Theses, Dissertations and Student Research Papers (College of Earth, Ocean and Atmospheric Sciences)

Post-fire vegetation response to snow in the western United States

2013-05-14T17:21:41Z

Post-fire vegetation response to snow in the western United States Blauvelt, Katie The western United States is experiencing significant changes in wildfire and snow regimes as a result of warming temperatures. An amplification of wildfire activity and reduction in snow water equivalent, snow covered area, and earlier spring snowmelt are documented trends that are projected to continue into the future. With an increase in wildfire activity, it is important to understand how a reduction in snow will impact regenerating vegetation in the western United States. The first objective of this study was to assess summer vegetation biomass response to antecedent winter snow on a local scale by determining the physiographic characteristics that influence the relationship between snow and vegetation in the case of the 2002 Biscuit Fire. The second objective was to assess the broad scale regional patterns of regenerating vegetation response to snow, by comparing the correlation between summer vegetation biomass and antecedent winter snow before and after large wildfires across the western United States. Remote sensing data and spatial-temporal statistics were used to analyze the relationship between snow and vegetation. In the local scale analysis, the 2002 Biscuit Fire was analyzed, which burned over 2,000 km² in southwest Oregon and northern California. Nonparametric Multiplicative Regression (NPMR) was used to explore the complex relationships between multiple predictor variables (winter snow frequency, elevation, slope, aspect, and burn severity) and the summer vegetation response variable (enhanced vegetation index, or EVI), before and after the Biscuit Fire burned. The burned area was subset by soil type to determine how soil texture influenced the snow and vegetation relationship. In the regional scale analysis, the Pearson's Correlation Coefficient was calculated to analyze the relationship between winter snow frequency and summer EVI before and after 23 wildfires across the western United States. In the case of the Biscuit Fire, summer EVI responded negatively to snow before the fire, and responded positively to snow after the fire. EVI in coarse-textured skeletal soils exhibited the clearest shift to a positive response to snow after the fire burned, while EVI in fine-textured clay soils did not exhibit this type of shift. The regional analysis proved that wildfire disturbances affect the relationship between snow and vegetation differently across the western United States. Seven fires clustered near the Biscuit Fire in northern California and southwestern Oregon behaved similar to the Biscuit Fire, shifting from a negative pre-fire snow and EVI correlation to a less negative or positive post-fire snow and EVI correlation. The majority of these fires had relatively low average elevations (430 to 1708 m) with greater than 80% forest land cover. Ten fire areas exhibited a significant positive pre and post-fire snow and EVI correlation. The majority of these fires had relatively high average elevations (1612 to 2291 m) and consisted of greater than 50% shrub, scrub, and grass land cover. The local scale analysis suggests that the condition of the vegetation (undisturbed vs. regenerating) and the soil texture in which it grows affects its response to winter snow. The low water holding capacity of coarse-textured soils and the short root-lengths of regenerating vegetation may result in greater dependence on snow as a water resource. Regionally, vegetation type and elevation may affect the vegetation's response to snow; short-rooted shrubs at higher elevations above the transient snow zone may be more dependent on snow as a water resource. These results suggest that the relationship between snow and vegetation is not constant, depending on the condition of the vegetation. Increases in wildfire activity and a reduction of snow in the future may impact successional trajectories in certain regions where vegetation may have historically relied on snowmelt to regenerate. Graduation date: 2013

Data assimilation for prediction of shallow water flows with uncertain bathymetry

2013-05-06T21:13:46Z

Data assimilation for prediction of shallow water flows with uncertain bathymetry Wilson, Gregory W. A new method is introduced for incorporating bathymetric uncertainty into predictions of nearshore and river flows (i.e., unstratified flows primarily forced by pressure and radiation stress gradients). The method involves the use of the ensemble Kalman filter (EnKF) as a parameter estimation scheme, where the parameter to be estimated is the spatial field of bathymetry. That is, bathymetry is treated as a slowly varying uncertain parameter in the model, which can be corrected via the assimilation of other available observations. The reason bathymetry is targeted is, as we show, it is often a limiting factor for accuracy in real-world modeling applications. Results are shown using data from four field experiments. Two experiments involve measurements in the nearshore (surf zone) ocean at Duck, NC. There, we show that bathymetric uncertainty due to rapid bathymetric change (time scale of days), or simply lack of available measurements, can cause significant error in model predictions of waves and currents. We demonstrate the ability of the EnKF to reduce this error by correcting the bathymetry, which we then cross-validate using in-situ measurements. Specifically, the correction is achieved by assimilating in-situ observations of alongshore current and significant wave height, as well as (in a separate experiment) remote-sensing observations of alongshore current, wave celerity, and location of shoreline. Similarly in a river environment (Snohomish River, WA, and Kootenai River, ID), we demonstrate the EnKF using twin tests, assimilating pseudo-observations of currents from a variety of hypothesized platforms (fixed in-situ gages, passive drifters, and Doppler radar). Again, the EnKF is found to yield accurate estimates of bathymetry. Graduation date: 2013

