Abstract:We evaluated patterns of denitrification and factors effecting denitrification in the upper Mississippi River. Measurements were taken over 2 years, during which river discharge ranged from record flooding to base flow conditions. Over the period of study, average denitrification enzyme activity was highest in backwater lakes and lowest in the main channel. Throughout the study reach, highest denitrification enzyme activity occurred during fall and lowest occurred in winter. Rates during spring floods (2001) were only slightly higher than during the preceding winter. Mean unamended denitrification rates ranged from 0.02 (fall 2001 in backwaters) to 0.40 µg N·cm -2 ·h -1 (spring 2001 in backwaters). Laboratory experiments showed that denitrification rates increased significantly with addition of NO 3 -regardless of sediment C content, while rates increased little with addition of labile C (glucose). Denitrification in this reach of the upper Mississippi River appears to be NO 3 -limited throughout the growing season and the delivery of NO 3 -is strongly controlled by river discharge and hydrologic connectivity across the floodplain. We estimate that denitrification removes 6939 t N·year -1 or 6.9% of the total annual NO 3 -input to the reach. Hydrologic connectivity and resultant NO 3 -delivery to high-C sediments is a critical determinant of reach-scale processing of N in this floodplain system. Résumé : Nous avons évalué les patterns de dénitrification et les facteurs qui opèrent la dénitrification dans le Mississippi supérieur. Les mesures ont été réalisées sur 2 années, pendant lesquelles le débit de la rivière a varié d'inondations record à des conditions d'étiage. Durant la période d'étude, l'activité enzymatique moyenne de dénitrifi-cation était maximale dans les lacs de la plaine de débordement et minimale dans le chenal principal. Dans toute la zone d'étude, les valeurs maximales de l'activité enzymatique moyenne de dénitrification ont été mesurées à l'automne et les valeurs minimales en hiver. Les taux durant les inondations du printemps (2001) étaient tout juste un peu plus élevés que l'hiver précédent. Les taux moyens non corrigés de dénitrification variaient de 0,02 (automne 2001 dans des eaux de la plaine de débordement) à 0,40 µg N·cm -2 ·h -1 (printemps dans des eaux de la plaine de débordement). Des expériences en laboratoire montrent que les taux de dénitrification augmentent de façon significative après l'addition de NO 3 -, quel que soit le contenu des sédiments en C; ces taux augmentent peu après l'addition de C labile (glucose). La dénitrification dans cette section du Mississippi semble être limitée par NO 3 -durant la saison de croissance et l'apport de NO 3 -est fortement contrôlé par le débit de la rivière et la connectivité hydrologique à travers la plaine de déborde-ment. Nous estimons que la dénitrification retire 6 939 t N·an -1 , soit 6,95 % de l'apport annuel de NO 3 -dans la section. La connectivité hydrologique et l'apport de NO 3 -aux sédiments riches en C qui en résult...

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1. Although it is well known that sediments can be hot spots for nitrogen transformation in streams, many previous studies have confined measurements of denitrification and nitrate retention to shallow sediments (<5 cm deep). We determined the extent of nitrate processing in deeper sediments of a sand plains stream (Emmons Creek) by measuring denitrification in core sections to a depth of 25 cm and by assessing vertical nitrate profiles, with peepers and piezometers, to a depth of 70 cm. 2. Denitrification rates of sediment slurries based on acetylene block were higher in shallower core sections. However, core sections deeper than 5 cm accounted for 68% of the mean depth-integrated denitrification rate. 3. Vertical hydraulic gradient and vertical profiles of pore water chloride concentration suggested that deep ground water upwelled through shallow sediments before discharging to the stream channel. The results of a two-source mixing model based on chloride concentrations suggested that the hyporheic zone was very shallow (<5 cm) in Emmons Creek. 4. Vertical profiles showed that nitrate concentration in shallow ground water was about 10-60% of the nitrate concentration of deep ground water. The mean nitrate concentrations of deep and shallow ground water were 2.17 and 0.73 mg NO 3 -N L )1 , respectively. 5. Deep ground water tended to be oxic (6.9 mg O 2 L )1 ) but approached anoxia (0.8 mg O 2 L )1 ) after passing through shallow, organic carbon-rich sediments, which suggests that the decline in the nitrate concentrations of upwelling ground water was because of denitrification. 6. Collectively, our results suggest that there is substantial nitrate removal occurring in deep sediments, below the hyporheic zone, in Emmons Creek. Our findings suggest that not accounting for nitrate removal in deep sediments could lead to underestimates of nitrogen processing in streams and catchments.

In-stream nitrogen processing in the Mississippi River has been suggested as one mechanism to reduce coastal eutrophication in the Gulf of Mexico. Aquatic macrophytes in river channels and flood plain lakes have the potential to temporarily remove large quantities of nitrogen through assimilation both by themselves and by the attached epiphyton. In addition, rooted macrophytes act as oxygen pumps, creating aerobic microsites around their roots where coupled nitrification-denitrification can occur. We used in situ 15 N-NO 3 -tracer mesocosm experiments to measure nitrate assimilation rates for macrophytes, epiphyton, and microbial fauna in the sediment in Third Lake, a backwater lake of the upper Mississippi River during June and July 2005. We measured assimilation over a range of nitrate concentrations and estimated a nitrate mass balance for Third Lake. Macrophytes assimilated the most nitrate (29.5 mg N m -2 d -1 ) followed by sediment microbes (14.4 mg N m -2 d -1 ) and epiphytes (5.7 mg N m -2 d -1 ) . Assimilation accounted for 6.8% in June and 18.6% in July of total nitrate loss in the control chambers. However, denitrification (292.4 mg N m -2 d -1 ) is estimated to account for the majority (82%) of the nitrate loss. Assimilation and denitrification rates generally increased with increasing nitrate concentration but denitrification rates plateaued at about 5 mg N L -1 . This suggests that backwaters have the potential to remove a relatively high amount of nitrate but will likely become saturated if the load becomes too large.

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