Amb aquest treball es preté comprovar la universalitat de la relació estequiomètrica de Redfield i recercar patrons consistents de les desviacions d'aquesta raó promig en ambients oligotrofics. Per tal de dur a terme aquest propòsit s'han desenvolupat relacions estequiomètriques en el compartiment particulat i dissolt orgànic i inorgànic per el C, N, P i Si.La estequiometria dels nutrients a l'oceà s'ha examinat al Mar Mediterrani i a l'Oceà Atlàntic subtropical, mentre que la zona costenera del Mar Mediterrani ha servit per estudiar aquestes relacions sota l'influència d'aports terrestres. En el segon capítol de la tesi es descriuen patrons meridionals del fluxe de nitrogen i fósfor vers la capa biogènica de l'Oceà Atlàntic Central. La raó promig entre el fluxe difusiu del nitrogen inorgànic dissolt (DIN) i del fósfor (DIP) es mostra similar a la raó de Redfield al llarg de l'Oceà Atlàntic Central, però tendeix a valors per sota dels establerts per Redfield a la part sud del trajecte i superiors a la raó de Redfield al centre del gir sudtropical. La raó N:P del fluxe difusiu i la raó N:P dels nutrients dissolts inorgànics en la capa biogènica es troben fortament correlacionats, mentre que no existeix cap correlació amb els valors de la raó N:P de les aigües intermitges. Els resultats trobats en aquest capítol de la tesi indiquen que la recirculació vertical de nutrients a la capa biogènica de l'oceà Atlàntic Central és capaç d'operar amb raons estequimètriques que difereixen de Redfield i per tant els components biogènics i biolítics s'adapten a les variacions locals de la raó de Redfield. La hipòtesis que existeixen raons estequiomètriques previsibles en la reserva oceànica de material dissolt orgànic que es troben en equilibri amb la reserva del material particulat orgànic i dissolt inorgànic, es corrobora en el tercer capítol d'aquesta tesi. La majoria del carboni orgànic present en aigües oligotrofiques del Mediterrani estratificat es troba en forma d'orgànic dissolt, mentre que el POC (carboni orgànic particulat) representa un percentatge menor. El nitrogen i fósfor orgànic dissolt que comprenen el 50-80% del "pool" total de P i N a la capa biogènica, decreix en percentarge a la capa biolítica. S'ha comprovat una distribució uniforme del nitrogen disolt total (TDN). L'increment en el percentatge de N inorgànic disolt i el decreixement en percentatge de N orgànic dissolt amb la fondària, és un indicador clar de l'equilibri dinàmic que existeix entre les reaccions bioquímiques entre les reserves oceàniques. Mitjançant un sistema de balanços, s'estableix un intercanvi de nutrients (exportació-importació) entre els pools dissolt orgànic i dissolt inorgànic en el sistema. S'ha comprovat que el fluxe de nitrogen orgànic dissolt (DON) excedeix al fluxe difusiu de nitrogen inorgànic i per tant els aports atmosfèrics i terrestres són importants en aquesta regió.En el quart capítol de la tesi es descriu el lligam existent entre el component pelàgic i els component del bentos d'una àrea litoral del Mar Mediterrani, és a dir un compartiment anabolic que produeix matèria orgànica i un de catabolic que actúa com agent oxidant de la reserva de matèria orgànica del sistema. El compartiment pelàgic es mostra heterotrofic. Al mateix temps, existeix una contribució important de material terrestre als sediments. Per contra, el compartiment bentònic és autotrofic on el dèficit en la producció grossa es compensa amb l'excés de producció neta del sistema. Mitjançant la quantificació simultànea dels fluxes anuals de sedimentació per al C, N, P i Si així com dels fluxes sediment-aigua de les espècies orgàniques i inorgàniques dissoltes, s'ha establert un sistema de balanços de matèria del sistema. El compartiment bentonic es configura com a sumider o exportador de matèria orgànica, degut als aports terrestres de carboni en el sistema.Els patrons de distribució de nutrients derivats dels resultats dels capítols anteriors es comproven mitjançant un experiment d'addició de nutrients. En el capítol cinquè d'aquesta tesi s'estudien els canvis en la distribució de nutrients en les diferents reserves nutricionals d'un sistema quan es troba sotmès a aports controlats de nutrients. S'ha comprovat que, mentre el tamany relatiu de la reserva de nutrients inorgànics dissolts no varia amb l'increment de nutrients en el medi. Hi ha una tendència a l'increment del tamany relatiu de la reserva del material particulat, paral.lela a un decreixement simultani de la reserva de material dissolt orgànic. Aquest experiment contribueix