We analyzed two large data sets (the National Stream Water Quality Accounting Network and the National Eutrophication Survey) to compare quantitatively the ecological structure and function of rivers, river impoundments, and natural lakes on a broad scale. Discriminant analysis with physical-chemical variables separated these three types of aquatic systems along a composite gradient that correlated with water residence time (r = 0.7), drainage area (r = 0.7), water depth (r = 0.6), stream flow (r = 0.6), and water clarity (r = 0.4). Natural lakes and rivers occupied opposite ends of this gradient, and reservoirs, on average, were intermediate, but natural lakes and impoundments overlapped extensively and did not form a useful dichotomy. Regression analysis showed algal abundance per unit phosphorus increasing in the sequence: rivers < impoundments < natural lakes, which paralleled intergroup differences in residence time. Algal abundance per unit phosphorus did not differ significantly among these three system types when systems with similar residence times were compared. Our results indicate that water residence time is a useful system-level index that has similar ecological implications for rivers, lakes, and reservoirs.

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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A review of the Long Term Resource Monitoring Program (LTRMP) water quality component was conducted in May-June 2002 and considered seven aspects of the program: (1) objectives of water quality monitoring relative to LTRMP objectives, (2) potential of the current sampling design to meet those objectives, (3) adequacy and suitability of the water quality procedures manual, (4) adequacy and efficiency of procedures for quality assurance and quality control in data collection and laboratory analyses, (5) operations of the analytical laboratory, (6) recent implementation of electronic data acquisition, and (7) communications and data access. The review was conducted by a panel of five external experts who reviewed program documents and then met at the Upper Midwest Environmental Sciences Center from 3-7 June 2002. The panel found that the objectives of water quality monitoring program are tightly interconnected and one element cannot be neglected without negatively affecting the others. The panel emphasized the need for increased analysis of existing data. It is only by analyzing the data and submitting the findings to external peer review that the final step in quality assurance is achieved, the adequacy of the design is fully tested, redundancies are revealed, and the actual value of the collected data is clearly shown and documented. The panel also emphasized that distribution of data and communication of findings are critical functions of the LTRMP. The panel found that the documentation of LTRMP water quality monitoring procedures (Soballe and Fischer 2004) is excellent, describing scientifically sound methods in appropriate detail to interpret the data collected under the program, to ensure standardization among sampling teams, and to duplicate the sampling program in other locations with other personnel. The panel agreed that the current combination of fixed site and stratified random sampling is well suited to examination of conditions at scales of whole pools or entire study reaches across multiple years. The panel found that data acquisition and data management employed by the LTRMP water quality component are exceptional, and that the quality assurance protocols are excellent, exceeding the levels used in many similar programs. This was not a water quality lab audit and the panel recommended that the laboratory continue its participation in such activities as the Standard Reference Sample Program of the USGS. The panel noted that LTRMP derived information is not being used to the extent possible by resource managers in the development and evaluation of resource

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