Thesis (Ph.D.). / Written for the Dept. of Geography. Title from title page of PDF (viewed 2008/08/04). Includes bibliographical references.

Thesis (Ph. D.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 173-191). Also available on the World Wide Web.

Master of Science / Division of Biology / Walter K. Dodds / Understanding the variability of ecological processes across spatial scales is a central issue in ecology, because increasing scale is often associated with increasing complexity. In streams, measurements of biogeochemical fluxes are important for determining ecosystem health and the downstream delivery of nutrients, but are often collected at scales with benthic areas measured in spatial areas from ~10 cm[superscript]2 to ~100 m[superscript]2 (referred to here as patch and reach, respectively), which are smaller than the scale that management decisions are made. Both biotic and abiotic factors will be important when attempting to predict (i.e. scale) biogeochemical rates, but few studies have simultaneously measured rates and their primary drivers at different spatial scales. In the first chapter, I used a conceptual scaling framework to evaluate the ability to additively scale biogeochemical rates by comparing measurements of ecosystem respiration (ER) and gross primary production (GPP) from patch to reach-scales across multiple sites over a two-year period in a prairie stream. Patch-scale measurements with and without fish (biotic factors) and abiotic factors measured simultaneously with metabolic rates suggest that abiotic conditions are stronger drivers of these rates. Patch-scale rates significantly overestimated reach rates for ER and GPP after corrections for habitat heterogeneity, temperature and light, and a variety of stream substrata compartments. I show the importance of determining abiotic and biotic drivers, which can be determined through observational or experimental measurements, when building models for scaling biogeochemical rates. In the second chapter, I further examined patch-scale abiotic and biotic drivers of multiple biogeochemical rates (ER, GPP, and ammonium uptake) using path analyses and data from chapter 2. Total model-explained variance was highest for ER (65% as R[superscript]2) and lowest for GPP and ammonium uptake (38%). Fish removal directly increased ammonium uptake, while all rates were indirectly affected by fish removal through changes in either FBOM and /or algal biomass. Significant paths of abiotic factors varied with each model. Large-scale processes (i.e. climate change and direct anthropogenic disturbances), and local biotic and abiotic drivers should all be considered when attempting to predict stream biogeochemical fluxes at varying spatial scales.

abstract: This dissertation is presented in two sections. First, I explore two methods of using stable isotope analysis to trace environmental and biogeochemical processes. Second, I present two related studies investigating student understanding of the biogeochemical concepts that underlie part one. Fe and Hg are each biogeochemically important elements in their own way. Fe is a critical nutrient for phytoplankton, while Hg is detrimental to nearly all forms of life. Fe is often a limiting factor in marine phytoplankton growth. The largest source, by mass, of Fe to the open ocean is windblown mineral dust, but other more soluble sources are more bioavailable. To look for evidence of these non-soil dust sources of Fe to the open ocean, I measured the isotopic composition of aerosol samples collected on Bermuda. I found clear evidence in the fine size fraction of a non-soil dust Fe source, which I conclude is most likely from biomass burning. Widespread adoption of compact fluorescent lamps (CFL) has increased their importance as a source of environmental Hg. Isotope analysis would be a useful tool in quantifying this impact if the isotopic composition of Hg from CFL were known. My measurements show that CFL-Hg is isotopically fractionated, in a unique pattern, during normal operation. This fractionation is large and has a distinctive, mass-independent signature, such that CFL Hg can be uniquely identified from other sources. Misconceptions research in geology has been a very active area of research, but student thinking regarding the related field of biogeochemistry has not yet been studied in detail. From interviews with 40 undergraduates, I identified over 150 specific misconceptions. I also designed a multiple-choice survey (concept inventory) to measure understanding of these same biogeochemistry concepts. I present statistical evidence, based on the Rasch model, for the reliability and validity of this instrument. This instrument will allow teachers and researchers to easily quantify learning outcomes in biogeochemistry and will complement existing concept inventories in geology, chemistry, and biology. / Dissertation/Thesis / Ph.D. Geological Sciences 2014