a verificar el paper del DOM (matèria dissolta orgànica) com a principal reserva nutricional en sistemes oliogotròfics. / The stoichiometric ratios are powerful tools to model basic biogeochemical patterns of the sea when the fluxes of a single element are known. It is, therefore essential to understand the full implications of variable stoichiometries to predict the effect of the living components of the ocean on biogeochemical processes. Here, the stoichiometry between C, N, P and Si in different chemical pools (particulate and dissolved organic and dissolved inorganic matter) were examined in contrasting oceanic and littoral ecosystems, and the changes in nutrient partitioning in response to nutrient inputs was tested through experimental research. In the Central Atlantic Ocean, the average ratio between dissolved inorganic nitrogen and phosphorus in the estimated vertical diffusive fluxes was similar to the Redfield ratio, but tended to be above the Redfield ratio at the center of the South subtropical gyre. The N:P supply ratio and the N:P ratio of dissolved inorganic nutrients in the biogenic layer were strongly correlated, but were not positively correlated to that in the intermediate waters. The vertical nutrient conveyor belt of nutrients in the upper waters operates relatively independently of the underlying waters in the Central Atlantic, so that both the biogenic and the biolythic components should be able to adapt to local variation about the Redfield ratio.In the stratified NW Mediterranean Sea, the stoichiometry between dissolved inorganic, organic and particulate organic matter pools indicated an excess nitrogen relative to phosphorus, particularly in the biolythic layer, as well as a general silicate deficiency relative to both N and P. Most (> 80 %) of the organic carbon was present as dissolved organic carbon, with POC representing a minor percent of total organic C throughout the water column. The increasing C/N ratio of DOM with depth indicates that N is recycled faster than C in the DOM. There exists a dynamic equilibrium between the biological transformations between these pools with depth, with a dominance of DON production in surface waters and remineralization in the underlying layers, from which dissolved inorganic nitrogen is re-supplied to the biogenic layer. Alocthonous N inputs must be important in the region since the downward DON flux exceeded the diffusive DIN supply. The coupling between anabolic and catabolic compartments of a littoral area in the NW Mediterranean Sea are characterized. The pelagic compartment was heterotrophic, supported by significant allochthonous inputs of land material, whereas the benthic compartment was autotrophic, with the excess net benthic community production balancing the deficit in pelagic community production, leading to a system in metabolic equilibrium. Sedimentary inputs of phosphorus and silicon were compensated by sediment release of phosphate and silicate, whereas nitrogen was lost or accumulated in the sediment compartment. Carbon inputs to the benthic compartment also exceeded requirements, due to the allocthonous subsidies to the system, so that the benthic compartment stored or exported organic carbon.Experimental nutrient additions lead to a parabolic change in C/N and C/P ratios in the dissolved organic matter with increasing nutrient inputs. The relative size of the dissolved inorganic nutrient pools did not vary, but there was a tendency towards an increase in the relative size of the particulate pool at the expense of a decrease in the relative importance of DOM as a reservoir of N, P and C, with increasing nutrient inputs.The relative importance of different nitrogen pools was examined in relation to the total nutrient stoichiometry of the oligotrophic system. The ratio of dissolved inorganic nutrients reported in the research presented is indicative of a general deficiency in nitrogen compared to the global ratios reported in literature. The dissolved organic matter was highly depleted in P relative to N and C at all locations investigated and the resulting POC:PON ratio (11.7) of this study in the particulate pool deviates greatly from the literature values which approximates Redfield value (5.5-6). The shift of the dominance of DON towards PON at TOC/TN values higher than 20 on the oligothrophic areas of the study, gives evidence of increasing carbon export fluxes in a system dominated by particulate pool and points to the effect of the biota on the gradient-driven export to sinking carbon fluxes in the ocean.

