High and low flow nitrate distribution and its implications for nitrate retention in Navigation Pool 8, Upper Mississippi River (UMR), and in a high-nitrate tributary, the Root River, USA

摘要

Excessive nitrate transport from large, agriculturally-dominated watersheds to estuaries and coastal waters is an emerging national (USA; Howarth et al. 1996) and global problem. In the United States high nitrate transport from the Mississippi River has been implicated in development of a large hypoxic zone in the Gulf of Mexico (Goolsby et al. 2001, Rabalais et al. 2002). The primary nitrate source (～40%) is the Upper Mississippi River Basin (UMRB), defined as the upstream portion of the watershed north of Cairo, Illinois (Strauss et al. 2004, Richardson et al. 2004). Statistical modeling (Alexander et al. 2000) predicts a high potential for nitrate retention in low-order streams. Of the various fates, denitrification, the reduction of nitrate to dinitrogen gas, is most effective, since loss to the atmosphere is a permanent sink. Numerous literature reports confirm high N retention in low-order streams (e.g. Peterson et al. 2001). However, some recent small stream studies in the UMRB indicate a limited ability for low-order agricultural streams to impact long term loads, despite high denitrification potential (Bolke et al. 2004, Royer et al. 2004; Schaller et al. 2004). On the other hand, some large river reaches, generally considered nutrient conduits, may be more effective nitrate sinks than previously expected. Navigation Pool 8, UMRB, is one example of a large river sink for nitrate (Richardson et al. 2004, D. Soballe and D. Wasley, pers. comm.). Seasonally, the greatest potential for processing N mass is during late spring, high flow. At other seasons, especially late fall through early spring, mass retention is low due to lower loads, concentrations and temperature. Our primary objective was to examine nitrate distribution in a highly complex, large river channel during spring high flow when input exceeds output, and in late summer when nitrate transport is more conservative. A second objective was to identify po- tential retention mechanisms by comparing and contrasting physical-chemical properties from a range of surface water and pore water environments, including a large tributary, the Root River.