Browsing by Subject "hypoxia"
Now showing 1 - 17 of 17
Results Per Page
Sort Options
Item Benthic Function and Structure in the Northern Gulf of Mexico Hypoxic Zone: Sediment Biogeochemistry and Macrobenthic Community Dynamics in the Dead Zone(2012-07-16) Nunnally, CliftonCoastal low oxygen areas are expanding globally and are predicted to increase in size and duration due to climatic changes associated with a warming ocean. The Gulf of Mexico Hypoxic Zone (GoMHZ) is the second largest regularly occurring hypoxic habitat in the world and has increased in size since it was first mapped in the 1980s. The Mississippi Atchafalaya River System (MARS) floods the Louisiana continental shelf with fresh water high in nitrogenous compounds enhancing primary production which sinks to the sea floor. Stratification that occurs as a result of density differences and coastal currents creates a strong pycnocline that prevents bottom waters from being aerated causing seasonally hypoxic bottom waters (< 2.0 mg L^-1). The Mechanisms Controlling Hypoxia (MCH) project (hypoxia.tamu.edu) made regular cruises during 2004-2005 and 2007-2009 to the GoMHZ performing shelf wide hydrographic surveys and occupying central mooring sites within theoretical zones of differing hypoxic potential. Sediment cores were collected for incubation experiments using Batch Microincubation Chambers (BMICs) to measure rates of sediment community oxygen consumption and nutrient regeneration. Results of incubation experiments characterized sediments as net sources of dissolved inorganic nitrogen, mostly ammonium, and silicate and a net sink of phosphate. Modeling simulations of benthic-pelagic coupling focused in the western study zones related field measurements of benthic nutrient regeneration and primary production to important processes that maintain summertime hypoxia when surface waters are nitrogen limited. After incubations were completed macrofaunal individuals were removed from the cores enumerated and identified to the lowest possible taxon. Macrofauna communities in 2004-2005 were dominated by a hypoxia tolerant community dominated by polychaetes. Hurricanes Katrina and Rita in August and September of 2005 drastically reorganized macrobenthic communities decreasing abundances and negatively impacting diversity. These new communities collapsed under hypoxic stresses potentially impacting the ability of demersal foragers to utilize an important food resource. Large variations in biogeochemical fluxes and patchy distribution of fauna impeded the delineation of significant zones in benthic function and structure.Item Boolean Network Model of Hypoxia Stress Response Pathway(2014-01-14) Varghese, Rajani RajanHypoxic stress is a consequence of the decrease in the oxygen reaching the tissues of the body. The coupling of energy with oxygen makes low tension oxygen sensing and adaptation very essential for survival. The intracellular partial pressure of oxygen is regularly measured by a family of hydroxylase enzymes named as prolyl hydroxylase domain containing proteins (PHD). Hypoxia Inducible Factor (HIF) is the transcription factor that controls the ability of the cell to balance between adaptation and cell death during hypoxia. During normoxia, HIF1? undergoes non-reversible hydroxylation in the presence of PHD2. The hydroxylated HIF1? interacts with von Hippel-Lindua tumor suppressor protein (pVHL) and is degraded by ubiquitination. During hypoxia, PHD2 is inhibited which results in HIF-1? stabilization. Stabilized HIF-1? enters the nucleus and heterodimerizes with Hypoxia Inducible Factor-1? and the dimeric transcription factor HIF-1 is formed which binds to the response elements of the target genes. HIF-regulated target genes enable cells to induce an adaptive response by increasing glycolysis; angiogenesis etc. or undergo cell death by promoting apoptosis or necrosis. In this work, a Boolean network is generated whose state transitions realize the hypoxic stress response pathway. The simulated behavior of the Boolean network obtained is consistent with the experimental results from the already published pathway literatures.Item Diel Temperature and Dissolved Oxygen Patterns in Sites with and without Planktonic Life Stage of Thompsodinium intermedium in Comal Springs, TX(2012-07-16) Gilpin, CherylBetween July 2009 and October 2011, a new habitat was found for a rarely reported freshwater dinoflagellate