Geology topics

D.M. Soballe

Publications and source records attributed to D.M. Soballe.

4 recordsLinked to original sources

USGS2008

Effects of residence time on summer nitrate uptake in Mississippi River flow-regulated backwaters

Nitrate uptake may be improved in regulated floodplain rivers by increasing hydrological connectivity to backwaters. We examined summer nitrate uptake in a series of morphologically similar backwaters on the Upper Mississippi River receiving flow-regulated nitrate loads via gated culverts. Flows into individual backwaters were held constant over a summer period but varied in the summers of 2003 and 2004 to provide a range of hydraulic loads and residence times (??). The objectives were to determine optimum loading and ?? for maximum summer uptake. Higher flow adjustment led to increased loading but lower ?? and contact time for uptake. For highest flows, ?? was less than 1 day resulting in lower uptake rates (Unet, < 300 mg m-2 day-1), low uptake efficiency (U% < 20%) and a long uptake length (Snet > 4000 m). For low flows, ?? was greater than 5 days and U% approached 100%, but Unet was 200 mg m-2 day-1. Snet was < half the length of the backwaters under these conditions indicating that most of the load was assimilated in the upper reaches, leading to limited delivery to lower portions. Unet was maximal (384-629 mg m-2 day-1) for intermediate flows and ?? ranging between 1 and 1.5 days. Longer Snet (2000-4000 m) and lower U% (20-40%) reflected limitation of uptake in upper reaches by contact time, leading to transport to lower reaches for additional uptake. Uptake by ???10 000 ha of reconnected backwaters along the Upper Mississippi River (13% of the total backwater surface area) at a Unet of ???630 mg m-2 day-1 would be the equivalent of ???40% of the summer nitrate load (155 mg day-1) discharged from Lock and Dam 4. These results indicate that backwater nitrate uptake can play an important role in reducing nitrate loading to the Gulf of Mexico. Copyright ?? 2008 John Wiley & Sons, Ltd.

River Research and Applications↗

USGS2004

Denitrification in the Upper Mississippi River: Rates, controls, and contribution to nitrate flux

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 ??g N??cm -2??h-1 (spring 2001 in backwaters). Laboratory experiments showed that denitrification rates increased significantly with addition of NO3- 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 NO3- limited throughout the growing season and the delivery of NO 3- is strongly controlled by river discharge and hydrologie 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 NO3- delivery to high-C sediments is a critical determinant of reach-scale processing of N in this floodplain system.

Canadian Journal of Fisheries and Aquatic Sciences↗

USGS2004

Nitrification in the Upper Mississippi River: Patterns, controls, and contribution to the NO3- budget

We measured nitrification rates in sediment samples collected from a variety of aquatic habitats in Navigation Pool 8 of the Upper Mississippi River (UMR) 7 times between May 2000 and October 2001. We also conducted nutrient-enrichment experiments and analyzed vertical profiles of sediment to determine factors regulating nitrification. Nitrification rates were relatively high compared to other ecosystems (ranging from 0-8.25 ??g N cm-2 h-1) and exhibited significant temporal and spatial patterns. Nitrification rates were greatest during the summer and spring compared to autumn and winter (ANOVA, p < 0.05) and were greater in contiguous backwater and impounded habitats compared to main and side-channel habitats (p < 0.05). Regression analysis indicated that nitrification rates were weakly (r 2 = 0.18, p < 0.0001) related to temperature and exchangeable NH4+ of the sediment. However, nutrient-enrichment experiments showed that NH4+ availability did not limit nitrification in 3 sediment types with variable organic matter. Vertical profiles of sediment cores demonstrated that oxygen concentration and nitrification had similar patterns suggesting that nitrification may be limited by oxygen penetration into sediments. We conclude that temperature and sediment NH4+ can be useful for predicting broad-scale temporal and spatial nitrification patterns, respectively, but oxygen penetration into the sediments likely regulates nitrification rates in much of the UMR. Overall, we estimated that nitrification produces 6982 mt N/y of NO3- or 7% of the total annual NO3- budget.

Journal of the North American Benthological Societ↗

USGS1998

Successful water quality monitoring: The right combination of intent, measurement, interpretation, and a cooperating ecosystem

Water quality monitoring is invaluable to ensure compliance with regulations, detect trends or patterns, and advance ecological understanding. However, monitoring typically measures only a few characteristics in a small fraction of a large and complex system, and thus the information contained in monitoring data depends upon which features of the ecosystem are actually captured by the measurements. Difficulties arise when these data contain something other than intended, but this can be minimized if the purpose of the sampling is clear, and the sampling design, measurements, and data interpretations are all compatible with this purpose. The monitoring program and data interpretation must also be properly matched to the structure and functioning of the system. Obtaining this match is sometimes an iterative process that demands a close link between research and monitoring. This paper focuses on water quality monitoring that is intended to track trends in aquatic resources and advance ecological understanding. It includes examples from three monitoring programs and a simulation exercise that illustrate problems that arise when the information content of monitoring data differs from expectation. The examples show (1) how inconsistencies among, or lack of information about, the basic elements of a monitoring program (intent, design, measurement, interpretation, and the monitored system) can produce a systematic difference (bias) between monitoring measurements and sampling intent or interpretation, and (2) that bias is not just a statistical consideration, but an insidious problem that can undermine the scientific integrity of a monitoring program. Some general suggestions are provided and hopefully these examples will help those engaged in water quality monitoring to enhance and protect the value of their monitoring investment.

Lake and Reservoir Management↗