Recent drought over West Africa (1970s-present) has been a global concern, and the ability to predict the frequency and severity of future droughts is important to mitigate the devastating socio-economic effects of drought. The Sahel region, situated at 10-20°N just south of the arid Sahara Desert and north of the forested Guinea Coast, is particularly vulnerable to drought periodicity because rainfall is already low at 400 mm yr-1. The ability to predict future climate variability depends on adequate knowledge of fluctuations in the past. In West Africa, meteorological records are too sparse and too short in duration to characterize the drought frequency. Consequently, climate reconstructions from lacustrine sediment records are increasingly recognized as an important source of information on past climate variability. Lake Bosomtwe, Ghana (6o30N and 1o25W) was formed over one million years ago by a meteorite impact crater in the Guinea Coast region, just south of the Sahel region. Lake Bosomtwe has a closed-drainage hydrology and lake levels are known to fluctuate with the net flux in rainfall inputs relative to evaporative outputs. In 2004, the International Continental Scientific Drilling Program recovered the complete sediment record for paleoclimatic reconstructions. However, very little has been studied of the limnological conditions that lead to the formation of laminated sediments in Lake Bosomtwe. This thesis has set out to understand the influence climate has on the physical, chemical and biological in-lake processes that generate sedimenting materials, which are preserved as laminated sediment layers. Two years of water column sampling of temperature, oxygen and nutrients at a central deep-water site (78 m water depth maximum) found that this quiescent crater lake is thermally stratified during much of the year, with anoxia persisting below 35 m water depth. During the short dry season of July and August, the monsoon rains that are associated with the intertropical convergence zone (ITCZ) are displaced northwards over the Sahel region (and away from lake Bosomtwe), and cool air temperatures and clear night skies lead to the disruption of the thermocline and circulation of dissolved nutrients nitrogen (N) and phosphorus (P) in Lake Bosomtwe. Phytoplankton primary productivity, as measured by particulate carbon and chlorophyll a concentrations, was found to increase markedly following the nutrient upwelling event in August. Sediment trap samplers deployed at 20 and 30 m water depth captured the pattern of organic matter deposition and a high flux of organic sediment was deposited shortly after the nutrient upwelling episode in August. The composition of these organic-rich sediments was distinguished by a marked depletion in δ13C and enrichment of δ15N, as compared to sediments deposited before and after this event. Spatial assessment of sediment cores identified that presently, visible laminations were preserved at and below 35 m water depth, but, not at shallower depths. Water depth was also positively correlated with the organic matter content in sediment records and could be used to reconstruct pre-historic lake levels down core. The relationship between lake level and organic content in sediments predicted that water levels were likely 22 m lower than present levels during the period ~1425-1610 CE, which corresponds with a climatic periods known as the Little Ice Age (LIA). The spatial sediment trends also revealed that inorganic sedimentation rates had increased since the onset of recent land clearance and road construction in the catchment, particularly to the north, near the town of Abono. For this reason, two cores from the central deep-water region of Lake Bosomtwe were analysed for organic and carbonate content, δ13C and δ15N, nutrients (C, N, P), magnetic susceptibility, greyscale imagery of the x-radiograph and micro-X-ray analysis of elemental constituents. Paleoenvironmental reconstructions during the past 550 years found that climate-driven lake level change was a prominent factor contributing to the organic content of sediments. High inorganic content, iron concentrations and depleted δ13C distinguished a low stand during the LIA (~1425-1610) when pelagic sediments were likely exposed to periodic oxygenation. High concentrations of organic matter, calcium (Ca) and strontium (Sr), enrichment of δ13C and low C:N ratios were indicative of wet years that likely increased lake levels and the depth of water column mixing. However, sediments with high organic content, depleted δ13C signatures and reduced Ca and Sr concentrations were suggestive of drought years that restricted the depth of seasonal water column mixing and nutrient circulation and did not necessarily result in pronounced lake-level change. During the past century, δ13C of bulk matter was positively correlated with the rainfall anomalies (r2 = 0.45, P < 0.002), indicating that droughts can result in reduced primary productivity, which may ultimately lower fishery yields. The communities living within the crater are dependent on subsistence fishing and farming, and predicting the drought frequency and magnitude in this region is essential to protecting both the ecosystem and the human population. Long-range climate forecasts for West Africa predict greater drought and increasing air temperature. However, with a detailed long-term paleoclimatic reconstruction from Lake Bosomtwe sediment records, potentially the accuracy of these predictions can be improved and better equip policy makers to enact a viable action plan in the best interests of the people.