species, Thompsodinium intermedium - Comal Springs (Comal County), Texas. In 2011, diel in-situ monitoring in monospecific blooms of this species revealed previously undetected negative impacts on endangered species habitat availability associated with conditions of low flow levels, recorded at the U.S. Geological Survey gage # 08169000 on Texas Commission on Environmental Quality river segment 1811 station 12655. During a period of low springflow in the summer of 2011, late afternoon and early morning measurements of dissolved oxygen and temperature and presence of dinoflagellate blooms were monitored at six sites. Significant differences in diel fluctuations were found in all of these parameters among sites with and without the planktonic blooms. These fluctuations increased risk of hypoxia and hyperthermia conditions at sites of planktonic bloom events. Arrays of in-situ continuous monitoring temperature/light probes were used inside and outside of blooms. Wildlife and human health implications are that hypoxia and hyperthermia are known to promote conditions favorable to harmful microbes which may be transported from springs to coastal bays. In-situ data demonstrated that T. intermedium blooms, hypoxia, and hyperthermia occurred in the upper Comal headwaters. These natural environmental stressors may be avoidable if adequate springflows are maintained to buffer against these impacts.Item Dissolved Organic Matter Cycling on the Louisiana Shelf: Implications for the Formation of Hypoxia(2012-02-14) Shen, LiAlthough there has been considerable work on the role of nutrient-derived (mostly nitrate) primary production in fueling hypoxia in northern Gulf of Mexico, very little is known about the relative importance of autochthonous versus allochthonous sources of dissolved organic matter (DOM). Moreover, even less is known about the importance of dissolved organic nitrogen (DON), a critical component of DOM (along with DOC) in supporting hypoxia in this region. Most nitrogen in marine organisms exists in the form of amino acids. Changes in the spatial and temporal distribution of amino acids in the Mississippi River Plume have been shown to be important in the dynamic microbial cycling in the plume. In this study, concentrations of amino acids, DON and DOC were linked with hydrography data (e.g., DO, salinity, temperature, fluorescence) to determine how these sources of DOM are related to seasonal and diurnal changes in hypoxia on the inner Louisiana shelf. The general working hypothesis of this work was that allochthonous and autochthonous sources of DOM on the Louisiana shelf have been largely underestimated in their role in fueling hypoxia in northern Gulf of Mexico. A positive correlation between DOC, DON and fluorescence demonstrated that the main source of both DOC and DON was likely to be in situ phytoplankton production. Surface waters in the near-field showed this relationship more than at stations to the west where a sub-surface chlorophyll peak near the pycnocline may also provide a source of DOC and DON in bottom waters. DFAA always had relatively low concentrations at all water depths, which further supports prior work which has shown rapid cycling and high consumption rate of DFAA by heterotrophic bacteria. In addition to biotic controls, selective adsorption of DFAA likely contributed to the dominance of aspartic and glutamic acids at our stations. Hypoxia was generally observed in bottom waters in both spring and summer 2010. Dissolved oxygen generally revealed a negative correlation with nitrate+nitrite concentrations. Based on other work, one possible reason for such linkages may be from NH4+ released from dissimilatory nitrate reduction to ammonium (DNRA). Another possible reason may be the high degradation of labile DOM (such as DFAA) as shown by high respiration in bottom waters in prior work by Amon and Benner (1998).Item Inhibition of cholesterol biosynthesis under hypoxia(Texas A&M University, 2006-04-12) Tan, QiulinOxygen balance is very important and tightly regulated in mammals. Under hypoxia, hypoxia inducible factor 1(HIF-1) dimerizes with hypoxia inducible factor 1? (HIF-) and activates expression of several genes. Using a mammalian two hybrid assay, we found that HIF-1 interacted with sterol response element binding protein 1a (SREBP1a). SREBP1a regulates transcription of HMG-CoA reductase via binding to the sterol response element (SRE) in the