Browsing by Subject "Hypoxia"
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Item Coastal Hypoxia on the Texas Shelf: An Ocean Observing and Management Approach to Improving Gulf of Mexico Hypoxia Monitoring(2013-05-07) Mullins, Ruth LouiseA combination of in situ sampling and real-time ocean observations was used to investigate the processes responsible for the formation and the areal extent of Texas coastal hypoxia from 2002 to 2011. In situ sampling, real-time mooring and buoy observations, and multivariate statistical modeling were used to investigate the physical processes driving hypoxia formation. Geostatistical interpolation (ordinary kriging) models were tested to compare the differences in annual hypoxia area on the Texas shelf. Results from these two sections were integrated into recommendations for improving federal hypoxia monitoring and mitigation strategies in the northwestern Gulf of Mexico. Winds, currents, temperature, salinity, and dissolved oxygen records revealed the annual, seasonal, and daily variability of hypoxia formation on the Texas coast from 2009 to 2011. Hypoxic events occurred from late May to late October lasting from hours to weeks. Hypoxia formation was either the result of salinity stratification, associated with the freshening of surface waters by the advection of Mississippi-Atchafalaya River freshwater westward or the wind- and current-driven upcoast or downcoast flow of Brazos River discharge. Records from 2010 and 2011 showed the variability and frequency of stratification development differs on the north and south Texas shelf. Multivariate linear model results showed contributing factors on the north Texas shelf vary annually and that primary factors for hypoxia development are near-surface current speeds and salinity-driven stratification. Interpolation models resulted in three size categories for hypoxia area: small (100 ? 1,000 km^2), moderate (1,001 ? 3,000 km^2), and large (3,001+ km^2). Moderate years include 2002, 2004, and 2007 and a large year was 2008. There was no increase in hypoxic area from years 2002 to 2011, but years 2007 and 2008 resulted in a hypoxic area over 5,000 km^2, which is the federally mandated hypoxia reduction target for the northwestern Gulf of Mexico. Geostatistical interpolators represent and predict the structure and spatial extent of the hypoxic area on the Texas shelf by accounting for the anisotropy of physical processes on the Texas shelf. Geostatistical interpolation models are preferred to deterministic models for developing and improving federal hypoxia monitoring and mitigation strategies on the northwestern Gulf of Mexico shelf.Item DEVELOPMENT OF A COASTAL MARGIN OBSERVATION AND ASSESSMENT SYSTEM (CMOAS) TO CAPTURE THE EPISODIC EVENTS IN A SHALLOW BAY(2010-07-14) Islam, Mohammad S.Corpus Christi Bay (TX, USA) is a shallow wind-driven bay which is designated as a National Estuary due to its impact on the economy. But this bay experiences periodic hypoxia (dissolved oxygen <2 mg/l) which threatens aerobic aquatic organisms. Development of the Coastal Margin Observation and Assessment System (CMOAS) through integration of real-time observations with numerical modeling helps to understand the processes causing hypoxia in this energetic bay. CMOAS also serves as a template for the implementation of observational systems in other dynamic ecosystems for characterizing and predicting other episodic events such as harmful algal blooms, accidental oil spills, sediment resuspension events, etc. State-of-the-art sensor technologies are involved in real-time monitoring of hydrodynamic, meteorological and water quality parameters in the bay. Three different platform types used for the installation of sensor systems are: 1) Fixed Robotic, 2) Mobile, and 3) Remote. An automated profiler system, installed on the fixed robotic platform, vertically moves a suite of in-situ sensors within the water column for continuous measurements. An Integrated Data Acquisition, Communication and Control system has been configured on our mobile platform (research vessel) for the synchronized measurements and real-time visualization of hydrodynamic and water quality parameters at greater spatial resolution. In addition, a high frequency (HF) radar system has been installed on remote platforms to generate surface current maps for Corpus Christi (CC) Bay and its offshore area. This data is made available to stakeholders in real-time through the development of cyberinfrastructure which includes establishment of communication network, software development, web services, database development, etc. Real-time availability of measured datasets assists in implementing an integrated