Effects of anthropogenic disturbances on the iron and nitrogen cycles in riparian wetland soils

Shrestha, Junu Princeton University 2008 해외박사(DDOD)

Wetlands provide very important ecosystem services, such as removal of excess nutrients from uplands, and governance of fate and transport of various contaminants. However, there has been a steady deterioration of such ecosystem services due to interference from anthropogenic factors, primarily rapid urbanization and intensive agricultural practices. Effects of such individual disturbances on natural wetland biogeochemistry have been widely studied, but the combined influence of the disturbances is yet to be explored. This dissertation examines the effects of the combination of extensive urban development and intensive agricultural practices on wetland biogeochemistry. We analyzed the influence of vegetation removal, representative of extensive urban development, and excess urea application, a significant indicator of intensive agricultural practices, on soil iron and nitrogen cycles in wetlands. Iron (Fe) is an abundant redox species in wetlands which plays a key role in wetland biogeochemistry, and in contaminant removal. Similarly, nitrogen cycle regulates nutrient processing in these ecosystems. For investigation these processes, experiments were conducted at different scales ranging from laboratory analysis of soil collected from Assumpink wildlife management area, a riparian wetland in New Jersey, to in situ experiments. Our studies showed urea-treated soils had elevated concentrations of bioavailable Fe(III), and a higher rate of Fe(III) production. Such results demonstrate that wetland soils with excess of urea runoff could potentially release contaminants bound to Fe(III) complexes due to their increased bioavailability for reduction. In terms of nitrogen species, when wetland soil was individually subjected to vegetation removal or urea application, only inorganic nitrogen pool was affected, whereas when applied together, the treatments affected the organic nitrogen pool as well. Our study conclusively demonstrates that depending on the type and intensity of disturbance, different components of nitrogen cycle are affected. Seasonal flooding is an important characteristic of wetland behavior. However, working of the nitrogen cycle in anaerobic conditions is poorly understood. In order to develop a deeper understanding of nutrient processing in anaerobic conditions, the dissertation analyzes ammonium oxidation in such conditions, and the role of iron in the process. Contrary to commonly accepted notion, nitrite was observed in the anaerobic wetland soil without any initial nitrate concentrations, and ammonium was confirmed to be the source of nitrite in such conditions. Determination of in-situ utilization rates of ammonium and nitrite demonstrated the observed nitrite concentrations to be in a steady state. Absence of manganese oxides and other oxidized nitrogen compounds, led to the hypothesis that iron hydroxides could be the oxidant for ammonium in the soil. Previously, cycle of oxic and anaerobic conditions in wetlands was considered to be a pre-requisite for oxidation of ammonium into nitrite or nitrate and then reduction to dinitrogen. These new results demonstrate the potential for complete removal of excess ammonium even in inundated anaerobic conditions. The dissertation establishes that urbanization and excessive fertilizer use alter the soil nitrogen and iron cycles, thereby changing the entire biogeochemistry of a wetland ecosystem. In addition, the thesis demonstrates the possibility of anaerobic ammonium oxidation in wetland soil. This enhances our knowledge of the nitrogen cycle, and reveals its previously unrealized potential for nitrogen removal in anoxic conditions.

Spatial and temporal variability of soil Carbon dioxide and Nitrous oxide fluxes in tropical forest soils: The influence of tree species, precipitation, and soil texture

van Haren, Joost Lambertus Maria The University of Arizona 2011 해외박사(DDOD)

