Theses/Dissertations - Ecological, Earth, and Environmental Sciences

Fire disturbance effects on regional carbon cycling in a sub-humid woodland.

2013-05-15T19:53:56Z

Fire disturbance effects on regional carbon cycling in a sub-humid woodland. Yao, Jian, 1984- Fire disturbance affects many ecosystem processes, especially carbon (C) cycling. In addition, fire is routinely used as a management tool in wildland ecosystems. In this study, I measured the fire-affected C storage, vegetation composition, habitat suitability for an endangered bird species (golden-cheeked warble), and charcoal content in juniperoak woodlands with documented past surface fires within Balcones Canyonlands National Wildlife Refuge. Using this information, I modified a process-based model of ecosystem biogeochemical cycling (Biome-BGC) to simulate the effects of different fire types, the production and loss of fire-derived charcoal, and the charcoal effects on soil water availability. From my field measurements, I found significantly different total aboveground biomass C with average values of 5.25, 6.86, and 9.18 kg m-2 for 60 plots with known fire histories that categorized into the recent (<40 year), old (>=40 year), and no fire group, respectively. These data also showed that higher oak recruitment was associated with fires that occurred in years with low summer precipitation. I concluded that fire may have a dual effect on habitat suitability with catastrophic wildfire damaging potential habitat and significantly reducing regional C storage, and moderate intensity fires in dense young juniper stands promoting tree species diversity. From the laboratory analysis of soils derived from these same plots, I estimated that the regional average soil charcoal concentration to be 1.40 g C kg-1 soil (20.8 g C kg-1 SOC) based on the methods of 13C nuclear magnetic resonance and mid-infrared spectroscopy, with fire-affected sites having significantly higher soil charcoal concentrations than the non-fire sites. I calculated the regional annual soil charcoal loss rate to be 4.7%, potentially due to soilerosion on steep slopes. Finally, the modified Biome-BGC model was able to reasonably simulate fire-affected C and charcoal storage changes. The model also indicated that the effects of fire on the ecosystem properties (vegetation C, leaf area index, and net ecosystem exchange) were closely associated with the severity of fire. The fire-derived charcoal did not appear to have significant effects on the simulated ecosystem properties. My results provided detailed ecological information regarding fire-affected processes in these woodland ecosystems.

Influence of pH and total dissolve solids on harmful algal blooms of Prymnesium parvum.

2013-05-15T18:49:45Z

Influence of pH and total dissolve solids on harmful algal blooms of Prymnesium parvum. Prosser, Krista Noel. Harmful blooms of Prymnesium parvum in inland waters continue to expand, in part, due to anthropogenic influences and climate change. This thesis examines influences of pH and total dissolved solids (TDS) on P. parvum blooms, growth, and toxicity to aquatic life. Chapter 1 evaluated pH effects on P. parvum bloom development and toxicity by manipulating in situ limnocorrals during 21 d pre-bloom and bloom development experiments. Though neutral pH levels preempted P. parvum bloom development, higher pH resulted in bloom formation and ambient toxicity. Chapter 2 examined whether TDS with major ionic composition associated with hydraulic fracturing influenced P. parvum growth and toxicity across nutrient conditions. Though no growth was observed at 130 mg/L, higher treatment levels of TDS stimulated P. parvum growth and increased acute toxicity to fish. Such novel observations provide pH and TDS thresholds for potentially supporting management of P. parvum growth and acute toxicity in inland waters.

Using multiple parameters to compare effluent quality of eight wastewater treatment systems.

2013-05-15T18:48:52Z

Using multiple parameters to compare effluent quality of eight wastewater treatment systems. Price, Amy E. Wastewater effluent qualities from two study sites in central Texas were compared using whole effluent toxicity (WET) tests and chemical indicator parameters. Three of the effluents were collected from the City of Whitney Wastewater Treatment Facility in Whitney, TX; the remaining five effluents were collected at the Waco Metropolitan Area Regional Sewerage System (WMARSS) and the Baylor Wastewater Research Program site near Waco, TX. The first hypothesis examined at the City of Whitney Wastewater Treatment Facility was that effluent water quality improves through a pond and wetland treatment system. The second hypothesis examined was that there is no difference of effluent water qualities between two seasons. The first hypothesis examined for the Waco effluents was that effluent qualities of the four on-site systems are comparable to a centralized municipal wastewater treatment effluent: WMARSS. The second hypothesis examined was that there is no difference of effluent water qualities between two seasons.

Advancing an understanding of ecological risk assessment approaches for ionizable contaminants in aquatic systems.

2013-05-14T14:33:58Z

Advancing an understanding of ecological risk assessment approaches for ionizable contaminants in aquatic systems. Valenti, Theodore Walter. Freshwater is increasingly becoming a finite resource in many regions of the world. Gaps between estimated water supply and demand continue to narrow and the prospects of acquiring additional sources of freshwater remain limited. Furthermore, economically efficient water resource management practices are perplexed by increasing urbanization and changing land-use in semi-arid regions. Although repeated use of water is a practical and effective means for easing strain on water supplies, there is concern that unnecessary contamination may diminish future value of this important resource. Some surface waters in semi-arid regions of the U.S. are effluent-dominated as flow is comprised of >90% treated wastewater. Ionizable compounds are chemicals often associated with urban development and examples include pharmaceuticals, agrochemicals, natural toxins, and other common contaminants (e.g. ammonia). Because continued population growth and urbanization are likely to increase contaminant release and alter dilution capacity of receiving systems, it is important that best management approaches are developed at the watershed scale to limit water quality degradation associated with ionizable compounds. Current methods for prospective and retrospective ecological risk assessments of ionizable compounds seldom consider site-specific conditions during the analysis of effects of phase. Ionization state is largely controlled by the acid/base dissociation constant (pKa) and pH of the solution where a compound resides. Stream water quality can therefore influence ionization state, which is important because the unionized forms a more lipophilic and have a greater propensity to cross cellular membranes. Consequently, the unionized forms are hypothetically more toxic. I completed toxicity tests in the laboratory using various contaminants as model ionizable compounds over a gradient of environmentally-relevant surface water pH and then related measured toxicological endpoints to observed pH of surface waters using both discrete and probabilistic ecological risk assessment approaches. The result of my studies clearly demonstrated that site-specific pH may influence the toxicity of ionizable contaminants. Potential modifications to conceptual frameworks of ecological risk assessment for ionizable contaminants are suggested so that uncertainty can be reduced.