promoter region. HMG-CoA reductase is the rate-limiting enzyme in cholesterol biosynthesis. The interaction between SREBP1a and HIF-1suggests that HIF-1 may play an important role in regulation of cholesterol biosynthesis. We tested the effects of hypoxia on the HMG-CoA reductase. We found that hypoxia caused suppression of SRE-driven luciferase reporter gene expression. HMG-CoA reductase mRNA levels decreased under hypoxia in both hepatoma cells and mouse primary hepatocytes. Electrophoretic mobility shift assay showed that HIF-1 blocked binding of SREBP1a to the SRE sequence in vitro. Ectopic expression of HIF-1 suppressed the SRE- driven luciferase reporter gene expression in BPR cells (HIF-1). Our results suggest that hypoxia inhibits cholesterol biosynthesis by suppressing SREBP1a-regulated gene expression and this suppression is caused by the blockage of SREBP1a binding to SRE sequence by HIF-1.Item Mechanisms of alcohol-induced neuroteratology: an examination of the roles of fetal cerebral blood flow and hypoxia(Texas A&M University, 2005-02-17) Parnell, Scott EdwardHypoxia (decreased tissue oxygen levels) has long been considered as a possible mechanism of alcohol-induced developmental deficits, yet research has not conclusively disproved this hypothesis, nor has it provided substantial evidence for a mechanism of developmental alcohol insults involving hypoxia. Previous research has shown that moderate acute doses of alcohol does not induce hypoxemia (decreased arterial oxygen levels), yet these same studies have shown that this same alcohol exposure does transiently decrease cerebral blood flow (CBF). This is significant because although developmental alcohol exposure did not result in hypoxemia, the decreases in CBF seen in these previous studies may induce hypoxia within the brain. Unfortunately, these experiments were only performed after acute doses of alcohol, so it is unknown if a more chronic or repeated alcohol exposure paradigm would have similar effects. The present study examined blood flow in the sheep fetus after repeated alcohol exposure in a bingelike paradigm throughout the third trimester. Additionally, this study examined the fetal neurovascular response to a subsequent infusion of alcohol after the repeated alcohol exposure. This latter experiment was designed to examine the hypothesis that alcohol exposure throughout the third trimester affects the normal responsiveness of the neurovasculature to alcohol (compared to previous research demonstrating acute alcohol-induced decreases in CBF). The results from the present experiments indicate that although few regions were significant, the majority of the regions (especially the brain regions) exhibited a trend for increases in blood flows after alcohol exposure. This phenomenon was especially prominent in the group receiving the lower dose of alcohol. Additionally, the data from this study demonstrated that after repeated alcohol exposures the near-term sheep fetus did not respond to a subsequent dose of alcohol in a similar manner seen in previous experiments when the acute alcohol exposure was administered in alcohol na?ve animals. After the final alcohol exposure the subjects in this study had either no effect in terms of blood flow or an increase in CBF. This is opposite to previous observations which demonstrated reduced blood flow in numerous brain regions. The present experiments suggest that alcohol does not induce fetal hypoxia, but does negatively affect the normal neurovascular response to alcohol. This latter phenomenon could have negative consequences on future development of the brain.Item Monomethylmercury concentrations on the eastern Texas-Louisiana shelf during the formation, peak, and disappearance of hypoxia(Texas A&M University, 2007-09-17) Keach, Sara ElizabethA study of monomethylmercury (MMHg) concentrations in the water and sediment of the hypoxic zone in the northeastern Gulf of Mexico was conducted on several cruises between April 2004 and May 2005. Surface water MMHg concentrations were low and constant throughout the sampling period. Bottom water concentrations displayed a seasonal trend: maximum MMHg concentrations were in June/July 2004, decreased to a minimum in October 2004, and in May 2005 concentrations had begun to increase. MMHg concentrations and MMHg as a percent of THg in surface sediment (0-2 cm) also followed this trend. Bottom water dissolved oxygen and