This thesis presents data collected and analyzed to determine the influence of above ground biological factors on the spatial and temporal variability of tropical soil biogeochemical processes. The tropics are the largest natural source of CO2 and N2O to the atmosphere and many tropical forests are changing due to climate or land-use change and forest fragmentation. To understand how these changes are affecting ecosystem feedbacks to climate change we need to understand the plant-soil interactions in tropical forests and how these scale to the whole forest and region. For this thesis, I completed four studies that investigated the interaction of the tree species and forest growth with soil trace gas fluxes within the Amazon basin. Locally, tree species do influence soil N2O fluxes, with fluxes close to four out of fifteen tree species consistently elevated above the overall mean and consistently low near two species. These results suggest that tree species composition can significantly influence soil biogeochemistry. Experimental sugar additions elevated N2O fluxes, suggesting that N2O cycling is mostly driven by heterotrophic (carbon-limited) denitrifiers, and that carbon transport into the soil by trees may present a mechanism for the observed differences. Alternatively, tree-soil competition for nutrients could explain the tree species related soil N2O flux differences. However, soil nitrate concentrations and low N2O fluxes associated with legumes, suggest that nitrogen is not limiting. This work provides evidence that vegetation species composition can be a controlling factor in overall trace gas emissions in tropical forests, although I found that only rather large (20%) change in species composition would cause an appreciable effect on the landscape-scale forest soil gas flux. In order to better isolate the effects of species on soil processes, I also investigated soil gas fluxes in a tropical plantation, where trees were grown in monoculture plots. As in the natural forest, different tree species were associated with flux differences, though the species N2 O flux rank order was unlike what I found in the forest, indicating that monoculture plantation plots are not generally informative for species influence on soil processes in diverse forests. Fast tree growth rates and overall lower fluxes of CO2 and N2O from plantation vs. natural forest or agricultural soils suggest that conversion of abandoned farmland to plantation may reduce greenhouse gas fluxes to the atmosphere in tropical systems, a net benefit from a climate change policy perspective. Finally, motivated by the finding that tree carbon export may influence N2O gas production in soils at the local scale, I investigated the effect of variation in growth rates of forest stands on regional variations in N2O production. I found that site-to-site and monthly flux variability within the Tapajos National Forest (south of Santarem, Brazil) and across the central-eastern Amazon basin is significantly and positively correlated with forest growth rates. I hypothesize that this relationship is a consequence of: (1) the dominance of denitrification (as opposed to nitrification) in driving N2O emissions in wet, clay-rich tropical soils that are conducive to anaerobic metabolism (especially in wet seasons); (2) the requirement of denitrifiers, as obligatory heterotrophs, for labile carbon, which is limited in tropical soil, and (3) the fact that downward sap flow in trees is the main source of carbon for both stem growth and root exudation. The plausibility of this hypothesis is supported by results from a process-based, numerical model (PnETDNDC), which, when trained to climatic and ecological data of our sites, reproduces the wood growth and soil N2O flux patterns we observed. Extrapolating the regionally observed correlation between stand-level tree growth and N2O fluxes, to the Amazon basin as a whole using wood growth measurements from forest inventory plots, yields a mean soil N2O flux of 2.6 kg-N ha-1 y-1 for the whole Amazon basin, higher than most previously published estimates. This suggests that tropical forests may be a bigger contributor to global N2O budget than previously thought, a consequence of accounting for the importance of processes that link vegetation carbon dynamics to soil biogeochemistry.

Anthropogenic changes to mercury and lead biogeochemistry in forest soils across the northeastern United States

Richardson, Justin B Dartmouth College 2015 해외박사(DDOD)

Understanding mercury (Hg) and lead (Pb) accumulation and retention in forest soils is needed to reduce their negative impacts on human and wildlife health. In addition, mercury and lead can be used to characterize the role of soil in terrestrial biogeochemistry. In this thesis, I assess mercury and lead concentrations and inventories in soil horizons across the northeastern United States to investigate how forest soils have responded to anthropogenic atmospheric deposition and perturbations. Mercury concentrations and pools were greater in Vermont and New Hampshire compared to western Pennsylvania, and corresponded with soil properties (pH, soil organic matter) and site characteristics (mean annual precipitation, mean annual soil temperature), based on stepwise and multiple regressions. Mercury in the mineral soil varied between soil types, with greater concentrations and inventories in Bhs horizons of Spodosols compared to Bw horizons in Inceptisols. Using previous lead data from the study sites and 207Pb/ 206Pb ratios, I conclude that lead was primarily gasoline-derived and is moving from the organic horizons into the mineral soils across the region. In addition to atmospheric deposition, humans are directly impacting forest soils through the introduction of exotic species and land-use change. Nine sites across northern Vermont and New Hampshire were studied to determine if exotic earthworms are affecting forest soil mercury and lead concentrations and inventories. Most of the exotic earthworm species had bioaccumulated potentially harmful concentrations of mercury and lead. Moreover, earthworms were a significant inventory of mercury and lead, with greater mercury pools in earthworm biomass than in the organic horizon. Lastly, I investigated the effect of clear-cutting on mercury and lead in forest soils along disturbance gradients at three study areas in the northeastern United States. Organic and mineral horizon inventories of mercury were lower at stands that had been clear-cut more frequently. However, this effect did not coincide with a difference in soil properties. These studies illustrate that soils in Vermont and New Hampshire have accumulated more mercury and retained lead longer than soils in Western Pennsylvania. Additionally, storage of lead and mercury in forest soils of Vermont and New Hampshire has been negatively impacted by anthropogenic disturbances.