sampling scheme for our monitoring systems installed at different platforms. With our integrated system, we were able to capture evidence of an hypoxic event in Summer 2007. Data collected from our monitoring systems are used to drive and validate numerical models developed in this study. The analysis of observational datasets and developed 2-D hydrodynamic model output suggests that a depth-integrated model is not able to capture the water current structure of CC Bay. Also, the development of a threedimensional mechanistic dissolved oxygen model and a particle aggregation transport model (PAT) helps to clarify the critical processes causing hypoxia in the bay. The various numerical models and monitoring systems developed in this study can serve as valuable tools for the understanding and prediction of various episodic events dominant in other dynamic ecosystems.Item Effect of instabilities in the buoyancy-driven flow on the bottom oxygen: Applications to the Louisiana Shelf(2009-05-15) Kiselkova, ValeriyaA combination of in situ sampling and numerical modeling was used to investigate the effects of mesoscale (<50 km) circulation patterns and stratification on the evolution of hypoxia on the Louisiana Shelf. Temperature, salinity, and dissolved oxygen concentrations records reveal the presence of an alongshelf meander, which is manifested vertically and horizontally as a wave-like distribution of the properties in the water column. The observations suggest the meander is a ubiquitous characteristic of the shelf with alongshore spatial scale approximately 50 km and less, which is consistent with the locations of sandy shoals along the coast and the local deformation radius. Twelve numerical experiments using an idealized three-dimensional shelf circulation model were performed to evaluate the relative importance of the variable bottom topography and freshwater forcing on the development, evolution, and scales of the dynamic instabilities. The inclusion of the shoals into the bottom topography showed the development of the dynamic instabilities as the flow passed over the shoals and downstream. Introduction of fresh water onto the shelf resulted in greater salinity differences, and, as a consequence in the formation of the dynamically unstable salinity fronts along the plume edge. The combination of the freshwater forcing and shoaling topography produced competing and complex interactions. Six numerical experiments were analyzed in order to investigate the effect of dynamic instabilities on spatial and temporal patterns of dissolved oxygen concentrations along the shelf. Although a linear relationship between Brunt-V?is?l? frequency and dissolved oxygen deficit was expected, a nonlinear loop-like relationship was discovered that reflects the response of biochemical properties to the alongshelf variability of the density field. Comparison of the numerical modeling runs to observations of density and dissolved oxygen concentrations on the Louisiana Shelf reinforces the importance of physical processes such as topographic steering and/or freshwater forcing on the alongshore distribution of physical and biochemical properties. It suggests that the time scales of respiration (~3 days) and buoyancy transfer processes (~5-7 days), associated with the physical processes that are responsible for water column stability and ventilation, are similar to the time scales associated with the benthic respiration rates.Item Elucidating the signal cascades induced by progestins that mediate sperm hypermotility in Atlantic croaker (Micropogonias undulatus) and southern flounder (Paralichthys lethostigma)(2013-12) Tan, Wenxian, active 21st century; Thomas, P. (Peter)The overall goal of this research was to verify the involvement of membrane progestin receptor alpha (mPRα) in mediating progestin-stimulated sperm hypermotility in the Atlantic croaker and southern flounder. Sperm motility in Atlantic croaker and southern flounder were tested with both the endogenous progestin, 17,20β,21-trihydroxy-4-pregnen-3-one (20β-S) or the selective mPRα agonist, 10-ethenyl-19-norprogesterone (Org OD 02-0). In croaker, the Pi3k/Akt/Pde and ErbB2/Mapk intracellular signaling pathways were examined. The role of mPRα in mediating sperm hypermotility and fertility in southern flounder was also studied. The effects of seasonal hypoxia on sperm motility in croaker were investigated in a field study in the northern Gulf of Mexico in the fall of 2010. Finally, the effects of acidified activator solution (simulating ocean acidification) were studied in the laboratory. In vitro, Org OD 02-0 mimicked the stimulatory actions of 20β-S in inducing sperm hypermotility and