temperature displayed inverse relationships with bottom water MMHg concentrations. This correlation between dissolved oxygen and MMHg is typical for low-oxygen waters, but the relationship between temperature and MMHg is relatively unique. A possible explanation is that warmer summer temperatures inhibited bacterial methylation. Stratification intensity (quantified as N2) was strongly correlated with bottom water MMHg concentrations, indicating either increased methylation at the pycnocline or that the pycnocline inhibited vertical mixing, thus limiting MMHg to the bottom water. Benthic flux estimations indicate that sediment release of MMHg could be a significant source of MMHg to bottom water. The presence of an oxygenated layer in the surface sediment could have played a role in inhibiting MMHg flux during oxic conditions; a decrease in the thickness of this layer under hypoxic conditions likely allowed MMHg to diffuse into the bottom water. Dissolved oxygen seemed to play an important role in controlling sediment MMHg concentrations with highest methylation rates in sediment under hypoxic water. Overall, sites closest to the Mississippi River mouth displayed the highest MMHg concentrations. Further research will need to be done in this area to fully characterize the relationship between biogeochemical parameters and MMHg concentrations.Item Nitrous Oxide Production in the Gulf of Mexico Hypoxic Zone(2010-10-12) Visser, Lindsey A.The Gulf of Mexico hypoxic zone is created by strong persistent water stratification and nutrient loading from the Mississippi River which fuels primary production and bacterial decomposition. The Texas-Louisiana shelf becomes seasonally oxygen depleted and hypoxia (O2 less than or equal to 1.4 ml l-1) occurs. Low oxygen environments are conducive for the microbial production of nitrous oxide (N2O), a powerful greenhouse gas found in the atmosphere in trace amounts (319 ppbv). Highly productive coastal areas contribute 61% of the total oceanic N2O production and currently global sources exceed sinks. This study is the first characterization of N2O produced in the Gulf of Mexico hypoxic zone. Because of enhanced microbial activity and oxygen deficiency, it is hypothesized that the Gulf of Mexico hypoxic zone is a source of N2O to the atmosphere. Seasonal measurements of N2O were made during three research cruises in the Northern Gulf of Mexico (Sept. 2007, April 2008, and July 2008). Water column N2O profiles were constructed from stations sampled over time, and bottom and surface samples were collected from several sites in the hypoxic zone. These measurements were used to calculate atmospheric flux of N2O. The Gulf of Mexico hypoxic zone was a source of N2O to the atmosphere, and N2O production was highest during times of seasonal hypoxia. N2O was positively correlated with temperature and salinity, and negatively correlated with oxygen concentration. Atmospheric fluxes ranged from -11.27 to 153.22 umol m-2 d-1. High accumulations of N2O in the water column (up to 2878 % saturated) were associated with remineralization of organic matter at the base of the pycnocline and oxycline. Seasonal hypoxia created a source of N2O to the atmosphere (up to 2.66 x 10-3 Tg N2O for the month of September 2007), but there was a slight sink during April 2008 when hypoxia did not occur. Large fluxes of N2O during the 3 to 5 month hypoxic period may not be counterbalanced by a 7 to 9 month sink period indicating the Gulf of Mexico hypoxic zone may be a net source of N2O to the atmosphere.Item Organic Carbon Cycling in East China Sea Shelf Sediments: Linkages with Hypoxia(2013-01-03) Li, XinxinThe Changjiang River provides the main source of sediment and terrestrial derived organic carbon (OC) to the Changjiang large delta-front estuary (LDE) in the East China Sea (ECS). This study analyzed bulk OC, biomarkers including lignin and plant pigment, black carbon (BC) on ECS sediments sampled in winter 2009 and 2010 in order to study the OC cycling under the influence of natural and anthropogenic disturbance. Low-oxygen tolerant foraminiferal microfossils were analyzed in another two sediment cores to study the historical hypoxia events in the Changjiang LDE. Bulk carbon to nitrogen (C/N) ratio and stable isotope ?13C in the surface sediment samples indicated a mixture source of terrestrial, deltaic and marine derived OC. Refractory BC and reworked marine OC seemed