Dissertations / Theses on the topic 'Environmental biogeochemistry'

Selenium (Se) is a trace element of great ecological importance whose environmental distribution is highly impacted by anthropogenic activity. In the 1980s, selenium was recognized as a major aquatic contaminant following widespread deformities and mortality among waterfowl hatchlings near the agricultural drainage evaporation ponds of the Kesterson Reservoir (CA, USA). Today, 400,000 km² in the Western United States are threatened by agricultural selenium contamination, as are parts of Canada, Egypt, Israel, and Mexico. From the soil aggregate to the watershed, from the soils of the Central Valley to the sediments of the Salton Sea, and from Environmental Science to Policy and Management, in this dissertation I explore agricultural selenium contamination across scales, ecosystems, and disciplines. I begin with a review of the science, policy, and management of irrigation-induced selenium contamination in California, the heart of worldwide research on the issue. I then delve into the physical and biogeochemical mechanisms that control selenium reduction and mobility within the structured surface soils that are the source of contamination, using an aggregate-scale combined experimental and reactive transport modeling approach. Finally, I present a diagenetic model for selenium incorporation into the sediment of the Salton Sea, which has been receiving seleniferous agricultural drainage over the last 100 years.

To extract lessons from the last 30 years of seleniferous drainage management and water quality regulation in California, I reviewed the history and current developments in science, policy, and management of irrigation-induced selenium contamination in California. Specifically, I evaluated improvements in the design of local attenuation methods and the development of programs for selenium load reductions at the regional scale. On the policy side, I assessed the site-specific water quality criteria under development for the San Francisco Bay-Delta in the context of previous regulation. This approach may be a landmark for future legislation on selenium in natural water bodies and I discussed challenges and opportunities in expanding it to other locations such as the Salton Sea. By combining proven management tools with the novel, site-specific policy approach, it may be possible to avoid future events of irrigation-induced selenium contamination. However, the majority of regional selenium load reductions in California were achieved by decreasing drainage volume rather than selenium concentrations. Thus, there appear to be opportunities for additional improvements through management practices that enhance selenium retention in source soils.

To quantify the likely implications of these experimental results for soils with different degrees of aggregation, I formulated a general mechanistic framework for aggregate scale heterogeneity in selenium reduction. Specifically, I constructed a dynamic 2D model of selenium fate in single idealized aggregates, in which reactions were implemented with double-Monod rate equations coupled to the transport of pyruvate, O₂, and Se-species (selenate, selenite, and elemental selenium). The spatial and temporal dynamics of the model were validated with the experimental data and predictive simulations were performed covering aggregate sizes between 1 and 2.5 cm diameters. Simulations predict that selenium retention scales with aggregate size. Depending on aeration conditions and the input concentrations of selenate and pyruvate, selenium retention was predicted to be 4-23 times higher in 2.5-cm-aggregates compared to 1-cm-aggregates. Under oxic conditions, aggregate size and pyruvate-concentrations were found to have a positive synergistic effect on selenium retention. Promoting soil aggregation on seleniferous agricultural soils may thus help decrease the impacts of selenium contaminated drainage on downstream aquatic ecosystems receiving it.

This work presents agricultural selenium contamination as a complex problem that crosses ecosystems, scales, and disciplines. From a management perspective, the tension between dispersed non-point sources and hotspots where elevated selenium concentrations and sensitive aquatic ecosystems converge is difficult to address. Differences in biogeochemical conditions and trophic transfer within food webs render traditional regulatory approaches ineffective and force regulators to engage with the science of site-specific selenium transfer between ecological compartments. At the same time, gaps still exist in our mechanistic understanding of selenium's environmental cycling and in our integration of scientific knowledge across different ecosystems and scales. Centimeter scale heterogeneity in the biogeochemical conditions within source soils may fundamentally control selenium emissions across large agricultural areas and thus determine the selenium loading of rivers, lakes, and estuaries. Within aquatic environments receiving seleniferous drainage, the first few centimeters of surface sediment may control selenium exposure for entire food webs. Improved understanding at this level holds the potential to simultaneously reduce selenium emissions and respond more effectively to pollution where it occurs. In order to preserve sensitive habitat while also meeting agricultural drainage needs in seleniferous regions we must bridge the gaps between ecosystems, scales, and disciplines.

(Abstract shortened by UMI.)