Biogeochemical Cycling in Pristine and Mining-Impacted Upland Fluvial Sediments

Saup, Casey Morrisroe ProQuest Dissertations & Theses The Ohio State Uni 2020 해외박사(DDOD)

Upland catchments play an outsized role in the processing and export of water, sediments, nutrients, and organic matter, thus strongly influencing downstream water quality. In Chapter 1, an overview of biogeochemical cycling within upland fluvial sediments is presented, focusing on key regional biogeochemical cycling patterns and solute export processes. Additionally, anthropogenic influences on this environment, such as historical mining activities and climate change, are briefly reviewed in this chapter.Chapter 2 explores the relationship between spatial hydrologic heterogeneity and microbial community assembly and functional potential within the hyporheic zone. The region of groundwater and river water mixing, known as the hyporheic zone, is a hotspot of microbial activity that influences solute export and cycling in rivers. Hyporheic mixing patterns can vary over small spatial scales, leading to heterogeneity in fluid chemistry and microbial community composition and function. Here, we integrate new mass-spectrometry data, metagenomic insights, and ecological models with previous analyses of microbial community composition and dissolved organic matter (DOM) quality to understand spatial relationships between hyporheic flow and microbial community assembly, metabolism, and DOM processing at high-resolution (100 locations) along a 200 m meander of East River, Colorado (USA). Ecological modeling revealed a strong linkage between community assembly patterns and underlying hydrologic and geochemical drivers, including the impact of physical heterogeneity (riverbed grain size) on microbial community structure. Geochemical profiles associated with upwelling groundwater suggest the influence of underlying geology, specifically Mancos-derived solutes, in driving community assembly. Distinct microbial community profiles and functional potential in zones of upwelling groundwater suggest that groundwater chemistry may have a greater influence on biogeochemical cycling within the river channel as periods of baseflow increase in both frequency and duration under future climate change scenarios.Chapter 3 investigates the seasonal impacts of snowmelt-dominated hydrology on hyporheic microbial community dynamics and associated biogeochemistry. Terrestrial and aquatic elemental cycles are tightly linked in upland fluvial networks. Biotic and abiotic mineral weathering, microbially-mediated degradation of organic matter, and anthropogenic influences all result in the movement of solutes (e.g., carbon, metals, nutrients) through these catchments, with implications for downstream water quality. Within the river channel, the region of hyporheic mixing represents a hotspot of microbial activity, exerting significant control over solute cycling. To investigate how snowmelt-driven seasonal changes in river discharge affect microbial community assembly and carbon biogeochemistry, depth-resolved pore water samples were recovered from multiple locations around a representative meander on the East River near Crested Butte, CO, USA. Vertical temperature sensor arrays were also installed in the streambed to enable seepage flux estimates. Snowmelt-driven high river discharge led to an expanding zone of vertical hyporheic mixing and introduced dissolved oxygen into the streambed that stimulated aerobic microbial respiration. These physicochemical processes contributed to microbial communities undergoing homogenizing selection, in contrast to other ecosystems where lower permeability may limit the extent of mixing. Conversely, lower river discharge conditions led to a greater influence of upwelling groundwater within the streambed and a decrease in microbial respiration rates. Associated with these processes, microbial communities throughout the streambed exhibited increasing dissimilarity between each other, suggesting that the earlier onset of snowmelt and longer periods of base flow may lead to changes in the composition (and associated function) of streambed microbiomes, with consequent implications for the processing and export of solutes from upland catchments.Chapter 4 explores the metal mobilization patterns that occurred as a result of a mine waste spill. The Gold King Mine spill in August 2015 released 11 million liters of metal-rich mine waste to the Animas River watershed, an area that has been previously exposed to historical mining activity spanning more than a century. Although adsorption onto fluvial sediments was responsible for rapid immobilization of a significant fraction of the spill-associated metals, patterns of longer-term mobility are poorly constrained. Metals associated with river sediments collected downstream of the Gold King Mine in August 2015 exhibited distinct presence and abundance patterns linked to location and mineralogy. Simulating riverbed burial and development of anoxic conditions, sediment microcosm experiments amended with Animas River dissolved organic carbon revealed the release of specific metal pools coupled to microbial Fe- and SO42--reduction. Results suggest that future sedimentation and burial of riverbed materials may drive longer-term changes in patterns of metal remobilization linked to anaerobic microbial metabolism, potentially driving decreases in downstream water quality. Such patterns emphasize the need for long-term water monitoring efforts in metal-impacted watersheds.Finally, Chapter 5 summarizes key findings and conclusions as well as outlines remaining questions that may be addressed in future work. The results presented in this study highlight the importance of seasonal and spatial hydrologic heterogeneity in upland watershed biogeochemistry. In collecting this information, I have expanded current knowledge of hyporheic hydrobiogeochemistry in upland fluvial systems. Particularly, I conclude that seasonally-driven dynamic water mixing patterns within the hyporheic zone create unique geochemical profiles that drive differences in microbial community composition, assembly, and functional potential. Our results provide insights into the relationship between hydrology and microbial community and metabolisms within river corridors, information which is critical to understanding water resources, particularly in our rapidly changing climate.