Recent research has shown that the Arctic may be a sink for mercury, however, the fate of this deposited mercury in the environment is not known. The objective of this project was to determine the factors affecting methyl mercury (MeHg) production in Arctic organic soil on the Truelove Lowlands, Devon Island, Canada. In the field we observed a steady decrease in MeHg over time, with MeHg concentration at many sampling locations declining below detection limits. This decrease did not correlate to any chemical or biophysical parameter measured. During the study the Lowlands appeared to be mildly reducing with dissolved Fe(II) being present in the porewater, however, no correlation was observed between MeHg production and the variables measured. The dissolved organic matter concentration of the porewater was quite high, the pH was circumneutral and it would seem that in the absence of more highly reducing conditions that mercury would be unavailable for methylation.<p> It seems likely under field conditions MeHg was much more bioavailable then inorganic mercury. This would lead to a higher rate of demethylation then methylation and a net decrease in MeHg. Little research has been done on demethylation and the effect of environmental conditions on demethylation, especially in arctic environments. However, it is possible that the rate of demethylation was not affected by changes in temperature or any other parameter measured over the course of the field study. <p> Laboratory microcosm studies using saturated soil from the organic horizons demonstrated little potential for unspiked organic soil to produce significant amounts of MeHg. The spiked treatment, however, had an eight fold increase in MeHg concentration and the sterile treatment showed no change in MeHg concentration over 40 days of freeze (-5 0C) and 59 days of thaw (4 oC). <p> Our data suggests that a combination of atmospheric and in-situ processes maintain a cycle of MeHg production (spring) and loss (summer) in arctic soils. It would seem that Arctic wetland soils are not a significant source of MeHg to the Arctic ecosystem and that snowmelt is the dominant source.

Human actions have led to the destruction or degradation of natural habitats in virtually all parts of the Earth. Ecosystem restoration is one method to mitigate the effects of habitat loss. But restoration ecology is a young discipline and there is much left to be learned about how to effectively restore ecosystem functioning. This dissertation examines how soil amendments and planted herbaceous species diversity affect the restoration of ecosystem functions in wetlands, while also testing basic ecological questions that help us understand ecosystem function. Using data from the greenhouse and from the biodiversity and ecosystem function field experiment in Duke Forest, in Durham, NC, I examine how plant trait diversity, average plant traits, and environmental conditions influence nitrogen (N) removal from restored wetlands. Field data collected from a restored wetland in Charlotte, NC, enables me to examine how soil organic amendments influence the development of soil properties, processes, and plant communities. Finally, combining field data from both sites, I compare how soil properties influence denitrification potential in both restored wetlands. One unanswered question in the research relating biodiversity and ecosystem function is whether species diversity or species traits are more important drivers of ecosystem function. The first portion of my dissertation poses several hypotheses about how plant traits, plant trait diversity (calculated as a multivariate measure of plant trait diversity), and environmental conditions are likely to influence two ecosystem functions, biomass N and denitrification potential (DEA), and then examines these hypotheses in a restored wetland in the Piedmont of N.C. Using multiple linear regression, I demonstrate that functional diversity (FD), of traits important for plant growth had no effect on biomass N, but two plant traits, leaf area distribution ratio (LADR) and water use efficiency (WUE), had strong negative effects. Soil inorganic N also had a positive effect. For DEA, FD of traits related to denitrification also did not have a significant effect, but there was evidence of a weak positive effect. Two plant traits had positive effects on DEA, aboveground biomass and aboveground biomass C:N ratio; two traits, belowground biomass C:N ratio and root porosity, had negative effects. Soil inorganic N and soil organic matter also had positive effects on DEA. Results from a Principal Components Analysis (PCA) clustering plant species in trait-space, suggest that <em>Carex</em>, <em>Scirpus</em>, and <em>Juncus</em> species tend to be associated with traits that maximize biomass N, while there is no specific region of trait