The characteristic glacial and interglacial cycles of the Pleistocene underwent a climatic transition at ~430 ka known as the Mid- Brunhes Event (MBE). Many studies, particularly from the Southern Hemisphere have noted that after this transition, the amplitude of the climatic cycles increased. Despite the indication of an MBE signal in many globally distributed paleoclimate records, the geographic extent of the climatic transition remains unknown and its presence in northern hemisphere and terrestrial records is debated. Lake El’gygytgyn is located in the far- east Russian Arctic and provides the longest, most continuous record of Arctic climate (3.6 Ma). This study examines organic biomarkers in the Lake El’gygytgyn sediment core to determine if the MBE is expressed in the terrestrial Arctic. The paleoclimate reconstruction spans the interval of 340- 730 ka at a resolution of ~3 ka. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are utilized to reconstruct temperature and plant leaf wax n-alkanes are used to examine vegetation changes. Statistical analysis of this, and other existing proxy data, indicates that a signal of the MBE is preserved in the Lake El’gygytgyn sediment record. BrGDGT temperature reconstructions suggest the terrestrial Arctic experienced both the warmest interglacials and coldest glacial periods after the MBE climatic transition. Arid glacial intervals and wetter interglacials are recorded by changes in the average chain length of n- alkanes, with wetter interglacials predominating after the MBE.

Microbial mats have played an important role in the carbon (C) and nutrient cycles since the Archean Eon and modern mats are important contributors to the biogeochemistry of intertidal wetlands. Microbial mats are flat assemblages of microbes that are currently found in many unvegetated habitats globally. Intertidal salt pans are a common habitat for microbial mats, however little is known about the distribution of microbial mats within the intertidal landscape. Understanding the spatial distribution of microbial mats is critical to developing quantitative estimates of the impacts of microbial mats on their ecosystems. We photographically measured the presence and density of microbial mats within 1 m2 quadrats across a landscape scale (~1000 Ha) on the Gulf Coast of Florida. The wide variety of metabolic processes that are found within microbial mats makes the net biogeochemical impacts of the microbial mats highly variable as well. To explore the biogeochemistry associated with microbial mats, we measured a suite of soil attributes under microbial mats and compared those measurements to nearby soils without microbial mats. We found that microbial mats are found on soils with biogeochemical attributes that are significantly different than soils without microbial mats. Soil organic matter, nitrate concentration, and soil temperature significantly increased in soils under microbial mats; pH was significantly lower in soils under microbial mats. Also notable was although the concentration of soil organic matter was higher, the bioavailability of that organic matter was significantly lower. Microbial mat presence is correlated with geomorphic variables such as proximal boundaries, as well as neighboring vegetation and other microbial mats.

Calcium stable isotope ratios are a relatively new tool that biogeochemists can use to investigate the biogeochemical cycle of calcium in terrestrial ecosystems, having seen widespread application only in the past 15 years. To advance the application of calcium isotopes in biogeoscience research, I conducted three investigations focused on interpreting calcium isotope ratios in streamwater and in the cation exchange pool of forest soils. In the first study, we observe a shift toward lower 44Ca/40Ca ratios in streamwater draining a New Hampshire watershed after an experimental clearcutting event. Isotope ratio measurements of ecosystem calcium pools indicate that enhanced leaching of the soil exchangeable pool produced the observed shift in 44Ca/40Ca ratios. A trend towards decreased 44Ca/40Ca ratios in soils in the years following the harvesting indicates that calcium leached from the soil exchangeable reservoir was likely replaced by calcium released by the decay of belowground biomass, maintaining pre-harvest levels of exchangeable calcium even in the face of a significant ecosystem disturbance. In a second study, we observed significant differences in the 44Ca/40Ca of the soil exchange pool between two neighboring tropical watersheds, although 44Ca/40Ca of calcium inputs (bedrock and atmospheric deposition) at the two sites were indistinguishable. Further, both sites had higher 44Ca/40Ca ratios compared with external inputs, a relatively rare observation globally. We propose that hurricane disturbance best explains the high 44Ca/40Ca at each site, and that the difference in 44Ca/40Ca between the two sites can be accounted for by the magnitude of disturbance at each site. Finally, a synthesis of our new data with previously published results shows that globally, soil exchangeable 44Ca/40Ca ratios can be higher, lower or equal to external inputs. Modeling work indicates that in addition to isotopic fractionation, the balance in fluxes between vegetation and soil is critical in determining how soil exchangeable 44Ca/40Ca ratios vary relative to external inputs. When plant uptake and return to the soil are equal, soil and external inputs 44Ca/40Ca are equal, while high soil 44Ca/40Ca ratios develop when uptake exceeds return. Soil develops low 44Ca/40Ca when biomass obtains calcium from sources other than the exchangeable reservoir.