Dreissenid-Mediated Energy and Nutrient Cycling in Profundal Regions of the Laurentian Great Lakes

Huff, Audrey University of Minnesota ProQuest Dissertations & T 2023 해외박사(DDOD)

In the Laurentian Great Lakes, invasive zebra and quagga (dreissenid) mussels have dramatically altered biotic community structure, primary productivity, and biogeochemistry since their introduction in the 1980s. Recently, quagga mussel (Dreissena rostriformis bugensis) populations have been expanding deeper into profundal regions of Lakes Michigan, Huron, and Ontario. These dense offshore populations have substantially altered offshore energy and nutrient cycling, but there are key gaps in our understanding of deep-water quagga mussel physiology and their impacts on pelagic biogeochemistry. Specifically, there is a lack of information on (1) quagga mussel tissue nutrient sequestration and regeneration rates, including variability in tissue stoichiometry (C:N:P molar ratios) and its influence on mussel excretion rates and excretion stoichiometry, (2) quagga mussel impact on offshore sediment geochemistry, including sediment mixing rate, sediment oxygen penetration, and dissolved nutrient dynamics at the sediment-water interface, and (3) quagga mussel population dynamics, including size distribution and growth rates, in deep, offshore lake regions. Presented here are the results of field (chapter 2), experimental (chapter 3), and modelling (chapter 4) studies I conducted to address these knowledge gaps about quagga mussel physiology and ecological impacts.To determine variability of quagga mussel tissue stoichiometry and its impact on mussel excretion (chapter 2), I measured mussel tissue and excretion carbon, nitrogen, and phosphorus content along depth (20 – 130m) and trophic gradients in Lakes Michigan and Huron during spring mixing and summer stratification periods of 2019. I found that mussel tissue C:N:P ratios varied substantially in Lakes Michigan and Huron, suggesting that quagga mussels have flexible internal homeostasis. I also found that tissue C:N:P stoichiometry was a significant driver of mussel excretion rates and excretion stoichiometry. When mussels had lower tissue C:P ratios than available seston, excretion C:nutrient (C:N and C:P) ratios decreased. Next, to investigate the influence of quagga mussels on offshore sediment geochemistry (chapter 3), I conducted a sixweek microcosm experiment. I incubated quagga mussels, Diporeia spp. (previously the dominant Great Lakes’ macroinvertebrate), and oligochaete worms (the second most common benthic macroinvertebrate in the Great Lakes). Species were incubated separately and in combination to determine varying organism impacts on sediment mixing and biogeochemistry as well as potential community interaction effects. To simulate deep, offshore conditions, I used low particulate organic matter (POM) sediment in the microcosms and kept them in the dark and at 4°C. I found that sediment mixing depth and intensity varied significantly among species, but that there were no significant differences in sediment oxygen penetration depth or nutrient dynamics. Additionally, I found no evidence for species interaction effects. Finally, I used a Dynamic Energy Budget (DEB) model to explore quagga mussel physiology and growth rates under variable temperatures and food quantities (chapter 4). First, I simulated quagga mussel growth at annual temperatures and food availability representative of oligotrophic, mesotrophic, and eutrophic conditions in nearshore, mid-depth, and offshore regions of the Great Lakes. I then simulated mussel growth under three climate warming scenarios (+0.5°C, +1°C, and +2°C water temperatures). Corresponding changes in lake stratification regime under warming scenarios included an increase in the duration of summer stratification and a decrease in the duration of winter stratification. I found that quagga mussel growth increased with warmer water temperatures and altered stratification regimes. I also found that relative importance of water temperature and food availability varied over trophic status and mussel age, with mussel sensitivity to food limitation increasing as mussels grew larger over time.The combined results from these three studies indicate that quagga mussel impacts on pelagic energy and nutrient dynamics are mostly due to direct mechanisms – including carbon and nutrient ingestion, sequestration, and regeneration – rather than altered sediment geochemistry. My results provide detailed information on quagga mussel physiology, including variability of internal stoichiometry and growth under a wide range of environmental conditions, which strongly influences mussel nutrient recycling. Together, these results improve the current understanding of quagga mussel biology and will help to inform estimates of quagga mussel impacts on biogeochemical cycling in the Great Lakes and other invaded ecosystems.