to comprise most of the OC pool with older, less reactive signatures as deduced from ?14C, and BC analyses. Winter wind/wave energy and hydrodynamic sorting had a substantial winnowing effect on surface sediment OC redistribution. As a result, the highest lignin concentration shifted to the south during the 2010 cruise after the summer flood event. In addition, algal inputs from local deltaic lakes due to eutrophication and/or lateral transport likely caused the observed lack of benthic-pelagic coupling of pigment concentrations between the surface sediments and the water column after the summer flood in 2010. For the down-core sediment, the mass accumulation rate distribution followed the dispersal pathway of the ECS sediment. Terrestrial and marine derived OC showed significant spatial and temporal distribution. Lignin rich materials were better preserved in sediments closer to the coast while offshore sediments tended to be composed of lignin-poor, degraded OC, that were likely hydrodynamically sorted to a long distance during transport. Besides eutrophication, plant pigments indicated that marine-derived OC was mostly deposited in the sediment mixed layer with decay in the underlying sediment accumulation layer. The total OC standing stock since 1900 is approximately 1.62?1.15 kgC m^-2, about 1/10 of the total OC stock in all the middle and lower lakes in the Changjiang catchment. There has been an increase in the number of hypoxic bottom water events on the Changjiang LDE over the past 60 yrs indicated from the increases in low-oxygen tolerant foraminiferal microfossils due to excess deposition of OC and summer stratification.Item Regulation of PERLECAN and 2OST Expression in Prostate Cancer Progression by Stress-activated Transcription Factors(2012-02-14) Ferguson, Brent WadeHeparan sulfate proteoglycans modulate many of the growth factor pathways that drive prostate cancer progression. Prior to being secreted into the extracellular matrix, the covalently attached HS chains are modified by sulfation which has been shown to increase the affinity of binding growth factors. The specific HSPG that I focus on in this dissertation is Perlecan (Pln). Previously, our group along with collaborators found that 54 percent of prostate cancer tumors had upregulated levels of Pln protein that correlated with increasing Gleason score [93]. The LNCaP-DU145-LN4 cell line series is introduced as a model for this subset of tumors because Pln levels increase 50-fold as the cells become more metastatic. It was found that three stress-induced transcription factors, HIF1?, NFkB, and ATF2, all stimulate Pln expression. ChIP analysis reveals that HIF1? and NFkB directly bind the Pln promoter while ATF2 does not. The ROS-generating NADPH Oxidase and the ROS-inducible p38 MAPK were also found to induce Pln expression. To address the subset of prostate cancer tumors that reach metastasis without upregulation of Pln, I focused on the 2-o-sulfotransferase enzyme and its effect on proliferation and invasion in the LNCaP-C4-2B cell model which does not show upregulation of Perlecan expression. 2OST RNAi resulted in a significant decrease in proliferation in each line of the series. 2OST RNAi in highly metastatic C4-2B cells caused a significant decrease in cell invasion. Cells with decreased levels of 2OST had increased accumulation of actin and E-cadherin suggesting the possible formation of adherens junctions. I also found that expression of 2OST increases four-fold as cells become more metastatic. I found HIF1? and ATF2 act in a direct manner while NFkB acts indirectly to stimulate 2OST expression. In summary, I have analyzed the effect of cellular stress on the expression of the Pln and 2OST genes and investigated the phenotype of 2OST knockdown in metastatic prostate cancer cells. These studies lead me to propose that the tumor stress response is necessary for prostate cancer progression due to the role of stress in the upregulation of extracellular HS that is required for growth factor signaling and metastatic behaviors.Item Relationships between nutrients and dissolved oxygen concentrations on the Texas-Louisiana shelf during summer of 2004(2009-06-02) Lahiry, SudeshnaHypoxia (dissolved oxygen concentrations less than 1.4 ml/l) is a recurrent seasonal phenomenon on the Louisiana Shelf, caused by the combined effects of nutrient loading by the Mississippi and Atchafalaya River System (MARS), and