No description available.

<p>Blue carbon systems (mangroves, salt marshes, and seagrass beds) sequester large amounts of carbon via primary productivity and sedimentation. Sequestered carbon can be respired back to the atmosphere, buried for long time periods, or exported (&ldquo;outwelled&rdquo;) to adjacent ecosystems. This study estimates the total outwelling of dissolved organic carbon (DOC) from the Neponset Salt Marsh (Boston, Massachusetts) as well as the major plant and sediment processes contributing to the overall flux. The total export was quantified via high-resolution <i> in situ</i> chromophoric dissolved organic matter (CDOM) measurements as a proxy for DOC using 12 years of transect data. Seasonal trends, alternate sources of fresh water, and long-term trends in DOC export will be discussed. To characterize the percentage of this flux attributable to marsh vegetation, the effects of sunlight, anoxia, plant species, biomass type, and microbes on plant leaching were studied using incubations of above- and belowground biomass over four seasons. Seasonal comparisons led to the &ldquo;Fall Dump&rdquo; hypothesis in which higher DOC concentrations are leached during the fall when marsh plants senesce for winter. In summing seasonal fluxes from vegetation, approximately 46% of the total DOC export from the marsh may be attributed to leaching from the three dominant plant species in the Neponset Salt Marsh. The influence of seasonality and climate change (e.g., drought) on both overland flow and deep sediment pore water leaching were also investigated. Depending on season and marsh condition, overland flow and sediment pore water leaching combined could contribute 8&ndash;16% of the total export from the marsh. Finally, the influence of natural sunlight irradiation and microbes on the release of dissolved organic matter (DOM) from resuspended surface sediments was studied and approximately 11&ndash;22% of the total export could be attributable to this flux. Approximately 49 mol C m^&minus;2 yr^&minus;1 are outwelled from the Neponset Salt Marsh and, using net primary productivity estimates from the literature, 16 &plusmn; 12 mol C m^&minus;2 yr^&minus;1 are buried in the Neponset Salt Marsh. </p>

<p> Regional climate models project that precipitation in the Great Plains of North America will become characterized by more intense rainfall events separated by longer dry periods. Changing seasonal precipitation patterns may differentially favor grassland productivity in ecosystems dominated by either cool or warm season grass species, and thus influence carbon uptake and loss in these systems. Furthermore, model estimates of ecosystem respiration based primarily on soil temperature could overestimate respiration by failing to account for the effects from saturated conditions during heavy precipitation events. This research contrasted water and carbon fluxes during two years with different intra-annual precipitation within a cool season mixed grassland and compared to a neighboring warm season grassland in Rocky Flats National Wildlife Refuge, Colorado, USA. Results from this study showed a significant positive relationship between the accumulated April/May precipitation and growing season carbon uptake in the cool season, smooth brome-dominated grassland. In addition, significant rainfall in the autumn of 2013 played a role in the early spring growth and carbon uptake in 2014. Comparisons between eddy covariance and soil flux-gradient observations and model estimates of soil respiration showed that during the extreme precipitation event in September 2013, processed-based models better characterized fluxes as compared to empirical models based on soil temperature. The study also found that the cool season grassland was a net sink of carbon during the spring and autumn whereas the neighboring warm season tallgrass prairie was a net sink during the summer. In addition, the study found that the grasslands had considerably different sensitivities to water limitations, with grasses in the tallgrass prairie having a higher water use efficiency (WUE). The comparison of the adjacent semiarid grasslands at Rocky Flats NWR improves our understanding of the response to changing precipitation between cool season and warm season dominated grasslands. This research underscores the importance of expanding grassland research to understand how the composition of grasses will influence carbon cycling, especially as precipitation patterns shift with changing climate. Moreover, this research will add to observations during extreme precipitation events, which can improve both empirical and process-based models of soil respiration.</p>