Physical Controls on Southern Ocean Biogeochemistry

Prend, Channing J ProQuest Dissertations & Theses University of Cali 2022 해외박사(DDOD)

The Southern Ocean plays an outsized role in the global overturning circulation and climate system by transporting mass, heat, and tracers between basins, as well as between the surface and abyssal oceans. Consequently, the Southern Ocean accounts for a disproportionately large percentage of the total oceanic carbon uptake and helps set global nutrient inventories. Therefore, understanding the coupling between physical and biogeochemical processes in this region is crucial to reducing uncertainty in future climate projections. Historically, studying the Southern Ocean has been limited by the paucity of observational data from this remote environment. However, recent advances in autonomous observing technology have provided unprecedented spatial coverage of subsurface biogeochemical measurements. This thesis uses data from an array of more than 200 autonomous profiling floats—in conjunction with satellite data, numerical models, and theory—to investigate the fundamental question: How do physical processes in the Southern Ocean drive variability of phytoplankton biomass and carbon system parameters? Naturally, the answer to this question will depend on the spatial and temporal scales of interest. Our approach is to consider multiple scales, with the central motivation of better understanding the carbon cycle on climatic timescales.First, we investigate regional patterns of phytoplankton seasonality in the Southern Ocean (Chapter 2). Results show that enhanced mixing at topographic features contributes to spatial variability in bloom magnitude and timing. Looking to smaller scales, we examine the generation of phytoplankton patchiness by turbulent stirring (Chapter 3). We find that parameterizing eddy transport as an enhanced diffusion requires timescale separation between the physical and biological processes, which raises concerns for the representation of subgrid scale primary productivity in coarse resolution climate models. Next, we turn to air-sea carbon fluxes. We show that carbon outgassing occurs preferentially in the Indo-Pacific sector of the Southern Ocean due to regional differences in the mixed-layer entrainment of upwelled carbon-rich deep water (Chapter 4). Finally, we quantify the relative importance of different frequency bands in driving year-to-year variations of Southern Ocean primary productivity. We find that changes in annual mean phytoplankton biomass are driven by intermittent sub-seasonal events associated with storms and eddies, rather than low frequency climate variability (Chapter 5). Together, these chapters use a novel combination of in situ measurements, satellite data, and model output to elucidate physical mechanisms that control Southern Ocean biogeochemistry. Understanding these drivers is necessary to improve climate models and predict the response of the ocean to climate change.