density stratification. In 2004, three shelf wide cruises (in April, June and August) were conducted on the Louisiana Shelf to understand the mechanisms controlling hypoxia on the shelf, and examine the relationship between dissolved oxygen and nutrient concentrations during the hypoxic periods. The shelf was divided into three geographically separate zones: A (off the mouth of the Mississippi River), B (off the Terrebonne Bay) and C (off the mouth of the Atchafalaya). Each zone was different in terms of the physical and biochemical processes occurring there. In April, no hypoxia was observed on the shelf because of water column mixing by winds, even though high discharge occurred from the MARS. Nutrients were abundant in the surface waters but present only in little amounts at the bottom. In June, the water column was highly stratified. Because of the presence of upwelling favorable winds no vertical mixing occurred and caused extensive hypoxia on the shelf. Dissolved oxygen concentrations were negatively correlated with nutrients at the bottom of the water column. Nutrients were considerably higher at the bottom than at the surface (except for zone A, where high nutrients were seen even at the bottom), indicating remineralization below the pycnocline. Resuspension of organic material and remineralized nitrogen were sustaining hypoxia far from the river sources. In August, hypoxia was patchy on the Louisiana Shelf. Correlations between dissolved oxygen and nutrient concentration varied seasonally with highest correlations occurring during hypoxic conditions in June and August. The spatial distribution of nutrients and other oceanographic parameters, such as light transmission, fluorescence, and dissolved oxygen concentrations, indicate seasonal variability of biochemical processes that are related to physical processes that affect stratification.Item Role of hypoxia and hypoxia induced factors in the development of breast cancer brain metastasis(2009-05-15) Lungu, Gina FlorentinaHere we studied the role of hypoxia and hypoxia-induced factors in the development of breast cancer brain metastasis by using ENU1564, a carcinogen-induced mammary adenocarcinoma cell line. We detected hypoxia noninvasively by using a novel spectroscopic photoacoustic tomography technology (SPAT). Sprague-Dawley rats inoculated intracranially with ENU1564, a carcinogen-induced rat mammary adenocarcinoma cell line, were imaged with SPAT three weeks post inoculation. Proteins important for tumor angiogenesis and invasion were detected in hypoxic brain foci identified by SPAT and were elevated compared with control brain. We showed that HIF-1?, MMP-9, VEGF-A, and VEGFR2 (Fkl-1) protein and mRNA expression levels were higher (P < 0.05) in brain tumor tissues compared to normal brain. We also found an increased expression of HIF-1? proteins, MMP-9, VEGF-A and VEGFR2 mRNA and proteins in hypoxic ENU1564 cells in vitro. We also demonstrated the involvement of PI3K-Akt pathway in hypoxic regulation of MMP-9 and VEGF but not VEGFR2 by using specific PI3K inhibitor. Using MEK1/2 inhibitor we showed that hypoxic regulation of MMP-9, VEGF-A and VEGFR2 also involve MEK1/2-ERK pathway. We also investigated the effect of fibroblast growth factor-1 (FGF-1), one of the factors known to be upregulated by hypoxia, on the expression of MMP-9 in ENU1564 cell line. We observed that FGF-1 induces an increase in MMP-9 mRNA, protein, and activity in ENU1564 cells. Next, we investigated the role of components of PI3K-Akt and MEK1/2-ERK signaling pathways in our system. We demonstrated that FGF-1 increases Akt phosphorylation, triggers nuclear translocation of NF-?Bp65, and enhances degradation of cytoplasmic I?B?. Pretreatment of cells with LY294002, a PI3K inhibitor, significantly inhibited MMP-9 protein expression in FGF-1-treated cells. Conversely, our data showed that FGF-1 increases ERK phosphorylation in ENU1564 cells, increases c-jun and c-fos mRNA expression in a time-dependent manner, and triggers nuclear translocation of c-jun. Pretreatment of cells with PD98059, a MEK1/2 inhibitor significantly inhibited MMP-9 protein expression in FGF-1 treated cells. Finally, we observed increased DNA binding of NF-?B and AP-1 in FGF-1-treated cells and that mutation of either NF-?B or AP-1 response elements prevented MMP-9 promoter activation by FGF-1.Item Stratification Limited Vertical Ventilation: Effects of