Marine aggregates are an important means of carbon transfers downwards to the deep ocean as well as an important nutritional source for benthic organism communities that are the ultimate recipients of the flux. During these last 10 years, data on size distribution of particulate matter have been collected in different oceanic provinces using an Underwater Video Profiler. The cruise data include simultaneous analyses of particle size distributions as well as additional physical and biological measurements of water properties through the water column. First, size distributions of large aggregates have been compared to simultaneous measurements of particle flux observed in sediment traps. We related sediment trap compositional data to particle size (d) distributions to estimate their vertical fluxes (F) using simple power relationships (F=Ad^b). The spatial resolution of sedimentation processes allowed by the use of in situ particle sizing instruments lead to a more detailed study of the role of physical processes in vertical flux. Second, evolution of the aggregate size distributions with depth was related to overlying primary production and phytoplankton size-distributions on a global scale. A new clustering technique was developed to partition the profiles of aggregate size distributions. Six clusters were isolated. Profiles with a high proportion of large aggregates were found in high-productivity waters while profiles with a high proportion of small aggregates were located in low-productivity waters. The aggregate size and mass flux in the mesopelagic layer were correlated to the nature of primary producers (micro-, nano-, picophytoplankton fractions) and to the amount of integrated chlorophyll a in the euphotic layer using a multiple regression technique on principal components. Finally, a mesoscale area in the North Atlantic Ocean was studied to emphasize the importance of the physical structure of the water column on the horizontal and vertical distribution of particulate matter. The seasonal change in the abundance of aggregates in the upper 1000 m was consistent with changes in the composition and intensity of the particulate flux recorded in sediment traps. In an area dominated by eddies, surface accumulation of aggregates and export down to 1000 m occured at mesoscale distances (<100 km).

Microbes have been suggested to have a strong impact on the transportation of chlorine in soils. There are speculations about environmental factors limiting microbial effect on chlorine movement and retention. For this study, a numerical hydrochemical model was built to describe microbial transformation of chlorine in a laboratory lysimeter experiment. Undisturbed soil cores used to set-up the experiment were collected from a coniferous forest soil in southeast Sweden. The lysimeters were modelled in groups depending on their different water and chloride treatments. Microbial transformation of chlorine was better described under high water residence times and high chloride loads compared to low water residence times and low chloride loads. Microbial activity was also shown to properly account for a sudden shift from net-chlorine retention to net chlorine release in most of the lysimeters. Oxygen proved to be very important in accounting for the short-term shift from chloride retention to release in all the lysimeters. Model outcome revealed that 0.02– 0.10 mg Cl- could be available per day in a coniferous soil depending on season and other soil conditions. This study shows that modeling enable a better understanding of chlorine biogeochemistry. It also confirms the speculated importance of microbial activities on chloride availability in soils.

Woody plant encroachment is a globally-prevalent vegetation change phenomenon that has shifted grass-dominated ecosystems to mixed grass and woody plant matrices over the last century. In the Rio Grande Plains of Texas, the introduction of N-fixing woody legumes has increased above- and belowground primary productivity and changed the litter chemistry of the system, accelerating rates of belowground biogeochemical processes. The purpose of this study was to assess the impact of grassland to woodland transition on i) P concentrations in soil physical fractions that differ in their organic matter turnover rates, ii) P availability within the soil over the course of woody encroachment and across the landscape, and iii) microbial community composition and diversity. Soil samples were collected in remnant grasslands and four woody landscape elements (clusters, groves, drainage woodlands, and playas) along a 135-yr chronosequence of woody plant encroachment. P was fractionated by the Hedley method and P concentrations were determined by alkaline oxidation and lithium fusion coupled with ascorbic acid colorimetry. Bacterial and fungal communities were characterized by molecular methods. Whole soil P concentrations were 2-5X greater in woody landscape elements than in grasslands, and nutrient concentrations increased linearly with time following woody plant invasion in all but the slowest-cycling physical fractions. Plant-available P and organic P increased dramatically with time following encroachment. Changes in P availability were more pronounced in drainages and playas than in upland clusters and groves. Analysis of the bacterial and fungal communities demonstrated that microbial communities in grasslands differ at both phylum and genus level from the flora of the wooded landscape elements. This study demonstrates that woody encroachment strongly influences the distribution and availability of soil P and indicates that nutrient cycles in the soil are closely linked and similarly affected by increased woody plant abundance. Microbial communities under woody species differ in composition from those of the grasslands, and are likely contributing to the observed changes in nutrient availability. Since N and P are generally the most limiting nutrients in terrestrial ecosystems, increased stores of P are likely to alter rates of microbial processes, plant-microbe and plant-plant interactions, and successional dynamics in this ecosystem and similar landscapes around the world.

