For organisms relying on an aerobic metabolism, a constant oxygen (O₂) supply must be available to energy demanding tissues. In this thesis. the effects of hypoxia exposure and altered ionoregulatory demands on O₂ uptake of the larval zebrafish (Danio rerio) were evaluated. In Chapter 2, it was hypothesized that a pre-exposure to hypoxia would alter the O₂ uptake capacity of 4- and 7-days post-fertilisation (dpf) larvae through a modified vasculature system. Additionally, using a genetic knockout line, the role of Hif-1α in regulating cutaneous O₂ flux (JO₂) was tested. It was predicted that hypoxia-exposed larvae would display a higher JO₂ across the body due to a hypoxic, acclimatory response, explained by an increased vascularity and supported by an increased whole-body O₂ consumption (ṀO₂) and decreased critical O₂ tension (Pcrit). Consequently, this response was expected to be negated in the Hif1aa⁻/⁻ab⁻/⁻ larvae. Ultimately, JO₂ measured using the scanning micro-optrode technique (SMOT) remained unchanged between WT and Hif1aa⁻/⁻ab⁻/⁻ and normoxia- and hypoxia-exposed larvae, a finding which was supported by an unchanged vascularity across all treatments. The results from this chapter suggest that changes in hypoxia performance mediated by Hif-1α are unrelated to cutaneous JO₂ and vascularity. In Chapter 3, the aerobic costs of ion transport in 4 dpf larval zebrafish was assessed. We hypothesized that changes in rates of Na⁺ uptake evoked by acidic or low Na⁺ rearing would result in changes in ṀO₂ and/or JO₂, measured at the ionocyte-expressing yolk sac epithelium using SMOT. Ultimately, it was found that the measured JO₂ and ṀO₂ did not correlate with the corresponding Na⁺ uptake rate triggered by the acidic and low Na⁺ rearing environment. Thus, we conclude that the aerobic costs of ion uptake by ionocytes in larval zebrafish, at least in the case of Na⁺, are below detection using whole-body respirometry or cutaneous SMOT scans, providing evidence for a low aerobic cost for ion regulation in zebrafish larvae.

The remarkable synchrony between changes in temperature recorded in Greenland ice cores and variations in N isotope records from sedimentary cores recovered from the Arabian Sea and the Eastern Tropical North Pacific (ETNP) has provided evidence for teleconnections between changes in marine denitrification in the tropics and climate variations in the northern high latitudes. Changes in tropical denitrification have been attributed to changes in productivity, changes in the source of intermediate waters and the flux of dissolved oxygen to suboxic zones. Variations in marine denitrification and anammox occurring at intermediate depths in proximity to productive continental margins have had profound effects on the N:P ratio of upwelled waters between stadials and interstadials, and may have indirectly affected carbon sequestration in the ocean by changing the balance of nutrients available to primary productivity. Competitive equilibrium, the changing stoichiometric balance of elements available as nutrients and the shorter residence time of N compared to P are factors that are believed to favour diazotrophs (N2-fixing organisms) during interstadials and shift the competitive advantage to non-N2-fixing ecosystems during stadials. This study presents a very high-resolution analysis of sedimentary nitrogen isotope records, phosphorus concentrations and bulk detrital element concentrations from two cores collected along the Pacific Mexican Margin. The results show that the oxygen minimum zone (OMZ) bathing intermediate waters in ETNP is modulated by the interaction of a Northern Hemisphere climate component with the “leakage” of heavy nitrate believed to derive from the Eastern South Pacific (ESP). This southerly component has a more “Antarctic” timing and is similar to records from the Peru-Chile margin. The sedimentary core recovered from the Mazatlan margin shows a “Greenland” timing of millennial-scale events, with reduced upwelling and reduced primary productivity, a less intense OMZ leading to reduced denitrification and a more southerly position of the mid-tropospheric subtropical ridge during stadials. This would have increased the onshore flow of moist air, ultimately leading to increased precipitation along the western Mexican Margin. Interstadials show a reversal of these conditions. In contrast to the Mazatlan core, the N isotope record from the core recovered from the Gulf of Tehuantepec records an element of “Antarctic” timing superimposed on local, millennial-scale variations in denitrification that are more similar in timing to Greenland temperature changes. In addition, the interpretation of observed variations in detrital elements from the Gulf of Tehuantepec highlights latitudinal displacements of the ITCZ that are consistent with those observed in the Cariaco Basin in Venezuela. Bulk P concentrations from both cores suggest that although phosphorite formation in the ETNP during interstadials is not as widespread as previously thought, the very high accumulation rates in the Gulf of Tehuantepec and Mazatlan Margin lead to total Holocene phosphorus burial rates that are up to 4-5 times higher than had been estimated in previous studies. These observations lead to the argument that the ETNP may play a more important role in regulating global P budgets than was previously thought and call for an improved appreciation of the benthic microbial communities that modulate biomes at tropical latitudes.

