Browsing by Subject "Stable isotopes"
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Item Controls on the stable isotopic composition of speleothems, Barbados, West Indies(2004) Mickler, Patrick John; Banner, Jay L.; Stern, LibbyApplications of speleothem calcite geochemistry in climate change studies require the evaluation of the accuracy and sensitivity of speleothem proxies to most correctly infer paleoclimatic information. The present study of Harrison’s Cave, Barbados, uses the analysis of the modern climatology and groundwater system to evaluate controls on the C and O isotopic composition of modern speleothem calcite. This new approach directly compares the δ18O and δ13C values of modern speleothem calcite formations with the values for their corresponding drip waters in order to assess the degree to which isotopic equilibrium is achieved during calcite precipitation. Carbon isotope values for the majority of modern speleothem calcite samples from Harrison’s Cave fall within the range of equilibrium values predicted from the combined use of 1) calcite-water fractionation factors from the literature, 2) measured temperatures, and 3) measured δ13C values of the dissolved inorganic carbon of drip waters. Enrichments and depletions in 13C, relative to equilibrium C isotopic compositions, are also observed. The 13C depletions are likely caused by kinetically driven departures in the fractionation between HCO3- (aq) and CaCO3 from equilibrium conditions, caused by rapid calcite growth. 13C enrichments can be accounted for by Rayleigh distillation of the HCO3- (aq) reservoir during degassing of 13C-depleted CO2. In contrast to the C isotopic results, most modern speleothem calcites from Harrison’s Cave are not in O isotopic equilibrium with their corresponding drip water and are enriched in 18O relative to equilibrium values. δ18O variations of modern calcite are likely controlled by kinetically driven changes in the fractionation between HCO3- (aq) and CaCO3 from inferred equilibrium conditions to non-equilibrium conditions, consistent with rapid speleothem calcite growth. In contrast to δ13C, the effects of Rayleigh distillation on the δ18O values of modern calcite are buffered by CO2 hydration and hydroxylation reactions that influence the O isotopic composition of the HCO3- (aq) reservoir. The effects of Rayleigh distillation manifest themselves in samples taken along a growth layer by producing a progressive enrichment away from the growth axis with a constant δ13C vs. δ18O slope. This observation has significance to ancient speleothem studies. A review of the literature has found that 62% of 141 studies show positive δ13C vs. δ18O correlation consistent with our non equilibrium models.Item Effects of the Arundo donax L. on Hydrological Regime of the Rio Grande Basin(2012-07-16) Li, FanThis study investigated the role of an invasive tall cane, Arundo donax L. (Arundo), in the riparian water cycle. Four 100 meter transects were arrayed perpendicular to the lower Rio Grande in southwest Texas. The first objective was to determine the primary water source for Arundo by using naturally occurring stable isotopes. Surface soil, river water, groundwater, precipitation and rhizome samples were collected every month during 2010 and 2011 growing seasons, which coincided with a major flood that saturated soils in the first year followed by extreme drought in the second year. The second objective was to characterize how Arundo water use varied with water availability gradients in the riparian zone. Leaf gas exchange and leaf delta13C were measured along potential moisture gradients. The third objective was to understand the interaction between groundwater and surface water, and whether Arundo water use affected daily groundwater fluctuations. The isotope ratio of rhizome water was consistent with shallow soil moisture uptake and with previous observations of a relatively shallow, fibrous root system. Floodwater from July 2010 persisted in the soil for at least a year despite a severe drought, and became the dominant water source for Arundo during much of the study period. Although the alluvial water table in this floodplain was shallow (< 6 m) and subject to changes in river level, groundwater seemed not to be an important source for Arundo, so long as the soil moisture was sufficient. In this study, Arundo was not found to experience soil moisture limitation, and the spatial variability of Arundo transpiration was not associated with any soil moisture availability gradients. Arundo was found to close its stomata in response to increasing vapor pressure deficit (VPD), causing declining transpiration rate and increasing leaf delta13C composition. Significant exchange between the river and the alluvial groundwater was reflected in the similarity of isotopic compositions and the high correlation between river and groundwater elevations. Cross correlation analysis showed that over 50% of the diurnal groundwater fluctuations were caused by river stage changes. Consistent with the above ecophysiological and stable isotope results, Arundo water use was not found to influence daily groundwater fluctuations.Item Freshwater contributions and nitrogen sources in a South Texas estuarine ecosystem : a time-integrated story from stable isotope ratios in the eastern oyster (Crassostrea virginica)(2012-05) Bishop, Karen Anne; McClelland, James W.; Dunton, Kenneth H.; Walter, BenjaminChanges in freshwater inputs due to water diversions and increased urbanization may alter the function and properties of estuarine