Selenium (Se) is a micronutrient, yet elevated Se can be toxic to aquatic organisms. The range of Se concentrations within which Se uptake goes from insufficient to toxic is very narrow. It is thus important to understand the Se biogeochemical cycle in aquatic systems. In this thesis, the study focuses on changes in Se speciation during uptake by green algae. An optimized method was adopted to quantify and speciate Se in water using flow-injection atomic fluorescence spectroscopy coupled with high-pressure liquid chromatography. Details on the method are given here. For the uptake experiments, the uptakes of four Se species (selenite (Se-IV), selenate (Se-VI), selenocystine (Se-Cys) and selenomethionine (Se-Met)) by the green algae Chlamydomonas reinhardtii were compared. This thesis reports that the algae take up higher amounts of organic Se than inorganic Se. Selenomethionine (Se-Met) had the most rapid uptake, during which Se-Cys was produced. For all experiments, Se-IV was produced and found to sorb onto the algae cells, revealing that Se-IV is an important intermediate compound. Mass balance calculations revealed that more than 90% of Se was lost during uptake, probably to the atmosphere. This study also investigated the release of Se during algae decay to simulate the fate of Se during early-diagenesis. Selenium-rich algae cells were mixed with estuarine sediments at the sediment–water interface in a series of column incubations experiments. During the 7-week incubations, Se speciation was measured at the water–sediment interface and in pore water samples. We found that all the Se released to the pore water was in the form of Se-Cys. Although preliminary, these results highlight the key role of organic-Se species in the biogeochemical cycle of Se in the aquatic environment.

Phosphorus (P) is a limiting nutrient regulating productivity in both freshwater and marine ecosystems. A full knowledge of the sources and pathways of the P cycle is essential for understanding aquatic ecosystem function and for managing eutrophication. However, two significant pathways are poorly understood or remain uncharacterized. First, aquatic metazoans represent a significant internal regenerative pathway of P through the mineralization, translocation (i.e., benthic pelagic coupling) and excretion of nutrients. Rates of P excreted are expected to vary across taxa (i.e., zooplankton vs. mussels vs. benthic macroinvertebrates vs. fish), yet the significance of any one group of taxa in supplying P to bacteria and algae is unknown. Therefore, I developed the first comprehensive set of empirical models of nutrient release for aquatic metazoans (zooplankton, mussels, other benthic macroinvertebrates, and detritivorous and non-detritivorous fish) and compared inter-taxonomic differences in P excretion. I demonstrated that detritivorous fish excrete P at rates greater than all other taxa (as a function of individual organism mass); whereas, mussels generally excreted P at rates less than other taxa. Significant differences in the rate of P excretion between zooplankton and non-detritivorous fish were not observed [i.e., the allometry of P excretion was similar between zooplankton and non-detritivorous fish (as a function of individual body mass)]. I subsequently applied the models to assemblage biomass and abundance data to examine and compare the relative contribution of each taxa to the internal supply of P, and to examine the turnover time of P bound in metazoan biomass. I clearly demonstrated a hierarchy in the contribution by different metazoan assemblages to P cycling (zooplankton > benthic macroinvertebrates > mussels > fish) and clarified the significance of different metazoan taxa in P cycling. Moreover, I demonstrated that the slow turnover time of P bound in fish biomass (relative to other metazoans) indicates that fish are important as sinks rather than sources of P. A second potentially significant P pathway is through the influence of ultraviolet radiation (UVR) on P cycling. UVR may alter P cycling abiotically through changes in P availability and biotically through changes in the acquisition and regeneration of dissolved P by plankton. However, the significance of P released from the photodecomposition of dissolved organic P compounds (DOP), and the effect of UVR on the uptake and regeneration of dissolved P, the turnover of particulate P, and on ambient phosphate (PO43-) concentration has not been investigated and remains unknown. Therefore, my initial experiments applied the novel use of radiophosphate uptake assays to quantify the significance of the photodecomposition of DOP to PO43-. I concluded that the liberation of PO43- through the photodecomposition of DOP is not a significant pathway. However, the photochemical liberation of PO43- from suspended sediments was evident and should be an important pathway supplying PO43- to plankton in shallow polymictic lakes. This represents the first study to identify this P pathway in lakes. The turnover time of the PO43- pool increased under UVR irradiance (i.e., uptake of P by plankton decreased), while the regeneration rate of dissolved P and turnover rate of planktonic P were generally not affected. The