Lipids preserved in soils and sediments are important proxies in paleoclimate research. However, various growth conditions that affect the organisms synthesizing the lipids can in turn affect the abundance and stable isotopic compositions of the lipids themselves, and, consequently, can introduce significant errors in the paleoclimatic inferences drawn from them. This work examines how two climate proxies based on lipids, namely, glycerol dialkyl glycerol tetraether (GDGT)-based paleotemperature proxies in soils, and paleohydrological proxies based on hydrogen isotopic composition of lipids, respond to variability in environmental and other growth conditions (e.g., carbon source).

In order to evaluate the role of annual precipitation amount on the distribution of soil GDGTs and on GDGT-based temperature proxies in soils, we studied GDGT distribution in soils collected from two environmental transects in the USA—a dry, western transect covering six western states and a wet, east coast transect from Maine to Georgia. Our results indicate a significant impact of precipitation amount on soil GDGT distribution, which is related to soil aeration that in turn depends on precipitation amount, and also to soil pH. Our results also indicate that below an annual precipitation of 700-800 mm yr^–1 the MBT/CBT-temperature proxy based on soil GDGTs is not applicable. Furthermore, due to the distinct GDGT distributions in soils under arid conditions, soil input into lacustrine or marginal marine environments cannot be estimated using BIT index.

In order to estimate the effects of variability in environmental conditions and utilization of different substrates on D/H_lipid, we studied two heterotrophic organisms—Haloarcula marismortui, a halophilic archaeon and Tetrahymena thermophila, a ciliated protozoan, in pure cultures. Our results from experiments with H. marismortui indicate that metabolism of different substrates leads to formation of reducing agents (mainly nicotinamide adenine dinucleotide phosphate or NADPH) with distinct D/H signatures, which is reflected in the significant D/H variations in isoprenoidal lipids (ca. 100‰). Growth temperature affects growth rate as well as enzyme activities, and salinity of the growth media affects mainly growth rate of H. marismortui, and both cause similar variations in D/H_lipid (ca. 20-30‰) that are smaller compared to the substrate-effect. T. thermophila , on the other hand, responds to variations in growth temperature in a different manner. Isoprenoid and fatty acids synthesized by T. thermophila generally become more D-enriched with temperature increase. The isoprenoid ranges from being D-depleted to D-enriched relative to water with temperature increase, but the fatty acids do not display similar patterns. Our results from T. thermophila culture experiments indicate that temperature has a critical control on the D/H ratios of NADPH and possibly also intracellular water, due probably to temperature effects on processes that are related to growth and metabolism of T. thermophila.

Nitrogen (N) fixing trees are commonly used to promote forest restoration in disturbed areas because they can quickly recreate forest canopy structure. That structure in turn is hypothesized to attract animal seed dispersers and create enough shade to reduce undesirable species (particularly grasses). Yet N-fixers tend to increase soil N availability, which could facilitate the spread of nitrophilous invasive species. This dissertation evaluates the long-term consequences for understory community composition of establishing three N-fixing tree species (Acacia koa, Sophora chrysophylla, and Morella faya) after exotic grass-fueled fire in the seasonally dry subtropical woodland in Hawaii. To understand the restoration potential of these species, I compared discrete single-species stands of N-fixing trees in burned areas to both an intact native woodland and burned, open sites with no tree cover. Although N-fixing species are often assumed to be ecologically similar, trait variation among N-fixing trees in this system was strong enough to differentiate understory communities among stands of the three N-fixer species. To understand the mechanisms driving differences in understory composition among site types, particularly among N-fixing trees, I characterized the abiotic environment created by these species in terms of light and N availability, both of which were important drivers of understory community composition. High light and N availability were associated with greater exotic species cover and unique exotic species. Surprisingly, N availability was highest and N cycled fastest beneath the relatively slow-growing S. chrysophylla despite having much lower litter-N inputs than the faster-growing A. koa and M. faya. In this study, fast N-cycling was associated with high specific leaf area, high foliar N content and low foliar lignin:N. These traits are consistent with fast leaf economic spectrum traits in the general ecological literature, but this approach has not previously been applied to distinguish among N-fixing trees. Native Hawaiian dry forest understory recovery, particularly that of woody species, was limited throughout the burned area regardless of canopy cover. To determine what limits native shrub recovery, I sampled the seed bank and recorded natural seedling germination. I also planted native seedlings into the understory of all site types and either removed or left intact the invasive grass grasses present in the understory. I found that native shrubs were limited by both seed availability and competition with exotic grasses. Although outplant survival did not vary by N-fixer species identity, differences in the mechanisms by which each N-fixing species limited native seedling survival likely play a role in understory community assembly long-term. When restoration occurs in the context of secondary succession, prioritizing the creation of forest structure using N-fixing trees, particularly open-canopied fast-cycling species, such as S. chrysophylla, could make full community recovery more difficult by promoting rather than suppressing exotic grasses.