Bubble plume oxygenation systems are commonly used to mitigate anoxia and its deleterious effects on water quality in thermally stratified lakes and reservoirs. Following installation, increases in sediment oxygen flux (JO2) are typically observed during oxygenation and are positively correlated with the bubble plume gas flow rate. Studies show that JO2 is controlled by the thickness of the diffusive boundary layer (DBL) at the sediment-water interface (SWI), which is in turn controlled by turbulence. As a result, JO2 can be quite spatially and temporally variable. Accurately predicting oxygenation-induced JO2 is vitally important for ensuring successful oxygenation system design and operation. Yet despite the current understanding of physical and chemical controls on JO2, methods for predicting oxygenation-induced JO2 are still based on empirical correlations and factors of safety. As hypolimnetic oxygenation becomes more widely used as a lake management tool for improving and maintaining water quality, there is a need to move from the current empirically based approach to a mechanistic approach and improve the ability to predict induced JO2. This work details field campaigns to investigate and identify appropriate models of oxygen supply to the SWI and oxygen demand exerted from the sediment, with the intent to use these models to predict oxygenation-induced JO2. Oxygen microprofiles across the SWI and near-sediment velocity measurements were collected in situ during three field campaigns on two oxygenated lakes, providing simultaneous measurements of JO2 and turbulence. Field observations show that oxygenation can increase JO2 by increasing bulk hypolimnetic oxygen concentrations, which increases the concentration gradient across the SWI. Oxygenation can also enhance turbulence, which decreases the DBL thickness and increases JO2. Existing models of interfacial flux were compared to field measurements to determine which model best predicted the observed JO2. Models based on the Batchelor scale, friction velocity, and film-renewal theory all agree reasonably well with field observations in both lakes. Additionally, the oxygen microprofiles were used to fit a transient model of oxygen kinetics in lake sediment and determine the appropriate kinetic model. Oxygen microprofiles in both lakes can be described using zero-order kinetics, rather than first-order kinetics. The interfacial flux and sediment kinetic models are incorporated into a coupled bubble plume and 3-D hydrodynamic lake model, allowing for spatial and temporal variation in simulated JO2. This comprehensive model was calibrated and validated to field data from two separate field campaigns on Carvin's Cove Reservoir, Virginia. Simulated temperature profiles agreed quite well with field observations, while simulated oxygen profiles differed from observed profiles, particularly in the bottom 1 m of the water column. The model overestimates oxygen concentrations near the sediment, which results in higher simulated JO2 than was observed during the field campaigns. These discrepancies are attributed to oxygen-consuming chemical processes, such as oxidation of soluble metals, which are not accounted for in the hydrodynamic model. Despite this, the model is still able to capture the impact of bubble plume operation on JO2, as simulated JO2 is higher when the diffusers are operating. With some additional improvements to the water quality modeling aspects of the model, as well as further calibration and validation, the model should be able to reproduce observed JO2 provided oxygen concentrations near the SWI are accurately reproduced as well. The current work is an attempt to push toward a comprehensive lake oxygenation model. A comprehensive model such as this should improve the ability to predict oxygenation-induced JO2 and lead to improvements in the design and operation of hypolimnetic oxygenation systems. / Ph. D.

This study was conducted to evaluate the change of chemical composition on rice grass Echinochloa crusgalli hay harvestad of 32, 46, 72 and 90 days of regrowth, and its effect on intake, digestibility, ruminal fermentation passage kinetics of solid phase through gastrointestinal tract (Experiment 1) and oxygen utilization by the visceral system in sheeps. In experiment 1 eight castrated lambs (mean live weight of 33± 3,3kg) were used in a 4 X 4 Latin Square double experimental design. .Experiment 2 was realized in two assays, using three castrated lambs fed CA cut with 46, 72 and 90 days of regrowth, with mean live weight of 38± 5 e 35± 4kg (in assay 1 and 2 respectively). These animals were chronically catheterized of portal, hepatic and mesenteric veins. Experimental diets were hay fed twice a day ad libitum . Intake of hay components linearly decreased P<0,05) with the increase of the regrowth age of the plant with the ecxeption of indigestible neutral detergent fiber were similar between treatments (P<0,05). Aparent digestibility of no nitrogen compounds of the hay, and aparent and true nitrogen digestibility, urinary excretion, N retention and microbial protein synthesis were linearly decreased (P<0,05) with the increase of the regrowth age of the plant. Sugar and aminoacids concentrations linearly decreased (P<0,05), but peptides concentrations were cuadratically releated with the increase of regrowth age of CA (P<0,05). pH values and N-amonia concentrations were similar between the treatments. Retention times in the caecumcolon were similar between the treatments. The passage rate of solid phase and retention time in the reticulo-rumen, and retention time of solid phase through gastrointestinal tract