Backwater of the Upper n Cycling and Fate in a Floodplain Mississippi River

PURPOSE: Backwaters connected to large rivers efficiently retain nitrate and can play an important role in reducing downstream loading to coastal marine environments. However, biotic assimilation and transient storage can account for a significant portion of the nitrate uptake ver-sus denitrification. The fate of this nitrogen (N) pool is largely unknown. Is assimilated nitrate stored temporarily and exported from the system later as another N species or is it eventually routed from recycling pathways to denitrification or burial? This information is critical for mod-eling and managing large river-floodplain systems. The objective of this research was to examine N fluxes and transformations in a backwater system of the Upper Mississippi River within the context of an N budget in order to provide more insight into the fate of nitrate. BACKGROUND: High nitrate loading from agriculturally dominated large river systems has contributed substantially to impairment of sensitive coastal marine ecosystems (Vitousek et al. 1997; Rabalais 2002; Turner et al. 2006). For example, expansion of bottom water hypoxia in the northern Gulf of Mexico in recent decades has coincided with accelerated nitrate loading from the Mississippi River (Goolsby et al. 2001). In particular, Alexander et al. (2000) suggested that nitrogen (N) loads reaching the Mississippi River are transported to the Northern Gulf of Mexic