Sediment bioturbators play an important ecological role and may both be affected by contaminants in the sediment and affect the fate and distribution of these contaminants. This is especially important for the many contaminants, like lead, for which sediments serve as a sink upon the contaminants’ release into the environment. In this study, I investigated the toxicity of sediment Pb to a freshwater bioturbator, the effect of bioturbation on the environmental distribution of the Pb, the effect of sediment characteristics on the bioturbation-mediated transfer of Pb from the sediment to the water column, and this transfer’s toxicological consequences for planktonic organisms. Experiments were conducted in microcosms with control sediment or Pb-spiked sediment, the freshwater oligochaete Lumbriculus variegatus served as the model bioturbator, and the water flea Daphnia magna served as the model planktonic organism. The rate of bioturbation of the oligochaete was quantified using luminophores.

The bioturbation resulted in the transfer of Pb from the sediment to the water column. However, it did not affect Pb levels in the worm tissue or in the sediment. The environmental distribution of Pb among water column, biota, and sediment in the presence of the bioturbator was dependent on sediment characteristic like organic content, silt/clay content, and the pH of the sediment. Bioturbation by L. variegatus increased bioaccumulation of Pb in D. magna; however, this Pb had no toxic effect on survival, reproduction, and biomass of D. magna under the specific conditions used here. Quantification of the bioturbation rates of L. variegatus showed that the intensity of the bioturbation was enhanced at higher densities of the oligochaete but reduced at high sedimentary Pb concentrations. Overall this study demonstrated that bioturbation by L. variegatus can transfer Pb from the sediment to the water column, and that this transfer is dependent on sediment characteristics. The Pb transferred as a result of the bioturbation can enhance Pb availability to organisms in the water column, and potentially cause toxic effects in these organisms.

Benthic macrofauna can play an important role in facilitating some of the microbial mediated processes of nitrogen cycling in estuarine sediments. Declines in benthic macrofauna, like polychaete worms, have been attributed to long-term increases in bottom water hypoxia in Chesapeake Bay. Utilizing a large monitoring dataset including benthic macrofaunal abundance, biomass, and concurrent measures of environmental parameters, I examined how environmental conditions regulate the densities of opportunistic polychaetes in a mesohaline estuarine system. This analysis points to a benthic community dominated by euryhaline, opportunistic polychaete worms (M. viridis, S. benedicti, H. filiformis, A. succinea) which have well adapted but varying responses to hypoxia and other stressful conditions. Results of two laboratory experiments with the opportunistic polychaete Alitta (Neanthes) succinea were used to quantify the short-term influence of density and size of surface-feeding polychaetes on sediment-water fluxes of inorganic nitrogen under varying oxygen conditions. Polychaete enhancements of O₂ and nitrogen fluxes were strongly correlated with total animal biomass. Solute fluxes were stimulated by presence of both larger and smaller worms, but per capita effects were greater for the deep-burrowing larger polychaetes. Utilizing a unique large-scale monitoring dataset collected in the Chesapeake Bay, I employed Classification and Regression Tree (CART) and multiple linear regression (MLR) analyses to assess the relationship between benthic biomass and NH₄ ⁺ efflux within different regions of the estuary by season. In addition to labile organic matter, oligohaline and mesohaline tributary temperature and salinity control the rate of nitrogen cycling and benthic macrofaunal biomass. In deeper regions of mesohaline tributaries and the mainstem Bay, dissolved oxygen was found to be the dominating parameter regulating sediment nitrogen pathways as well as the structure of the benthic macrofaunal community. With increased macrofaunal biomass, spring regressions indicated an enhancement of NH₄⁺ efflux. In contrast, fall regressions indicated the enhancement of fixed nitrogen removal from sediments. Summer data lacked a significant relationship, but high NH₄ ⁺ effluxes under hypoxic/anoxic conditions suggested dissolved oxygen is the primary driver of summer nitrogen cycling. This study, using field and laboratory data, concludes that a complex balance between seasonal and regional dissolved oxygen, temperature and salinity conditions shape not only the benthic community but also the relationship between macrofaunal biomass and sediment nitrogen flux in this eutrophic estuarine system.

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 (“outwelled”) 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 in situ 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 “Fall Dump” 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 8–16% 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 11–22% of the total export could be attributable to this flux. Approximately 49 mol C m⁻² yr⁻¹ are outwelled from the Neponset Salt Marsh and, using net primary productivity estimates from the literature, 16 ± 12 mol C m ⁻² yr⁻¹ are buried in the Neponset Salt Marsh.