space or set of species that correspond to high DEA. Instead, there are multiple plant trait combinations that can lead to high DEA. These results suggest that, even though plant diversity (as measured by FD) does not significantly influence biomass N or denitrification, plant trait diversity is important to maintaining multiple ecosystem functions simultaneously. Restored wetlands tend to have lower levels of soil organic matter than natural reference wetlands. Low soil organic matter can limit nutrient cycling as well as plant survival and growth in restored wetlands. In the second portion of my dissertation, I examine how soil compost amendments influence the development of soil properties and processes as well as plant communities at a restored wetland in Charlotte, NC. Using two-way analyses of variance, multiple comparisons of means, and regression, I determine that available N and phosphorus (P) increase with increasing soil organic matter in both the low and high marsh. Total microbial biomass (MB) and microbial activity (measured by denitrification potential (DEA)) also significantly increase with increasing organic matter in both marsh communities, as does soil moisture. Neither total plant biomass (in the low marsh), nor plant species richness (in the high or low marsh) demonstrate any consistent patterns with soil organic matter level in the first three years post-restoration. These results suggest that compost amendments can positively influence some soil properties (i.e. soil available N, P, microbial biomass, and soil moisture) and some ecosystem functions including nutrient cycling (such as denitrification potential), but may have limited early impacts on plant communities. In restoration ecology there is a general assumption that restoring ecosystem structure will also restore ecosystem function. To test this fundamental assumption, I examine whether two restored wetlands demonstrate similar general relationships between soils variables (i.e. do the two systems have similar soil ecosystem structure), and whether the importance of each soil relationship is the same at both systems (i.e. do the two systems demonstrate the same soil function). I use structural equation modeling to both pose hypotheses about how systems function and to test them using field data. I determine that the same model structure of soil relationships is supported by data from these two distinct, yet typical urban restored wetland ecosystems (that is, the two systems have similar soil structure). At both systems higher soil organic matter is the most important predictor of higher DEA; however, most of the other relationships between soils variables are different at each system (that is, the two systems are not functioning in the same way). These results suggest that some fundamental relationships between soil properties and microbial functioning persist even when restored wetlands have very different land-use histories, plant communities, and soil conditions. However, restoring similar soil ecosystem structure does not necessarily lead to the restoration of similar soil function. Ultimately, I hope this research advances our understanding of how ecosystems function and improves future wetland restoration efforts. / Dissertation

<p><p>Invasive species are rapidly expanding in riparian wetlands while concurrently anthropogenic causes are increasing nitrogen (N) into these ecosystems. <italic>Microstegium vimineum (Microstegium) </italic> is a particularly abundant invasive grass in the Southeast United States. To evaluate impacts of <italic>Microstegium</italic> on both plant diversity and N cycling in a riparian floodplain, paired plots of <italic>Microstegium</italic> hand-weeded and unweeded were established for three years. Plots without <italic>Microstegium</italic> increased from 4 to 15 species m<super>-2</super> and 90% of the newly establishing species were native. The <italic>Microstegium</italic> community accumulated approximately half the annual N in biomass of the diverse community, 5.04 versus 9.36 g-N m<super>-2</super> year<super>-1</super>, respectively (p=0.05). Decomposition and release of N from <italic>Microstegium</italic> detritus was much less than in the diverse community, 1.19 versus 5.24 g-N m<super>-2</super> year<super>-1</super>. Rates of soil N mineralization estimated by in-situ incubations were relatively similar in all plots. While <italic>Microstegium</italic> invasion appears to greatly diminish within-ecosystem circulation of N through the under-story plants, it might increase ecosystem N losses through enhanced denitrification (due to lower redox potentials under Microstegium plots). Microstegium removal ceased in the fourth growing season and formerly weeded plots increased to 59% (&plusmn; 11% SE) Microstegium cover and species richness decreased to <8 species