Water-column Stabilities on the Formation of Hypoxia on the Texas-Louisiana Shelf(2013-12-02) Li, BoI examine the vertical structure of water-column stability and its relationship to near-bottom dissolved oxygen (DO) concentration in the hypoxic zone of the Texas-Louisiana Shelf using observations collected between year 2003 and 2009. A threshold of N=0.06 s^(-1) is defined to represent the lower limit of the stratification strength for existence of hypoxia. An idealized one-dimensional model with parameterized respiration was applied to study stratification structure associated hypoxia development. The simulation results showed that the bottom mixed layer could enhance the bottom DO depletion and favor the development of hypoxia. A secondary pycnocline was above the bottom mixed layer and performed as a barrier to inhibit the vertical extension of hypoxia and vertical DO flux through it. Furthermore, I discussed possible mechanisms causing bottom mixed layer on the continental shelf, including return flow caused by upwelling, inertial motions and tidal currents. Diapycnal diffusivity was calculated using hourly continuous observations in the hypoxic zone on the Texas-Louisiana Shelf. The estimated time-averaged diapycnal diffusivity was 3 ? 10^(-6) m^(2) s^(-1) along the pycnocline. Averaged cross-pycnocline vertical dissolved oxygen flux was calculated in a two-layer stratified water column with subpycnocline hypoxia. The estimated cross-pycnocline dissolved oxygen flux was 156.8 ml l^(-1) d^(-1), which resupplied 45% dissolved oxygen consumption in the subpycnocline layer. Continuous observations at the South Marsh Mooring showed the first highly resolved observations of short timescale fluctuations in the DO concentrations in the seasonal hypoxic waters on the Louisiana Shelf. There were 19 ventilation events in the DO records with a time period of 1~3 days. Analysis of the time series of DO, salinity and temperature at different levels of the mooring demonstrated that most of the ventilation-intervals were associated with increased vertical mixing, which contributed 58% of the total ventilation intervals. It suggests that mixing events dominate the ventilation-intervals. Comparison of the time derivative of the low-frequency part of the near-bottom DO concentration to observations of the wind and significant wave height suggests that local wind events are responsible for the enhanced vertical mixing.Item Temporal influences of seasonal hypoxia on sediment biogeochemistry in coastal sediments(Texas A&M University, 2004-11-15) Sell, Karen S.Bottom water hypoxia and its influence on the environment have been topics of increasing concern for many coastal regions. This research addresses both spatial and temporal variability in sediment biogeochemistry at the southeastern region of Corpus Christi Bay, TX, where seasonal (summer) hypoxia occurs. Traditional techniques for determination of a variety of dissolved and solid components, benthic oxygen demand, and sulfate reduction rates were augmented by measurements using solid state microelectrodes to simultaneously determine concentrations of dissolved O2, Mn2+, Fe2+, and [sigma]H2S in multiple small - interval (1 mm) depth profiles of sediment microcosms. Oxygen concentrations in the overlying water were manipulated in the sediment microcosms and electrode depth profile measurements were made over ~ 500 hours of experimentation. Laboratory and field microelectrode results were in good agreement for both norm - oxic and anoxic time periods. Results indicated that iron (Fe2+) and sulfide ([sigma]H2S) were the redox reactive species in these sediments. During hypoxic conditions an upward migration of dissolved Fe2+and [sigma]H2S through the sediment column and, at times, into the overlying water was observed as the dissolved oxygen concentrations decreased. A corresponding decline in the vertical extent of these redox species occurred when the overlying water was re-oxidized. When both dissolved iron and sulfide coexisted, FeS minerals were formed in the sediment, preventing sulfide diffusion into the overlying water. However, after a long duration of hypoxia (> 200 hours) this buffering capacity was exceeded and both iron and sulfide penetrated into the overlying waters. Results indicated that iron may have a greater influence on hypoxia than sulfide because its concentration in the overlying waters during induced hypoxia was an