net effect of UVR was an increase in steady state PO43- concentration (ssPO43-). Alkaline phosphatase activity (APA) in the dissolved and particulate fractions was significantly reduced in UVR treatments, but unrelated to changes in P uptake as proposed in the literature. This is the first study to comprehensively investigate the biotic effects of UVR on P cycling and represents a major advancement in the field of photobiology. In summary, I have characterized several poorly understood pathways in the P cycle of lakes. With the models I have developed, aquatic metazoans can now be integrated into the P cycle of lakes, for example, with other internal and external sources of P (e.g., from inlets, lake sediments and the atmosphere). This will advance our knowledge of P cycling, and will provide researchers with a better understanding of the nutrient pathways supporting primary production.

Phosphorus (P) is a limiting nutrient regulating productivity in both freshwater and marine ecosystems. A full knowledge of the sources and pathways of the P cycle is essential for understanding aquatic ecosystem function and for managing eutrophication. However, two significant pathways are poorly understood or remain uncharacterized. First, aquatic metazoans represent a significant internal regenerative pathway of P through the mineralization, translocation (i.e., benthic pelagic coupling) and excretion of nutrients. Rates of P excreted are expected to vary across taxa (i.e., zooplankton vs. mussels vs. benthic macroinvertebrates vs. fish), yet the significance of any one group of taxa in supplying P to bacteria and algae is unknown. Therefore, I developed the first comprehensive set of empirical models of nutrient release for aquatic metazoans (zooplankton, mussels, other benthic macroinvertebrates, and detritivorous and non-detritivorous fish) and compared inter-taxonomic differences in P excretion. I demonstrated that detritivorous fish excrete P at rates greater than all other taxa (as a function of individual organism mass); whereas, mussels generally excreted P at rates less than other taxa. Significant differences in the rate of P excretion between zooplankton and non-detritivorous fish were not observed [i.e., the allometry of P excretion was similar between zooplankton and non-detritivorous fish (as a function of individual body mass)]. I subsequently applied the models to assemblage biomass and abundance data to examine and compare the relative contribution of each taxa to the internal supply of P, and to examine the turnover time of P bound in metazoan biomass. I clearly demonstrated a hierarchy in the contribution by different metazoan assemblages to P cycling (zooplankton > benthic macroinvertebrates > mussels > fish) and clarified the significance of different metazoan taxa in P cycling. Moreover, I demonstrated that the slow turnover time of P bound in fish biomass (relative to other metazoans) indicates that fish are important as sinks rather than sources of P. A second potentially significant P pathway is through the influence of ultraviolet radiation (UVR) on P cycling. UVR may alter P cycling abiotically through changes in P availability and biotically through changes in the acquisition and regeneration of dissolved P by plankton. However, the significance of P released from the photodecomposition of dissolved organic P compounds (DOP), and the effect of UVR on the uptake and regeneration of dissolved P, the turnover of particulate P, and on ambient phosphate (PO43-) concentration has not been investigated and remains unknown. Therefore, my initial experiments applied the novel use of radiophosphate uptake assays to quantify the significance of the photodecomposition of DOP to PO43-. I concluded that the liberation of PO43- through the photodecomposition of DOP is not a significant pathway. However, the photochemical liberation of PO43- from suspended sediments was evident and should be an important pathway supplying PO43- to plankton in shallow polymictic lakes. This represents the first study to identify this P pathway in lakes. The turnover time of the PO43- pool increased under UVR irradiance (i.e., uptake of P by plankton decreased), while the regeneration rate of dissolved P and turnover rate of planktonic P were generally not affected. The net effect of UVR was an increase in steady state PO43- concentration (ssPO43-). Alkaline phosphatase activity (APA) in the dissolved and particulate fractions was significantly reduced in UVR treatments, but unrelated to changes in P uptake as proposed in the literature. This is the first study to comprehensively investigate the biotic effects of UVR on P cycling and represents a major advancement in the field of photobiology. In summary, I have characterized several poorly understood pathways in the P cycle of lakes. With the models I have developed, aquatic metazoans can now be integrated into the P cycle of lakes, for example, with other internal and external sources of P (e.g., from inlets, lake sediments and the atmosphere). This will advance our knowledge of P cycling, and will provide researchers with a better understanding of the nutrient pathways supporting primary production.