intracellular signaling cascades in croaker and flounder sperm, indicating that mPRα is the mediator of progestin signaling in the sperm of these species. In croaker sperm, both the Pi3k/Akt/Pde and ErbB2/Mapk intracellular signaling pathways were shown to be important mediators of progestin-induced sperm hypermotility, suggesting novel functions of G [subscript olf] βγ-subunits in teleost sperm. In flounder sperm, mPRα was shown to be important in mediating sperm hypermotility as only high motility sperm with high expression of mPRα were responsive to progestin stimulation, resulting in higher fertilization success compared to low motility sperm. A single LHRHa injection resulted in increased sperm motility and fertility, associated with an increase in mPRα expression in the sperm plasma membrane. The results also suggest that the mPRα/Acy/cAMP pathway first described in croaker sperm is present in flounder sperm. Field studies of male Atlantic croaker exposed to chronic seasonal hypoxia showed that hypoxia exposure resulted in smaller gonads, lower spermatogenesis, reduced testicular mPRα expression, and in some sites, reduced sperm motility. Studies with croaker sperm using acidified activator solution to simulate ocean acidification indicated that croaker sperm were sensitive to environmental insult. Furthermore, the results suggested that the progestin signaling mechanism is more sensitive to changes in ocean pH levels than the mechanism that controls sperm motility.Item Examination of the relationship of river water to occurrences of bottom water with reduced oxygen concentrations in the northern Gulf of Mexico(Texas A&M University, 2007-04-25) Belabbassi, LeilaSix years of comprehensive data sets collected over the northern continental shelf and upper slope of the Gulf of Mexico during the LATEX-A and NEGOM-COH programs showed that low-oxygen waters (<2.4 mL????L-1) are found only in spring and summer and only in water depths between 10 and 60 m. Four regions in the northern Gulf show considerable differences in the occurrence of low-oxygen waters. Lowoxygen waters are observed almost exclusively in regions subject to large riverine influences: the Louisiana and Mississippi-Alabama shelves. Hypoxic waters (oxygen concentrations <1.4 mL????L-1) are found only over the Louisiana shelf. No low-oxygen water is found over the Florida shelf which has minimum riverine influence. Lowoxygen water is found at only one station on the Texas shelf; this is during spring when the volume of low-salinity water is at maximum. The distributions of low-salinity water influenced the different distributions of low-oxygen and hypoxic waters in the four regions. Low-oxygen occurrences are clearly related to vertical stratification. Lowoxygen occurred only in stable water columns with maximum Brunt-V????is????l???? frequency (Nmax) greater than 40 cycles????h-1. When Nmax exceeded 100 cycles????h-1 in summer over the Louisiana shelf, oxygen concentrations dropped below 1.4 mL????L-1, and the bottom waters became hypoxic. Salinity is more important than temperature in controlling vertical stratification. Locations where temperature influence was larger were found in summer in water depth greater than 20 m over the Louisiana shelf, along the near shore areas of the Mississippi-Alabama shelf west of 87????W, and in the inner shelf waters of the Texas shelf. The extent of oxygen removal at the bottom of these stable water columns is reflected in the amount of remineralized silicate. Silicate concentrations are highest closest to the Mississippi River Delta and decrease east and west of the Delta. EOF analyses show that more than 65% of the oxygen variance is explained by the first mode. The amplitude functions of the first EOF modes of bottom oxygen, water column Brunt- V????is????l???? maxima, and bottom silicate are well correlated, indicating that much of the variance in bottom oxygen is explained by water column stratification and bottom remineralization.Item Hypoxia modeling in Corpus Christi Bay using a hydrologic information system(2009-05) To, Sin Chit; Maidment, David R.Hypoxia is frequently detected during summer in Corpus Christi Bay, Texas, and causes significant harm to benthic organism population and diversity. Hypoxia is associated with the density stratification in the Bay but the cause of stratification is uncertain. To support the study of hypoxia and stratification, a cyberinfrastructure based on the CUAHSI (Consortium of Universities for the Advancement of Hydrologic Science, Inc) Hydrologic Information System (HIS) is implemented. HIS unites the sensor networks in the Bay by providing a standard data language and protocol