Soil microbial dynamics and biogeochemical cycling in moist tropical forests

Cleveland, Cory Christopher University of Colorado at Boulder 2001 해외박사(DDOD)

The overall objective of my dissertation research was to elucidate some of the links between microbial community structure and function and biogeochemistry. I used natural gradients in phosphorus (P) fertility created by soils of widely varying ages (oxisols and mollisols) to investigate the factors that regulate the structure and function of the soil microbial community, and the possible linkages between microbial community structure and function and biogeochemical cycling in both natural (forest) and managed (cattle pasture) systems in the humid tropics. In Chapter 1, I used the natural P-fertility gradient, combined with direct manipulations of carbon (C) and P supply, to test the effects of P availability on the decomposition of multiple forms of C, including dissolved organic carbon (DOC) and soil organic C. Results from a combination of laboratory and field experiments suggested that C decomposition in old, highly weathered oxisol soils is strongly constrained by P availability, with potentially profound impacts for C and P cycling in this system. In Chapter 2, I explored the seasonal variability in the structure and function of the microbial community. I found strong seasonal patterns in both the magnitude and activity of the soil microbial community, and results suggested that in contrast to many temperate ecosystems, where N appears to regulate microbial processes, P availability may ultimately control microbial dynamics in tropical ecosystems. Finally, in Chapter 3, I again used forest and pasture sites on the natural soil gradient to investigate how conversion of tropical rain forest to cattle pasture affects the size and function of the microbial community, and to explore possible relationships between microbial dynamics and biogeochemistry following conversion. I found that land conversion led to fundamental changes in the magnitude and activity of the soil microbial community. Microbial biomass was consistently higher in forests than in pastures, particularly in the oxisol sites, where it was more than twice the pasture value. Forest sites were also characterized by a microbial community that was more active and responded more rapidly to carbon substrate additions, and was functionally different from pasture sites in important ways. My results provide evidence that changes in biogeochemical cycling following land conversion observed here and elsewhere may be directly related to changes in microbial community structure and function. (Abstract shortened by UMI.).

Microscale nutrient cycling in biological soil crusts of the Colorado Plateau

Johnson, Shannon Lyn Arizona State University 2005 해외박사(DDOD)

Biological soil crusts (BSCs) are cm-thin, cryptic, topsoil organo-sedimentary microbial communities that develop where plant cover is restricted, and attain maximal extent in arid lands. They stabilize soils against erosion, and are thought to play a significant role in local biogeochemistry, particularly regarding N-cycling in desert areas, since they are primary microsites for biological dinitrogen-fixation. Yet, N-limitation is characteristic of BSCs, indicating that net N exports are important in their biogeochemistry. I attempted to ascertain the fate of this unaccounted N, and the mechanism behind it, by studying N-transformations and the bacterial populations responsible in model crusts from the Colorado Plateau. I conducted microchemical, microbiological and geochemical analyses at scales commensurate with their small size. Biomass, oxygen, pH and nitrogenous compounds varied strongly with depth at the mm scale. Microbial populations were also organized vertically. Potential dinitrogen-fixation rates were high, and congruent with previous reports. Ammonium oxidation, though limited by oxygen supply, accounted for a significant proportion of net N-inputs in all crusts tested, and occurred preferentially 1--3mm below the surface. Recoverable populations of ammonium-oxidizing bacteria were vertically stratified, peaking at depths of maximal activity. Molecular fingerprinting using the amoA gene identified a previously unknown Nitrosospira-like organism as the crust's principal nitrifier. This organism was found in crusts from E Oregon to S Arizona. Phylogenetic analyses revealed a marked biogeographical separation among crust Nitrosospiras, which constitutes an unprecedented example of bacterial biogeographic differentiation in the absence of apparent dispersal barriers. Despite theoretically optimal conditions, denitrification rates in BSCs were extremely low and thus were irrelevant for N cycling. Congruently, recovered denitrifier populations were small and lacked vertical stratification. While the reason for this break in the N-cycle remains unclear, the lack of denitrification implies that N losses to the atmosphere are trivial. By contrast, downward diffusional export of dissolved ammonium and nitrate were comparable to N-inputs. My results indicate that BSCs act as net exporters of dissolved N downwards. Given their great horizontal expanse, this speaks for a major role of BSCs in the fertility of arid lands in general, and provides grounds for renewed efforts in their preservation.