<p>Although microbes may strongly influence wetland phosphorus (P) cycling, specific microbial communities and P metabolic processes have not been characterized in wetlands, and microbial P cycling is poorly understood across global ecosystems, especially in soils. The goal of this work is to test the effects of stress and growth factors on microbial communities in wetlands, and on microbial P metabolism and P cycling at ecosystem scales in wetland soils and beyond. I conducted field and laboratory research experiments in wetland soils, which by definition lie along gradients between terrestrial and aquatic ecosystems, and I explicitly compared results in wetlands to adjacent ecosystems to improve inference and impact. </p><p> To test relationships between microbial communities, soil stress and resource supply, I compared the distribution and abundance of uncultured bacterial communities to environmental factors across a range of wetland soils including a well-characterized P enrichment gradient, and restoration sequences on organic soils across freshwater wetland types. The strongest predictor of bacterial community composition and diversity was soil pH, which also corresponded with the abundance of some bacterial taxa. Land use and restoration were also strong predictors of bacterial communities, diversity, and the relative abundance of some taxonomic groups. Results from wetland soils in this study were similar to both terrestrial and aquatic ecosystems in the relationship of pH to microbial communities. However, patterns of biogeography I observed in wetlands differed from aquatic systems in their poor relationships to nutrient availability, and from terrestrial ecosystems in the response of microbial diversity to ecosystem restoration.</p><p> Accumulation of inorganic polyphosphate (PolyP) is a critical factor in the survival of multiple environmental stresses by bacteria and fungi. This physiological mechanism is best characterized in pure cultures, wastewater, sediments, and I used 31P-NMR experiments to test whether similar processes influence microbial P cycling in wetland soils. I surveyed PolyP accumulation in soils from different wetland types, and observed PolyP dynamics with flooding and seasonal change in field soils and laboratory microcosms. I found PolyP accumulation only in isolated pocosin peatlands, similar to patterns in the published literature. I observed rapid degradation of PolyP with flooding and anerobic conditions in soils and microcosms, and I characterized the biological and intracellular origin of PolyP with soil cell lysis treatments and bacterial cultures. While degradation of PolyP with flooding and anaerobic conditions appeared consistent with processes in aquatic sediments, some seasonal patterns were inconsistent, and experimental shifts in aerobic and anaerobic conditions did not result in PolyP accumulation in soil slurry microcosms. Similar to patterns in wetlands, I found prior observations of PolyP accumulation in published 31P-NMR studies of terrestrial habitats were limited to acid organic soils, where PolyP accumulation is thought to be fungal in origin. Fungal accumulation of PolyP may be useful as an alternative model for PolyP accumulation in wetlands, although I did not test for fungal activity or PolyP metabolism. </p><p> To evaluate relationships between microbial P metabolism and growth, I compared concentrations of P in soil microbial biomass with the soil metabolic quotient (qCO2) by compiling a large-scale dataset of the carbon (C), nitrogen (N) and P contents of soils and microbial biomass, along with C mineralization rates across global wetland and terrestrial ecosystems (358 observations). The ratios of these elements (stoichiometry) in biomass may reflect nutrient limitation (ecological stoichiometry), or be related to growth rates (Biological Stoichiometry). My results suggest that the growth of microbial biomass pools may be limited by N availability, while microbial metabolism was highly correlated to P availability, which suggests P limitation of microbial metabolism. This pattern may reflect cellular processes described by Biological Stoichiometry, although microbial stoichiometry was only indirectly related to respiration or metabolic rates. I found differences in the N:P ratios of soil microbial biomass among ecosystems and habitats, although high variation within habitats may be related to available inorganic P, season, metabolic states, or P and C rich energy storage compounds. Variation in microbial respiration and metabolic rates with soil pH suggests important influences of microbial communities and their responses to stress on metabolism and P cycling.</p><p> My dissertation research represents early contributions to the understanding of microbial communities and specific processes of microbial P metabolism in wetlands, including PolyP accumulation and Biological Stoichiometry, which underpin microbial cycling of P and C. Together, my research findings broadly indicate differences in microbial P metabolism among habitats in wetlands and other ecosystems, which suggests the prevailing paradigm of uniform P cycling by microbes will be inadequate to characterize the role of microbes in wetland P cycling and retention. While I observed some concomitant shifts in microbial communities, PolyP accumulation, and microbial stoichiometry with soil pH, land use, and habitat factors, relationships between specific microbial groups and their P metabolism is beyond the scope of this work, but represents an exciting frontier for future research studies.</p> / Dissertation