for transferring data. Thus hypoxia-related data from multiple sources can be compiled into a structured database. In Corpus Christi Bay, salinity data collected from many locations and times are synthesized into a three-dimensional space-time continuum using geostatistical methods. The three dimensions are the depth, the distance along a transect line, and time. The kriged salinity concentration in space and time illuminates the pattern of movement of a saline gravity current along the bottom of the Bay. The travel time of a gravity current in the Bay is estimated to be on the order of one week and the speed is on the order of 1 km per day. Statistical study of high-resolution wind data shows that the stratification pattern in the Bay is related to the occurrence of strong, southeasterly winds in the 5 days prior to the observation. This relationship supports the hypothesis that stratification is caused by the wind initiating hypersaline gravity currents which flow from Laguna Madre into Corpus Christi Bay. An empirical physical hypoxia model is created that tracks the fate and transport of the gravity currents. The model uses wind and water quality data from real-time sensors published by HIS to predict the extent and duration of hypoxic regions in the Bay. Comparison of model results with historical data from 2005 to 2008 shows that wind-driven gravity currents can explain the spatially heterogeneous patterns of hypoxic zones in Corpus Christi Bay.Item Modeling a gravity current in a shallow fluid system(2011-12) Kulis, Paula Sharon; Hodges, Ben R.; Maidment, David R.; Katz, Lynn E.; Reible, Danny D.; Raman, VenkatramanCorpus Christi Bay in Texas is a wind driven system, and under most conditions winds over the bay mix the water column vertically. However, seasonal, episodic, bottom-water hypoxia has been observed in the bay in conjunction with vertical salinity stratification. This stratification may be caused by dense gravity currents entering the bay. Understanding and modeling the mechanisms that result in stratification in Corpus Christi Bay may help predict hypoxia, and for this reason that is the focus of this dissertation. An evaluation of existing gravity current modeling techniques shows that most currently available models are designed to capture either phenomena local to a gravity current, such as gravity current entrainment and spreading, or larger scale phenomena such as wind mixing and large-scale circulation, but not both. Because gravity current mixing in Corpus Christi Bay is enhanced by wind-induced turbulence, both local gravity current physics and wind mixing effects are critical elements governing gravity current propagation in Corpus Christi Bay. As existing models do not represent gravity current entrainment and wind mixing together, this dissertation develops a coupled model system that accounts explicitly for turbulent wind mixing of a bottom-boundary layer, in addition to representing other local features of dense gravity current propagation such as entrainment and spreading. The coupled model system consists of a 2D depth-averaged hydrodynamic model that calculates gravity current mixing and spreading, coupled with a 3D hydrodynamic model whose domain includes a lighter ambient fluid surrounding the gravity current. The coupled models have flexible boundary conditions that allow fluid exchange to represent mixing from both gravity current entrainment and wind mixing. The coupled model system’s development, verification and application in Corpus Christi Bay advances understanding of gravity current mechanisms, and contributes to our scientific understanding of hypoxia in Corpus Christi Bay. This modeling technique has the flexibility to be applied to other density-stratified systems that are shallow and potentially wind-driven, such as shallow desalination brine disposal sites.Item Modulation by hypoxia of membrane steroid receptor expression and functions in ovaries of Atlantic croaker (Micropogonias undulatus)(2015-12) Ondricek, Kathryn Elise; Thomas, P.