ecosystems in South Texas. Freshwater and nitrogen inputs from the Mission and Aransas rivers to the federally designated Mission-Aransas National Estuarine Research Reserve (Mission-Aransas NERR) have received considerable attention in the past few years. However, freshwater inputs from two rivers (the San Antonio and Guadalupe rivers) that combine and drain into a neighboring bay (San Antonio Bay) may also provide a substantial nitrogen source to Aransas Bay, which is within the boundaries of the Mission-Aransas NERR. In order to study the influence of the San Antonio and Guadalupe rivers, an oyster species, Crassostrea virginica, was chosen to provide time-integrated information about freshwater contribution as a nitrogen source within the bays. Chapter One addresses variations in isotope values ([delta]¹⁵N and [delta]¹³C) in oyster adductor muscle tissue from 2009-2011 along a sampling transect from the head of San Antonio Bay through Aransas Bay. Stable carbon isotope values increased linearly from approximately -25 % to -17 %, while stable nitrogen isotope values decreased from approximately +16 % to +10 % along this transect. The patterns in stable carbon and nitrogen isotope values are consistent with substantial mixing of river-supplied water and nitrogen from San Antonio Bay into Aransas Bay. Variations in nitrogen isotopic signature between periods of sustained drought and flood conditions were relatively small, suggesting that riverine nitrogen contributions were similar regardless of the amount of freshwater inflow observed during the time frame of this study. Chapter Two addresses the isotopic equilibration time for adult oyster adductor muscle tissue using a year-long transplant experiment (November 2010-November 2011). Full representation of ambient water isotopic composition in oyster adductor muscle tissues was determined to occur roughly a year after transplant. Oyster adductor muscle could therefore be useful for long-term monitoring of nitrogen contribution from freshwater sources, and would be valuable to include in concert with water sampling and analysis of other tissues that have shorter integration rates for a comprehensive view of an estuarine system.Item Habitat use and trophic structure of Atlantic tarpon (Megalops atlanticus) inferred from geochemical proxies in scales(2016-05) Seeley, Matthew Edward; Black, Bryan A.; Walther, Benjamin D.; Fuiman, Lee AAtlantic tarpon, Megalops atlanticus, are highly migratory euryhaline predators that occupy different habitats throughout life. Atlantic tarpon are known to inhabit oligohaline waters, although the frequency and duration of movements across estuarine gradients into these waters are poorly known. This species supports over a two billion dollar industry within the Gulf of Mexico and is currently listed as vulnerable by the International Union for the Conservation of Nature (IUCN). Analysis of trace element and stable isotope compositions of growth increments in fish scales is a non-lethal method for reconstructing migrations across estuaries in vulnerable species. We analyzed Atlantic tarpon scales from the Texas coast to validate this method using inductively coupled plasma mass spectrometry (ICP-MS) for trace elements and isotope ratio mass spectrometry (IR-MS) for stable isotope ratios. Multiple scales and otoliths were taken from the same individual to confirm the consistency of elemental and isotopic uptake within the same individual and between structures. Results show that scale Sr/Ca and δ13C are effective proxies for salinity, while increases in δ15N are consistent with known trophic shifts throughout life history. Patterns of elemental concentrations and isotope values across scales within an individual were consistent with each other. Scale and otolith transects contained the same overarching trend with comparable shifts in elemental concentrations across growth increments in the two structures. Migratory contingents, or groups within distinct populations that exhibit different patterns of habitat use and movement across salinity gradients, were identified. The distribution of contingents indicated that migratory behavior is highly variable, with some, but not all fish transiting estuarine gradients into oligohaline waters. Yet, the majority of individuals sampled exhibited early life residency in oligohaline waters. This work demonstrates the use of low salinity habitats by Atlantic tarpon. Our validation of the methods for analyzing scales will provide novel opportunities to monitor fish migrations across salinity gradients.Item Hydrological and ecological observations along the eastern Beaufort Sea coast of Alaska(2015-12) Harris, Carolynn Maxene; Dunton, Kenneth H.; McClelland, James W; Hardison, Amber KThe US Beaufort Sea coast is fringed by barrier islands, which enclose numerous bays and lagoons that provide habitat for migratory fish and waterfowl that are essential to the subsistence and culture of Iñupiat communities of northern Alaska. In the eastern Beaufort, in particular, residents of Kaktovik rely heavily on the high benthic productivity of these lagoon systems to support the fish and bird populations that they depend on for subsistence. We monitored aspects of hydrology and ecology in several lagoons in the eastern Alaskan Beaufort Sea coast from 2011 to 2014. Our hydrographic data from continuous in situ measurements of temperature and salinity revealed that lagoons possess unique regimes that vary seasonally and reflect unique hydrologic and geomorphic characteristics. H2O-δ18O and salinity measurements