaters

Abstract

PURPOSE:In-stream uptake and processing of nitrate nitrite-N may be improved in large river systems by increasing hydrological connectivity between the main channel and adjoining backwaters, wetlands, and floodplain areas. Engineering designs to increase connectivity and loading to backwaters need to consider nitrate nitrite-N uptake capacity and efficiency in relation to hydraulic loading and residence time in order to optimize in-stream N processing. These relationships were examined during three summer periods for a series of backwater systems on the Upper Mississippi River that received flow-regulated nitrate nitrite-N loads via gated culverts.BACKGROUND: Nitrogen (N) runoff to receiving streams and rivers, particularly in the form of nitrate-nitrite-N, has increased several-fold in recent decades (Justic et al. 1995, Vitusek et al. 1997, Goolsby and Battaglin 2001. A consequence of accelerated N mobilization and transport has been water quality degradation of coastal areas and estuaries sensitive to N inputs (Nixon 1995). For instance, increased N loading from the Mississippi River basin has been associated with the development of extensive areas of anoxia and hypoxia (Rabalais et al. 1994) and declines in fish and invertebrate abundance (Pavela et al. 1983) in the Gulf of Mexico. Continued unchecked N loading to coastal systems could lead to significant declines in the diversity and abundance of higher trophic levels and increased bloom frequency of noxious and toxic algae (Vitusek et al. 1997).