(Peter),; Black, Bryan; Walther, Ben; Rahman, Md SaydurHypoxia is an endocrine disruptor, altering estrogen, testosterone, and progestin hormone levels and stunting gonadal growth in Atlantic croaker. Steroids act through specific hormone receptors to alter reproductive functions, and the hormonal response is dependent on the concentrations of these receptors. However, information is currently lacking on the effects of hypoxia on expression and functions of membrane receptors mediating rapid, non-genomic steroid actions such as final oocyte maturation and apoptosis. Atlantic croakers were exposed to normoxia (7.0 mg DO/L) or hypoxia (1.7 mg DO/L) for 6 weeks during their period of gonadal recrudescence (October-December). Relative gene expression was quantified using quantitative real-time PCR (qRT-PCR). mRNA expression of the membrane androgen receptor, ZIP9, was increased in hypoxia-exposed fish compared to normoxia-exposed controls, whereas mRNA expression of the membrane estrogen receptor, GPER, and membrane progestin receptor, mPRα, was decreased in hypoxia-exposed fish compared to controls. mRNA expression of pro-apoptotic factors Bax and p53 was also measured and expression of both genes was increased in hypoxia-exposed fish compared to controls. Relative protein expression of these receptors was quantified using Western blotting and the results were consistent with the qRT-PCR findings. Oocytes from both hypoxia-exposed and control fish were tested in an in vitro final oocyte maturation (FOM) assay to examine possible alterations in receptor functions. When oocyte maturation was stimulated with progestin, which acts through mPRα, significantly fewer oocytes of hypoxia-exposed fish underwent FOM compared to oocytes of normoxia-exposed controls. These results are consistent with the decrease in mPRα expression following hypoxia exposure. Ovaries were sectioned and stained with hematoxylin and eosin, and the proportions of perinucleolar stage, tertiary yolk stage, and atretic oocytes were determined. Ovaries from fish exposed to hypoxia showed an increase in the proportion of perinucleolar stage and atretic oocytes and a decrease in the proportion of tertiary yolk stage oocytes compared to controls. Finally, apoptotic cells in ovarian tissue sections were labeled using in situ TUNEL staining. Ovaries from fish exposed to hypoxia showed an increased proportion of TUNEL-positive ovarian follicle cells compared to controls. Collectively, these results show the concurrence of increased ZIP9 expression and apoptotic follicle cells in ovaries of Atlantic croaker exposed to hypoxia in vivo.Item Stable Isotope Characterization and Proxy Records of Hypoxia-Susceptible Waters on the Texas-Louisiana Shelf(2012-02-14) Strauss, JosiahHypoxia, with dissolved oxygen levels < 1.4 ml L-1, is a recurring summer feature of Louisiana shelf bottom waters. Stable isotope characterization (delta^18O and delta D) of surface waters over the hypoxic zone shows a shift of dominant river influence from the Mississippi River during April to the Atchafalaya in July. Carbon isotopes of dissolved inorganic carbon (?13CDIC) in bottom waters reveal the respiration of terrestrial organic carbon (OC) at inshore localities of 10 m depth and the respiration of marine OC at depths equal to and greater than 20 m. delat^18O and delta^13C profiles of Louisiana shelf Conus shells collected in 1972 show no evidence for summer hypoxia. Comparison with modern Conus records reveal a delta^13CDIC reduction during the last four decades associated with intrusion of ^13C-depleted fossil fuel CO2. Summer delta^13C reductions in Texas shelf Pteria shells may imply dissolved oxygen (DO) was reduced by ?0.7 ml L-1, although this may be attributed to influence of Brazos River discharge on shell delta^18O and delta^13C. Foraminifera fauna measured in age-calibrated sediments from the Texas shelf reveal a low oxygen conditions on between 1960 and modern sediments. From 1950 to 1960, fauna indicate oxygenated bottom waters. Contemporaneous increases of foraminifera delta^13Cand delta^18O suggest this event is associated with severe drought (the Little Dust Bowl). The synchronicity of these data suggests a link between Brazos River discharge and shelf hypoxia.Item Statistical and Realistic Numerical Model Investigations of Anthropogenic and Climatic Factors that Influence Hypoxic Area Variability in the Gulf of Mexico(2012-07-16) Feng, YangThe hypoxic area in the Gulf of Mexico is the second largest in the world, which has received extensive scientific study and management interest. Previous modeling studies have concluded that the increased hypoxic area in the Gulf of Mexico was caused by the increased anthropogenic nitrogen loading of the Mississippi River; however, the nitrogen-area relationship is complicated by many other factors, such as wind, river discharge, and the ratio of Mississippi to Atchafalaya River flow. These factors are related to large-scale climate variability, and thus will not be affected by regional nitrogen reduction efforts. In the research presented here, both statistical (regression) and numerical models are used to study the influence of anthropogenic and climate factors on the hypoxic area variability in the Gulf of Mexico. The numerical model is a three-dimensional, coupled