m<super>-2</super>. </p></p><p><p>To learn how <italic>Microstegium</italic> responds to increased N, we conducted a greenhouse competition experiment between <italic>Microstegium</italic> and four native plants across an N gradient. There was a unique competition outcome in each species combination, yet <italic>Microstegium</italic> was most dominant in the high levels of N. </p></p><p><p>Last, we disturbed a floodplain similar to wetland restoration disturbance and tracked available N. We also established a native community of plants with and without <italic>Microstegium</italic> in three levels of N. Disturbance to the floodplain dramatically increased inorganic N, especially in the form of NO₃ which was five times higher in the disturbed floodplain than the undisturbed floodplain. N levels remained elevated for over a year. <italic>Microstegium</italic> was N responsive, but did not show negative effects to the planted vegetation until the second year. Ironically, restoration activities are increasing available N, and favoring invasive species which in turn detracts from restoration success.</p> / Dissertation

Trace metals (e.g. Fe, Mn, Zn, Cu, Cd, Ni) are important micronutrients that have historically been regarded as toxic pollutants rather than essential components of riverine and estuarine environments. The toxicity and behavior of trace metals, in response to physical and biogeochemical processes, are determined by their individual physico-chemical properties. In this dissertation, the vertical transformation of trace metals across oxic-anoxic interfaces was investigated at two sites, a Fe-rich freshwater river with minimal sulfide and an estuary with elevated Mn and H2S concentrations. Sediment profiles obtained from the Chattahoochee River showed that dissolved arsenic, present as As(V) only, is scavenged by Fe-oxides and accumulates directly below the sediment-water interface. Depth profiles also indicate that As(V) fluxes into the overlying water during baseflow conditions as well as after storm events. The significant correlation between Fe(II) and As(V) suggest that As(V) is released from Fe-oxides during their microbial reduction. By implementing a series of sediment incubations under increasing As(V) loads, it was determined that adsorption onto Fe-oxides and microbially mediated reductive dissolution of these mineral phases drive arsenic cycling in this sediment. These incubations also reveal for the first time that arsenic, even in low concentrations, n turn, arsenic loading impacts iron cycling by stimulating anaerobic respiration of Fe-oxides and promoting recrystallization of authigenic Fe-oxides, up to a toxicity threshold up to a few micromolar in concentrations. A combination of in situ measurements with discrete water sampling was utilized to determine the effects of tidal cycling on the distribution of trace metals under changing redox conditions during two consecutive tidal cycles at Station 858 in the Chesapeake Bay. Estuarine circulation patterns driven by tidal oscillations, a defined pycnocline, and the shallow sill (~20 m) of the Chesapeake Bay promoted bottom water anoxia during the summer months that allowed dissolved sulfide and reduced manganese to accumulate below the oxycline. The distribution of barium (conservative freshwater tracer) and uranium (conservative seawater tracer) across the pycnocline over the two tidal cycles indicated that the source of dissolved species was surficial sediments. During ebb and flood tides, the shear stress from the bottom waters flowing over the sediment seems to episodically promote the advection of porewaters enriched in dissolved sulfide, manganese, uranium, barium, lead, chromium, and copper. The selective enrichment of these trace metals appears to be controlled by their reactivity with sulfide. In contrast, cobalt and nickel are retained in sediments by adsorbed or incorporated in FeS and FeS2, while arsenic co-precipitates with sulfide or iron sulfide minerals. Overall, this study demonstrates that natural aquatic systems are complex environments where the interplay between biological, chemical, and physical processes affects the distribution of trace metals over short time scales. While a great wealth of knowledge can be obtained by laboratory experiments with synthetic solutions or pure cultures of organisms, a combination of in situ measurements and incubations with real samples in necessary to characterize the processes regulating the cycling of trace metals in aquatic systems.

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Thesis (PhD)--Montana State University--Bozeman, 2008. / Typescript. Chairperson, Graduate Committee: Brian L. McGlynn. Includes bibliographical references.

Thesis (Ph. D.)--College of William and Mary. / Vita. Includes bibliographical references. Also available online.