were cuadratically related with the regrowth age of CA. The nutritional value of the hay of grass rice was affected negatively by the age of regrowth of the plant. The consumption was limited by the fiber consumption in of indigestible neutral detergent, but it was not related with the passage rate of solid phase through gastrointestinal tract of the animals. Around 9% and 19% of the consumption of energy metabolic it was liberated as heat by the organs that compose the systems portal and visceral, respectively, directly related with the hay consumption and of nutrients, but not with the maturity of the plant. / Este estudo foi conduzido com o objetivo de avaliar a mudança na composição química do feno de capim arroz (CA) cortado com 32, 46, 72 e 90 dias de rebrota, e, avaliar seu efeito sobre o consumo, digestão, fermentação ruminal, a taxa de passagem da digesta pelo trato digestivo (Experimento 1) e a utilização do oxigênio pelo sistema visceral em ovinos (Experimento 2). No 1º experimento, foram utilizados oito ovinos machos castrados, com peso vivo (PV) médio de 33± 3,3kg, em um delineamento duplo quadrado Latino 4 X 4. O 2º experimento foi realizado em dois ensaios, utilizando-se três ovinos consumindo CA cortado com 46, 72 e 90 dias de rebrota, com PV médio de 38±6 e 35±4kg (1º ensaio e 2º ensaio, respectivamente), em um delineamento Quadrado Latino 3 X 3. Nestes animais foram implantados catéteres permanentes nas veias porta, mesentérica e hepática. As dietas experimentais foram constituídas de feno fornecido duas vezes ao dia em quantidades suficientes para haver sobras de 10 a 20% do oferecido. Os consumos dos vários componentes do feno de capim arroz diminuíram linearmente com o avanço da idade de rebrota da planta (P<0.05), com exceção do consumo de FDN indigestível que foi similar entre os tratamentos. A digestibilidade dos componentes não nitrogenados do feno, digestibilidade aparente e verdadeira do nitrogênio (N) assim como, a síntese de proteína microbiana, excreção urinária e a retenção do N diminuíram linearmente (P<0.05) com o aumento da idade de rebrota da planta. As concentrações de açúcares e aminoácidos, diminuiram linearmente (P<0,05) enquanto, as concentrações de peptídeos variaram quadraticamente com o aumento da idade de rebrota da forrageira. Já os valores de pH e de N-amoniacal foram similares entre os tratamentos. O tempo de retenção e passagem no ceco-colon proximal (TRcc,TPcc, respectivamente) foram similares entre os tratamentos (P>0,05). No entanto, a taxa de passagem (TPrr) e o tempo de retenção no retículorúmen (TRrr), assim como o tempo de retenção da fase sólida da digesta pelo trato gastrintestinal total (TRT) variaram quadraticamente com a idade de rebrota do CA. O valor nutritivo do feno de capim arroz foi negativamente afetado pela idade de rebrota da planta. O consumo foi limitado pelo consumo de fibra em detergente neutro indigestível, mas não foi relacionado com a taxa de passagem da digesta pelo trato digestivo dos animais. Em torno de 9% e 19% do consumo de energia metabolizável foi liberado como calor pelos órgãos que compõem os sistemas portal e visceral, respectivamente, sendo diretamente relacionado com o consumo de feno e de nutrientes, mas não com a maturidade da planta.

Turbulent dynamics at two sites (C and D) in a hypoxic zone on the Texas- Louisiana continental shelf were studied by investigating turbulence quantities i.e. turbulence kinetic energy (TKE), dissipation rate of TKE (E), Reynolds stress (τ ), dissipation rate of temperature variance (χ), eddy diffusivity of temperature (ν't), and eddy diffusivity of density (ν'p). Numerical models were also applied to test their capability of simulating these turbulence quantities. At site D, TKE, E, and τ were calculated from velocity measurements in the bot- tom boundary layer (BBL), using the Kolmogorov’s -5/3 law in the inertial subrange of energy spectra of vertical velocity fluctuations in each burst measurement. Four second-moment turbulence closure models were applied for turbulence simulations, and modeled turbulence quantities were found to be consistent with those observed. It was found from inter-model comparisons that models with the stability functions of Schumann and Gerz predicted higher values of turbulence quantities than those of Cheng in the mid layer, which might be due to that the former stability functions are not sensitive to buoyancy. At site C, χ, E, v’t, and ν’p were calculated from profile measurements throughout the water column, and showed high turbulence level in the surface boundary layer and BBL, as well as in the mid layer where shear stress was induced by advected non-local water above a hypoxic layer. The relatively high dissolved oxygen in the non-local water resulted in upward and downward turbulent oxygen fluxes, and the bottom hypoxia will deform due to turbulence in 7.11 days. Two of the four models in the study at site D were implemented, and results showed that turbulence energy resulting from the non-local water was not well reproduced. We attribute this to the lack of high-resolution velocity measurements for simulations. Model results agreed with observations only for χ and E simulated from the model with the stability function of Cheng in the BBL. Discrepancies between model and observational results lead to the following conclusions: 1) the stability functions of Schumann and Gerz are too simple to represent the turbulent dynamics in stratified mid layers; 2) detailed velocity profiles measurements are required for models to accurately predict turbulence quantities. Missing such observations would result in underestimation,