hydrological-biogeochemical model (ROMS-Fennel). Results include: (1) the west wind duration during the summer explain 55% of the hypoxic area variability since 1993. Combined wind duration and nitrogen loading explain over 70% of the variability, and combined wind duration and river discharge explain over 85% of the variability. (2) The numerical model captures the temporal variability, but overestimates the bottom oxygen concentrations. The model shows that the simulated hypoxic area is in agreement with the observations from the year 1991, as long as hypoxia is defined as oxygen concentrations below 3 mg/L rather than below 2 mg/L. (3) The first three modes from an Empirical Orthogonal Function (EOF) analysis of the numerical model output results explain 62%, 8.1% and 4.9% of the variability of the hypoxic area. The Principle Component time series is cross-correlated with wind, dissolved inorganic nitrogen concentration and river discharge. (4) Scenario experiments with the same nitrogen loading, but different duration of upwelling favorable wind, indicate that the upwelling favorable wind is important for hypoxic area development. However, a long duration of upwelling wind decreases the area. (5) Scenario experiments with the same nitrogen loading, but different discharges, indicate that increasing river discharge by 50% increases the area by 42%. Additionally, scenario experiments with the same river discharge, but different nitrogen concentrations, indicate that reducing the nitrogen concentration by 50% decreases the area by 75%. (6) Scenario experiments with the same nitrogen loading, but different flow diver- sions, indicate that if the Atchafalaya River discharges increased to 66.7%, the total hypoxic area increases the hypoxic area by 30%, and most of the hypoxic area moved from east to west Louisiana shelf. Additionally, if the Atchafalaya River discharge decreased to zero, the total hypoxic area increases by 13%. (7) Scenario experiments with the same nitrogen loading, but different nitrogen forms, indicate that if all the nitrogen was in the inorganic forms, the hypoxic area increases by 15%. These results have multiple implications for understanding the mechanisms that control the oxygen dynamics, reevaluating management strategies, and improving the observational methods.Item Sub-lethal effects of hypoxia on harpacticoid copepod reproduction(2010-05) Ryckman, Laura Yukiko Chu; Buskey, Edward Joseph, 1952-; Montagna, Paul A.; Gardner, Wayne S.; Thomas, Peter; Fleeger, John W.Areas of hypoxia are found in coastal areas worldwide, and have become increasingly widespread. These areas vary in their duration and dissolved oxygen concentration from occasional diurnal hypoxia, as found in Corpus Christi Bay, Texas, seasonal hypoxia as in the northern Gulf of Mexico, to continuous hypoxia as found in oceanic oxygen minimum zones. The effects of exposure to low dissolved oxygen (DO) depend on the duration of exposure, the DO concentration and an organism’s tolerance to hypoxic conditions. Most studies have focused on lethal effects of hypoxia by comparing the abundance of benthic organisms and the species composition of benthic communities between hypoxic and normoxic areas. Sub-lethal effects of such as changes in reproduction may occur at less severe hypoxic conditions (by definition), but may still have effects at the population level. The goal of this study is to examine the sub-lethal reproductive effects of low DO on harpacticoid copepods. The life-history traits and reproductive biology of meiobenthic harpacticoid copepods make this group of organisms useful as test organisms to measure the sub-lethal effects of hypoxia on reproduction. It is hypothesized that changes in reproductive traits may be observed at DO concentrations higher than those that cause lethal effects because of the high energetic cost of female harpacticoid reproduction which may result in reduced fitness. Laboratory studies were conducted to examine the effects of low DO concentrations on survival, egg production, and the number and size of eggs. Harpacticoid population abundance, biomass and the abundance of ovigerous females were measured from field samples collected across sites that varied in their degree and duration of exposure to low DO. To contrast the effects of hypoxia in environments with different hypoxic exposures, field studies were carried out in the northwestern Gulf of Mexico in an area with near constant summer hypoxia, and in southeast corner of Corpus Christi Bay, Texas which experiences intermittent hypoxia often for less than one hour in the early morning hours during summer. At both field sites total copepod abundance, biomass and