order of magnitude greater than those of sulfide. Moreover, in the southeastern region of the Bay, where mixing was minimal and the water column was shallow, the sediments alone may have caused the onset of the hypoxic event in a relatively short time period (< 5.5 days). These results demonstrated that in shallow marine environments where seasonal hypoxia occurs, such as Corpus Christi Bay, the associated major changes that take place in the sediment biogeochemistry must be included in benthic - pelagic models for overlying water hypoxia.Item The Relationships of Particulate Matter and Particulate Organic Carbon with Hypoxic Conditions Along the Texas-Louisiana Shelf(2014-08-06) Zuck, Nicole AThe Mississippi-Atchafalaya river system discharges into the northern Gulf of Mexico and peaks during the spring freshet bringing high levels of nutrients that spur eutrophication in surface waters, often resulting in hypoxic (dissolved oxygen concentrations less than 1.4 mL/L) sub-pycnocline conditions. Hypoxia is generally manifest seasonally along the Louisiana coast over the shelf. In summer 2011, high rainfall in the Mississippi-Atchafalaya watershed caused high discharge into the northern Gulf of Mexico. In summer 2012 drought conditions in the watershed significantly reduced the discharge. Summer 2013 conditions returned to the climatologic average river discharge. Discrete samples were collected and continuous measurements were made via an onboard surface-water flow-through system, CTD casts, and by an undulating towed vehicle. Total particulate matter and particulate organic carbon samples were obtained from Niskin bottles on CTD casts. Samples were also taken to measure dissolved oxygen concentration and nutrients along with other hydrographic parameters. Water-column particulate matter and particulate organic carbon were analyzed to investigate the relationship between hypoxia and the composition of the particulate matter along the Texas-Louisiana shelf during different discharge rates in summer 2011 (for C:N of particulate matter), 2012 and 2013 for a larger suite of particle composition and relationship to hydrographic conditions of the Mississippi-Atchafalaya river system. The goal of this research is to determine if a statistically significant relationship is manifest, between shelf hypoxia and total particulate matter, as well as between particulate organic carbon and hypoxia, during periods of different river discharge (i.e. average, flood, or drought conditions). Correlations were identified between some variables, but no direct relationship between particulate matter and hypoxia was observed. There were however, some statistically significant changes in several parameters between summer 2013 and summer 2013.Item Wastewater Discharge, Nutrient Loading, and Dissolved Oxygen Dynamics in a Shallow Texas Bay(2014-05-07) Schroer, Lee AllenIn Oso Bay, a wastewater treatment plant acts as a source of eutrophication and may have measureable impact on the health of the bay. The objectives of this study were to create a model for modeling dissolved oxygen concentrations over time and to determine if eutrophication caused by the wastewater treatment plant is harmful to the bay. Continuous monitoring of environmental variables was carried out at 6 stations in Oso Bay over a 9-month period beginning in February and ending in December of 2013. Variables measured were water temperature (oC), pH, salinity (ppt), conductivity (mS), depth (meters), turbidity (nephilometric turbidity units), dissolved oxygen in both % saturation and concentration (mg/L), and chlorophyll-? concentration (?g/L). Grab samples of chlorophyll concentration (?g/L), total suspended solids (mg/L), and nutrient concentrations (?M) were also taken throughout the sampling period. Nutrients of interest were phosphate (PO_(4)), silicates (SiO_(4)), ammonium (NH_(4)), and nitrate+/-nitrite (NO_(x)). Hypoxia was observed at each of the stations in the bay and fluctuated on a diel cycle. Temperature, salinity, and temporal variability were significant factors in explaining the variance in dissolved oxygen concentrations (P < .0001) and were used to model dissolved oxygen variance (R^(2) = .7810). It is likely that the respiratory patterns of phytoplankton and bacteria also influence dissolved oxygen concentrations in Oso Bay, and that this is an indirect result of the discharge from the wastewater treatment plant.