<p>Human activities are increasingly altering the ways in which energy and elements cycle within and move between ecosystems. Through fossil fuel combustion and the use of synthetic fertilizers we continue to expose much of the biosphere to new rates and ratios of essential element supply. We are also shifting climate patterns on local, regional and global scales in ways that affect reaction rates and residence times of elements within ecosystems. Even the simplest ecosystems are usually too complex to predict many of the potential consequences that human activities will have on their sustained functioning. Because of this, we often monitor ecosystems as integrated wholes, looking to explain processes that account for important patterns observed across space and time. This dissertation consists of 3 data chapters, all of which use the small watershed ecosystem as the principal unit of study for understanding how human activities have altered element cycling in temperate forests in the southern Appalachian Mountains. </p><p>In Chapter 2, we present results from repeated synoptic surveys of streamwater chemistry for ~30 watersheds spanning one of the largest nitrogen (N) deposition gradients in North America, located within the Great Smoky Mountains National Park. We primarily focus on patterns in dissolved organic matter (DOM) concentrations and composition across the N gradient, with particular attention given to dissolved organic nitrogen (DON). DON dominates the global flux of N between terrestrial and aquatic systems, yet we have little understanding of how this prevailing N form responds to human N pollution. We found that DON concentrations often declined significantly with increasing catchment N loading and, through laboratory bioavailability assays, found that when N limitation is alleviated increased microbial demand for labile carbon (C) may drive this pattern. We use these findings to suggest a new hypothesis for the potential responses of DON to anthropogenic N pollution that accounts for the dual role that DON plays in both C and N cycles. </p><p>Chapter 3 is an extension of Chapter 2, in which we attempt to assess the role of DON as either a C or N source within an entire stream reach through a series of independent manipulations of labile C and inorganic N availabilities. In the second order reach of Walker Branch, a well-studied stream in eastern Tennessee, we performed a series of progressive (i.e., sequentially increasing concentrations), kinetic (i.e., very short duration), enrichments of acetate and nitrate on two successive days during April of 2009 before the tree canopy emerged and when in-stream algal production was high. In this system and on these short timescales, we were unable to elicit the same responses observed at sites across the chronic N deposition gradient in Chapter 2. We did, however, observe that DOM processing and composition was significantly altered. Using fluorescence characterization of DOM, we found that adding acetate displaced heterotrophic demand for terrestrially derived DOM. Conversely, nitrate additions stimulated production of highly bioavailable autochthonous DOM within the stream channel, which resulted in an indirect displacement of demand for terrestrially derived DOM. Understanding DOM dynamics in streams has long been a priority for stream ecologists because it represents an important energy and nutrient source fueling stream metabolism. Our results provide new insight into the processes controlling DOM concentrations and composition in Walker Branch, as well as demonstrate the potential of this method for future investigations of DOM in stream ecosystems. </p><p>Chapter 4 deviates from the preceding chapters' focus on N availability and ecosystem DOM dynamics, instead assessing the role of climate change on long-term streamwater concentrations and fluxes from the West Fork of the Walker Branch watershed. At this site, mean annual temperatures have increased by ~2&#730;C, while mean annual precipitation and runoff have declined by ~20% and >40%, respectively, since 1989. We use weekly streamwater samples to assess trends in concentrations and fluxes for 9 different