the abundance of ovigerous females were reduced at the most hypoxic site type compared the reference sites. In Corpus Christi Bay, total and ovigerous female abundance varied with exposure to hypoxia, but the relative declines in abundance and biomass were less dramatic than in the northern Gulf of Mexico. In the northern Gulf of Mexico, the percentage of ovigerous females within the total harpacticoids collected was reduced (p<0.001) to 3% at the Intermediate and 1% at the Hypoxic sites during the summers, but in the fall of 2007 at all transects had similar percentages (p = 0.81) of ovigerous females which ranged from 13 to 16%. Reference sites had a higher number of families with ovigerous females, and the relative abundances of those families were more evenly distributed at Reference sites compared to Hypoxic and Intermediate transects. Patterns of ovigerous female abundance with respect to DO concentration were similar across numerous harpacticoid families found at the study sites. Harpacticoid copepods in laboratory studies tolerated near anoxic DO conditions for longer than 120 hours. Average survival rates of the harpacticoids Schizopera knabeni and Nitokra affinis during periods of low DO with elevated ammonium concentrations were less than 50%, while survival rates for the other treatments were near 100%. The formation of an egg mass was reduced in the near anoxia treatment, but egg clutches did not form at all in the treatment that combined near anoxia and 10 micromolar ammonium. The DO concentration and presence of ammonium were important factors to the severity of population decline that were illustrated by population estimate calculations. Population estimates based on laboratory measurements of Nitokra affinis resulted in calculations of reduced survival and reproductive rates related to hypoxic exposure greatly decrease the potential population of the group, which may affect the copepod population’s ability to recover from hypoxic events. The effects of hypoxia on harpacticoid copepods are the result of the length of exposure, the concentration of low dissolved oxygen and exposure to other chemical fluxes that increase during periods of hypoxia. The measurement of reproductive indicators, such as the presence of an egg mass and the proportion of ovigerous females in a population, could be used to indicate the adverse environmental effects of low dissolved oxygen exposure. The studies from this dissertation are the first to document reproductive effects of low dissolved oxygen on harpacticoid populations in the field. Laboratory studies within this dissertation indicate the interaction between ammonium and hypoxia on survival and reproduction in harpacticoids. Future studies are needed to further determine the effects of hypoxia on the whole-life cycle of harpacticoid copepods.Item The Role of Particulate Matter in the Development of Hypoxia on the Texas-Louisiana Shelf(2013-07-31) Cochran, Emma MaryIn the northern Gulf of Mexico, hypoxia occurs annually during the summer on the Texas-Louisiana shelf. This study examines the distribution of particulate and dissolved components relative to hydrography, to better understand the processes controlling the development of hypoxia. Particulate matter on the Texas-Louisiana Shelf has three major sources ? river plumes, primary production, and resuspended sediments. The sources and processes controlling distribution and transport of particles are investigated using optical proxies (backscattering, chlorophyll fluorescence, Colored Dissolved Organic Matter fluorescence (CDOM)), temperature, salinity, dissolved oxygen (DO), and in-situ sampling during June and August 2011 cruises of the Mechanisms Controlling Hypoxia program (hypoxia.tamu.edu). Discrete samples of particulate matter (PM) and particulate organic carbon (POC) concentration were obtained for analysis and calibration of optical instruments interfaced with a profiling CTD, a towed undulating CTD (Acrobat), and the ship?s flow-through system along the shelf from south of Galveston, Texas, to east of the Mississippi delta. The results of this study support a previously hypothesized concept of three primary areas of organic and inorganic particle composition and processes that dominate those areas ? river-dominated water, highly productive surface waters, and clear, nutrient-poor low-productivity surface waters. The distribution and bulk composition of particulate matter in the northern Gulf of Mexico, plus the distribution of chlorophyll fluorescence and CDOM suggest that subpycnocline primary production plays a role in determining oxygen concentration in subpycnocline waters away from the river-dominated water.