Salt marsh food webs are complex systems, with high levels of in situ primary production supporting a wealth of resident and migratory species. In this study, we use stable isotopes as a tool to trace organic matter utilization within the Nueces Marsh food web. Specifically, we were interested in (1) the use of treated wastewater as a ¹⁵N tracer, and (2) seasonal and interannual variation in food web structure. We hypothesized that treated wastewater would selectively label detrital resourse use within the food web, allowing us to trace grazing vs. detrital pathways within the marsh system. We also hypothesized that marsh consumers would exhibit distinct differences in isotopic composition between summer and winter, and between different years. We found that the Nueces Marsh food web consists of 3.5 consumer trophic levels. The [delta]¹³C values of consumer organisms were similar across the spatial extent of the low marsh, regardless of proximity to wastewater inflow. However, a majority of the organisms collected from the wastewater channel were significantly (p<0.05) enriched in ¹⁵N compared to their reference counterparts. We propose that ¹⁵N-enriched nitrogen is entering the Nueces Marsh food web through detrital rather than grazing-based pathways, making wastewater effluent an effective tracer of detrital integration into a marsh food web system. Hydrologic data indicate that isotopic shifts between seasons and between years reflected larger scale shifts between drought and wet years. During drought years, decreased production by phytoplankton and emergent plants led consumers to rely more heavily on ¹³C-enriched cyanobacterial carbon. In contrast, wet years encourage phytoplankton and emergent plant production, making cyanobacterial carbon relatively less exploited. While the Nueces Marsh food web is supported by a stable detrital carbon pool, it may still be susceptible to larger scale hydrologic events. / text

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Prepared for U.S. Army Corps of Engineers, Los Angeles District Office, Contract No. DACW-09-80-C-0071

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

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The viscoelastic and creep properties of salt create challenges in the design of salt mines. Salt undergoes steady state creep for a long period of time, and the time of failure is not easily predicted. Developing functions for creep behavior is important in predicting the deformation of salt pillars. Through literature reviews, it was found that there are many relationships to determine the deformation rate of salt specimens through constitutive models. Mine panels have also been modeled to understand the stress and deformational behavior of the pillars. The purpose of this was project was to develop a relationship that determines the convergence rate from knowing the pillar width to pillar height ratio and thickness of the salt strata immediately above and below the mine. The third power law was adopted in the modeling of salt pillars, which is applicable to low stresses of less than 10 MPa that is typical of salt mine conditions. The finite difference software, FLAC3D was used for the simulations of salt pillar models. A square pillar was modeled using four pillar width to pillar height ratios from 1.5 to 4.6. In mining practices, the pillar width to pillar height ratios are designed to be 1.0 to 5.0. Three sets of pillar dimensions were used for each pillar width to pillar height ratio, this was done to determine whether different room and pillar dimensions for each pillar width to pillar height ratio resulted in different convergence rates. Eight salt thicknesses of 0 m to 26 m were modeled for each set of pillar dimensions, which was sufficient to determine the effect of salt thickness on convergence rate. From the modeled results, general trends among the various pillar width to pillar height ratios were observed. The convergence rate increased as the pillar width to pillar height ratio decreased. In addition, an exponential relationship was found between the convergence rate and the pillar width to pillar height ratio. There was a strong correlation between convergence values calculated from the developed function and the modeled values for the power law exponent of three. The developed expression can be used to estimate the convergence rate due to pillar compression and room convergence.

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Microelectrodes have been found to be a valuable tool in a variety of analytical studies. Their advantages over macro-sized electrodes are well known, including their enhanced mass transport properties (due to their ubiquitous hemispherical diffusion) which lead to steady state responses without external convection. They also exhibit high signal-to-noise ratios (greater sensitivities), furthering their analytical application. Microelectrode arrays are analytical devices with multiple electrodes. There are suitable for practical sensing with all the benefits of microelectrodes but with greater currents, leading to greater ease of measurement. To produce a reliable electroanalytical device the microelectrode response must be reproducible, a fundamental property based on the quality control of their production. Square microelectrode and array fabrication techniques have been developed for this purpose. This research discusses the fabrication and development of closely spaced arrays of square microelectrodes. Simulated and measured responses are compared and used to characterize electrode and array responses by cyclic voltammetry, electrical impedance spectroscopy and current-time transients. Measurements on variably spaced arrays allow insight into overlap of hemispherical diffusion from individual electrodes and the subsequent effect including peak current output on the array device. By studying these devices key insights into the mass transport properties of single square microelectrodes and microelectrode arrays were gained. This study also prepares and develops microelectrodes from materials appropriate for use in the extreme environments of molten salts and concentrated nitric acid solutions. These robust electrodes were developed for use in hydro- and pyro-chemical techniques for nuclear fuel reprocessing. These results demonstrate the practical uses for microelectrode systems across a wide range of chemical systems and in extreme conditions.