solutes over this period and, using wet deposition data, also evaluate changes in approximate watershed input-output budgets. The observed change in runoff was accompanied by a change in the proportional contributions of different soil flowpaths to streamflow generation through time, with deep groundwater playing an increasingly important role in recent years. Solutes that increase in concentration deeper in the soil profile exhibited significant increases in streamwater concentrations through time, while solutes with higher concentrations in soil solution in the upper profile decreased in concentration. Nutrient solutes, which exhibit much less variation across soil flowpaths, typically display large seasonal patterns in streamwater concentrations that are driven by in-stream biological uptake. However, most nutrient solutes exhibited little or no trend in concentrations through time, indicating that the biological controls on these solutes have remained relatively unaltered by the observed changes in climate over the 20-year period. On shorter timescales, changes in the frequency or severity of multi-year droughts, as well as changes in the frequency or intensity of storms that disrupt in-stream uptake, can have large impacts on watershed input-output budgets of nutrient solutes even if the effects do not manifest as linear trends through time. Our results demonstrate the important role that changing climates can have on watershed element cycles, illustrating that climate effects can manifest through either changes in hydrologic regime or through changing biogeochemical process rates. </p><p>Taken together, these chapters illustrate that human activities are indirectly but substantially changing biogeochemical cycles in temperate forests throughout the Southern Appalachians. Ecosystem structure and function depends on the ways in which energy and elements move within and between ecosystems. We rely on the sustained integrity of ecosystems for their many services and, because of this, it is essential that we understand ecosystem responses to current and future human impacts.</p> / Dissertation

<p>Human welfare depends heavily on ecosystem services like water purification and nutrient cycling. Many of these ecosystem services, in turn, rely on reactions performed by microbes and yet remarkably little is known about how anthropogenic impacts are affecting the structure and function of microbial communities. To help address this knowledge gap, this dissertation uses field surveys and laboratory experiments to examine how watershed urbanization affects microbial communities in receiving streams. We focus on a specific functional group and its associated function - the denitrifying bacteria and denitrification. Denitrifying bacteria use reactive nitrogen and organic carbon as substrates to perform denitrification. Denitrification is one of the few ways to permanently remove reactive nitrogen from ecosystems. Since excess reactive nitrogen in water contributes to serious water quality and human health problems like toxic algal blooms and bowel cancer, denitrification in streams can be considered a valuable ecosystem service. Watershed urbanization, however, may alter the structure of denitrifying bacteria communities in ways that constrain their capacity to remove reactive nitrogen from streams. </p><p>Watershed urbanization leads to drastic changes in receiving streams, with urban streams receiving a high frequency of scouring flows, together with increased nutrient (nitrogen and carbon), contaminant (e.g., heavy metals), and thermal pollution. These changes are known to cause significant losses of sensitive insect and fish species from urban streams. Microbes like denitrifying bacteria may be similarly affected. In the first part of this dissertation, we describe results from four repeated surveys of eight central North Carolina streams affected to varying degrees by watershed urbanization. For each stream and sampling date, we characterized both overall and denitrifying bacterial communities and measured denitrification potentials. Differences in overall and denitrifying bacteria community composition were strongly associated with the urbanization gradient. Denitrification potentials, which varied widely, were not significantly associated with substrate supply. By incorporating information on the community composition of denitrifying bacteria together with substrate supply in a linear mixed-effects model, we explained 45% of the variation in denitrification potential (p < 0.001). Results suggest that 1) watershed urbanization can lead to significant changes in the composition of bacterial communities in streams and 2) such changes may have important functional consequences.