Eelgrass and Epiphytes As Ecosystem Engineers in a Eutrophic Environment: Sediment Accumulation and Biogeochemistry

Haviland, Katherine Cornell University ProQuest Dissertations & Theses 2023 해외박사(DDOD)

Seagrasses are marine angiosperms widely regarded as ecosystem engineers by providing habitat for a variety of species and for their ability to alter sediment biogeochemistry through trapping of suspended sediment, release of oxygen from roots and rhizomes, and excretion of organic carbon from roots and rhizomes. Seagrass ecosystems are threatened by eutrophication tied to coastal development, which results in limited light reaching the benthos, as well as the build-up of toxic porewater sulfide in the sediments underlying the meadow, which has resulted in mass mortality events. In this dissertation, I assess how the dominant seagrass in West Falmouth Harbor, on Cape Cod, MA (Zostera marina, also known as eelgrass), alter their environment at varying stages of degradation, from a formerly-vegetated region over a decade post-mortality of seagrass, to a dense, healthy meadow. This site is a well-studied eutrophic estuary that experienced a large and continuing increase of nitrogen (N) loading since the early part of this century. In chapter 2, I assess patterns of seagrass trapping of sediment and carbon across meadow edges, and find enhanced C accumulation just outside of seagrass meadows. Additionally, I find significant impacts of tropical storms on sediment accumulation, with both enhanced deposition and erosion of sediment, depending on storm intensity. In chapter 3, I assess the role of epiphytic organisms on seagrass leaves in altering seagrass rhizosphere biogeochemistry. I report differences in the relationship between sulfide buildup and light depending on epiphyte presence, and isotopic evidence of the role epiphytic Nfixation plays in maintaining seagrass health in high sulfide environments. In chapter 4, I assess long-term alterations in iron (Fe) mineral speciation following seagrass meadow mortality and high-sulfide. I find evidence of legacy effects of eutrophication and sulfide on formerly-vegetated regions, evidenced by high FeS2 even after soluble sulfide concentrations have returned to healthy values. The altered Fe mineral pool presents an impediment to future restoration efforts, as we found in a follow-up mesocosm experiment, as lower levels of reactive Fe in the sediment may lead to spikes in sulfide concentrations and reduced sediment N-fixation.

Significance of microscale interactions in bacterial ecology and carbon biogeochemistry in the ocean

Malfatti, Francesca University of California, San Diego 2009 해외박사(DDOD)

Marine bacteria are important players in regulating the pelagic ecosystem structure and functioning and in biogeochemical cycles of carbon, nitrogen, phosphorus and other bio-elements in the ocean. Although a major role of bacteria in ecosystem function and carbon cycle is well established we still know little about the underlying in situ mechanisms. A fundamental problem concerns the nature and the strength of coupling between bacteria and organic matter. Individual bacteria in seawater must exert their actions on organic matter---mostly polymeric and particulate---at the microscale, e.g. with cell-surface associated hydrolytic enzymes, to create hotspots of growth substrates. Microscale microbial ecology in the pelagic ocean is currently little explored. The general premise of this research is that microscale imaging of in situ interactions, along with ecosystem-level studies of bacteria-mediated carbon cycling, will help constrain hypotheses on the underlying mechanisms and their biogeochemical consequences. Atomic Force Microscopy, epifluorescence microscopy, radiotracers approaches and molecular techniques were used to generate an integrated basis to answer specific questions on nanoscale to microscale ecology of marine bacteria and their implications for ocean-basin-scale-carbon biogeochemistry. Field studies during the summer and winter in the Southern Ocean indicated that the degree of coupling between phytoplankton production and bacteria carbon demand is critically important for ecosystem functioning during the long austral winter when primary production is negligible. We discovered that in the austral winter, in the Drake Passage, a significant amount of dissolved organic carbon produced during the summer supports an active microbial loop with important consequences for ecosystem functioning, carbon cycling and climate. Atomic Force Microscopy combined with Epifluorescence Microscopy enabled us to discover bacteria-bacteria and bacteria-Synechococcus associations---putative symbioses---in the pelagic ocean. This has implications for primary productivity and nutrient cycling. Further, at the nanometer to micrometer scale a substantial fraction of bacteria generally considered free-living were interconnected within organic matrixes forming bacterial networks suggesting potential for concerted metabolisms and biogeochemical activities as well as a role in marine aggregation. Finally, imaging of live pelagic bacteria by Atomic Force Microscopy provided insights on cell volumes based on measured height unbiased by fixation and drying. The new measurements provide a more reliable basis for quantifying bacteria-mediated carbon fluxes and the role of bacteria in pelagic marine ecosystems.