A study on the effects of enhanced pCO2 on open ocean diazotrophic assemblages

2013-05-02T20:27:32Z

A study on the effects of enhanced pCO2 on open ocean diazotrophic assemblages; A study on the effects of enhanced pCO₂ on open ocean diazotrophic assemblages Gradoville, Mary Rose Biological di-nitrogen (N₂) fixation is a key process in open-ocean ecosystems, where the new nitrogen (N) provided by marine diazotrophs can support a large fraction of primary productivity and carbon (C) drawdown. Recent laboratory studies have shown that elevated pCO₂ enhances the rate of N₂ fixation by select laboratory isolates of Trichodesmium and Crocosphaera. While both N₂-fixing cyanobacteria, these groups differ widely in their cell size, maximal growth rates, and diel periodicity of N₂ fixation. It is unclear whether the CO₂ enhancement shown by these species in laboratory settings will hold for diverse diazotrophic communities under natural ocean environments. The aim of this thesis was to investigate how open ocean diazotrophs respond to elevated pCO₂ and whether this response is modulated by environmental conditions. I used laboratory and field approaches to address this problem. Laboratory experiments were designed to test the impact of changing pCO₂ on growth, C and N accumulation by Crocosphaera strain WH8501. Elevating pCO₂ above present-day conditions increased growth rates and net C and N accumulation rates. Monitoring C and N accumulation over a diurnal cycle showed that cultures had a distinct rhythm of C and N accumulation; elevating pCO₂ changed this pattern by extending the period of high N accumulation rates. However, the ecologically relevant question is not how a single organism responds to elevated pCO₂ in a controlled setting, but rather how diverse microbial communities will respond in their natural habitats. Accordingly, field experiments were conducted on three cruises at Station ALOHA in order to study how natural assemblages of Trichodesmium and co-associated organisms respond to elevated pCO₂. In contrast to the laboratory results, we observed no consistent pCO₂ enhancement of C or N₂ fixation rates on these cruises. This response was not affected by amending seawater with P or Fe or by changing the light levels of deckboard incubations. Ultimately, DNA sequences from Trichodesmium colonies provided evidence suggesting that biological diversity of marine diazotrophs in our samples is the underlying reason for the lack of pCO₂ enhancement observed in the field. In other words, pCO₂ may upregulate metabolic processes in certain species but not others, leading to no measurable CO₂ enhancement of N₂ fixation by real-environment, diverse diazotrophic assemblages. Graduation date: 2013

Remineralization of marine particulate organic matter

2013-04-16T23:13:02Z

Remineralization of marine particulate organic matter Burkhardt, Brian Gary Marine microorganisms play a significant role in the cycling of nutrients in the open ocean through production, consumption, and degradation of organic matter (OM). Carbon (C), nitrogen (N), and phosphorus (P) are essential ingredients in every known recipe for life. However, the cycling of each of these elements proceeds at different rates such that the ratio of C:N:P can vary widely between particulate, dissolved, organic, and inorganic pools. To better understand the mechanisms controlling these transformations, this study investigated the bacterial remineralization of photosynthetically-derived organic matter derived from cultures of Trichodesmium IMS101, Thalassiosira weissflogii, Prochlorococcus MED4, and particulate material collected from the surface waters of an upwelling regime. Experiments were conducted at sea for a short duration (<6d) and in the laboratory for longer periods (<150 days). In all treatments, across experiments, we observed rapid and selective P remineralization independent of the type of organic material added. Full solubilization and remineralization of P typically occurred within a week. Conversely, N remineralization was slower, with only 39-45% of particulate N (PN) remineralized in shorter (6d) experiments and 55-75% of PN remineralized in <150d experiments. Nitrification was observed after 70-98 days depending on the remineralizing bacteria (isolated from either the Oregon coastal upwelling regime or the North Pacific Subtropical Gyre (NPSG). Notably, these events did not transform the full complement of ammonium to nitrate. This differential lability between N and P led to rapid changes in the N:P ratio of inorganic pools as organic matter was depolymerized by varying bacterial populations. The variable input of potentially limiting elements could have consequences for primary productivity and particle export. Finally, we observed that in short-term experiments with heterotrophic bacteria collected from the NPSG, the N:P ratio of remineralization (11 ± 2.2) was independent of the N:P of added organic material (5-23). This uniformity of inorganic ratios implies differential lability and N:P composition of residual semi-labile and refractory organic matter. Formation of refractory C and N rich organic matter, often termed the microbial pump, is a significant pathway for the transport and sequestration of elements in the aphotic zone of the ocean interior. The experimental results reported here suggest that differential supply of POM leads to rapid and preferential P remineralization, N:P remineralization independent of the N:P of added substrates, and variable N:P of residual organic matter. These findings help constrain our knowledge of elemental cycling in the marine environment. Graduation date: 2013