</p><p>The second part of this dissertation examines how urbanization-driven changes to the structure of denitrifying bacteria communities might affect the way they respond to stress or disturbance. Some communities can resist changes to functionality in response to disturbance, potentially as a result of previous exposure and subsequent adaptation (legacy hypothesis) or high diversity (insurance hypothesis). We compare the resistance of two structurally distinct denitrifying bacteria communities to experimental disturbances in laboratory microcosms. Communities originated from either a polluted, warm urban streams or a relatively pristine, cool forest stream. In this case, the two communities had comparable compositions, but forest communities were more diverse than their urban counterparts. Urban communities experienced significant reductions in denitrification rates in response to the most severe increased pollution and temperature treatments, while forest communities were unaffected by those same treatments. These findings support the insurance, but not the legacy hypothesis and suggest that the functioning of urban streams may be more susceptible to further environmental degradation than forest streams not heavily impacted by human activities. </p><p>In the third part of this dissertation, we discuss results from a one-time survey of denitrifying bacteria communities and denitrification potentials in 49 central North Carolina streams affected to varying degrees by watershed urbanization. We use multivariate statistics and structural equation modeling to address two key questions: 1) How do different urban impacts affect the structure of denitrifying bacteria communities and 2) How do abiotic (e.g., temperature) versus biotic (denitrifying bacteria community structure) factors affect denitrification potentials in urban streams? Denitrifying bacteria community structure was strongly affected by the urban impacts measured. Community composition responded to increased temperatures, substrate supply, and contamination, while diversity responded negatively to increased temperatures and hydrologic disturbance. Moreover, increased temperatures and substrate supply had significant positive effects, while urbanization-driven changes to denitrifying bacteria community structure had significant negative effects on denitrification potential. The structural equation model captured 63% of the variation in denitrification potential among sites and highlighted the important role that microbial community structure can play in regulating ecosystem functioning. These findings provide a novel explanation for recent observations of decreasing denitrification efficiency with increasing urbanization. Ultimately, we hope findings from this dissertation will help inform more effective stream management and restoration plans and motivate ecologists to consider including microbial community structure in ecosystem models of microbe-mediated processes.</p> / Dissertation

Our ability to understand and predict the fate and transport of contaminants in natural systems is vital if we are to be successful in protecting our water resources. One important aspect of understanding chemical fate and transport in natural systems is identifying key kinetic controls on important redox reactions such as sulfate reduction. Anaerobic microbial activities like sulfate reduction are of particular interest because of the important role they play in the degradation of contaminants in the subsurface. However, current rate estimates for sulfate reduction have a wide range in the literature making it difficult to determine representative rates for a given system. These differences in rate data may be explained by varying kinetic controls on reactions. Push-pull tests were used to evaluate sulfate reduction rates at the wetland-aquifer interface. Anaerobic aquifer water containing abundant sulfate was injected into sulfate-depleted wetland porewater. The injected water was subsequently withdrawn and analyzed for geochemical indicators of sulfate reduction. Complexities in rate data, such as presence of a lag phase, changing rate order and spatial variability, were observed and are hypothesized to be linked to activities of the native microbial population. Subsequent experiments explored the response of native microorganisms to geochemical perturbations using a novel approach to measure directly the effects of a geochemical perturbation on an in situ microbial population and measure rates of resulting reactions. In situ experiments involved colonization of a substrate by microorganisms native to the wetland sediments followed by introductions of native water amended with sulfate and tracer. Experimental results showed that higher sulfate concentrations and warmer seasonal temperatures result in faster sulfate reduction rates and corresponding increases in sulfate reducing bacteria. Findings from this research provide quantitative evidence of how geochemical and microbiological processes are linked in a system not at equilibrium.