revealed that meteoric water is the dominant source of low-salinity water in all lagoons in June and August, though sea ice melt water was also present. Most differences among lagoons were caused by variation in circulation and connection to adjacent marine waters. We also used stable C and N isotopic analysis to determine trophic structure and assess dependence of fauna on terrestrial (CT) vs. marine sources of carbon, with particular focus on animals widely used by local subsistence hunters. Our mixing model results suggest that terrestrial carbon (CT) may be assimilated by upper trophic level consumers, though marine-derived C sources dominate. For example, 15 - 70% of polar bear and 0 - 60% of beluga whale carbon was of terrestrial origin. Our results suggest that 1) CT assimilated by benthic omnivorous invertebrates is transferred to the highest trophic levels in the Beaufort Sea, and 2) arctic cod are the most likely intermediary for transferring CT from lower to upper trophic levels.Item Individual specialization and assortative mating in undifferentiated populations(2012-12) Snowberg, Lisa Kathryn; Bolnick, Daniel; Kirkpatrick, Mark, 1956-Individual specialization occurs when individuals selectively consume a subset of their population's diet. Intraspecific diet variation can stabilize population and community dynamics, promote species coexistence, and increase ecosystem productivity. Ecological variation also provides the variability necessary for natural or sexual selection to act. Individual threespine stickleback select different prey from a shared environment, and this variation is not simply a result of sex, size, or spatial heterogeneity. I use longitudinal observation of stickleback foraging microhabitat to support more commonly used cross-sectional metrics. Among recaptured individuals there were correlations between microhabitat use and functional morphology, and microhabitat use and long term dietary differences between individuals. I quantify individual specialization across populations using cross-sectional sampling to understand how and why ecological variation may itself be variable. All populations showed significant individual specialization. Specialization varied between populations and this variation seems to be a long-term property of populations. Overall morphological variance was positively correlated with ecological variation. Ecological variation, like all types of heritable variation, provides raw material for evolutionary change. For example, lacustrine populations of stickleback are commonly under disruptive selection due to intraspecific competition for prey resources. Speciation with gene flow may be driven by a combination of positive assortative mating and disruptive selection, particularly if selection and assortative mating act on the same trait. We present evidence that stickleback exhibit assortative mating by diet, using the isotopes of males and eggs within their nests. In concert with disruptive selection, this assortative mating should facilitate divergence. However, the population remains phenotypically unimodal, highlighting the fact that assortative mating and disruptive selection do not guarantee evolutionary divergence and speciation. There are several not-mutually-exclusive mechanisms by which assortative mating by diet may occur in these populations, such as shared microhabitat preference among individuals of similar diet. Stable isotopes reveal diet differences between different nesting areas and among individuals using different nest habitat within a nesting area. Spatial segregation of diet types may generate some assortative mating, but is insufficient to explain the observed assortment strength. We therefore conclude that sticklebacks' diet-assortative mating arises primarily from behavioral preference rather than from spatial isolation.Item Investigating pedogenic carbonate formation by measuring the stable isotope composition of water in Vertisols(2014-05) Okafor, Brandon Jerrod; Breecker, Dan O.; Young, Michael H; Banner, JayThe oxygen isotope compositions of pedogenic carbonates in paleosols are used to reconstruct paleoelevations, paleoatmospheric circulation, paleotemperatures, and paleoprecipitation. The oxygen isotope compositions of pedogenic carbonates are controlled by temperature and the oxygen isotope composition of soil water, which predominately originates from precipitation. In most calcic soils studied, pedogenic carbonates record the oxygen isotope composition of summer precipitation and/or mean annual precipitation subjected to evaporation. However, due to the complex hydrological properties of Vertiols, which are abundant in the rock record, the isotopic composition of soil water could potentially vary and could influence the isotopic composition of pedogenic carbonate. Furthermore, it is well established that soils contain multiple pools of water with different stable isotope compositions but little work has been done to investigate which pools are recorded by pedogenic carbonates. Therefore, the isotopic composition of soil water in modern Vertisols was monitored and compared with the oxygen isotope composition of pedogenic carbonate in the same soils to investigate if the oxygen isotope composition of pedogenic carbonates in Vertiols record mobile or immobile water. The isotope composition of soil water was determined in four ways: 1) measurement of isotope composition of water collected by vacuum distillation of soil samples collected by auger, 2) calculation from measured oxygen isotope compositions of soil CO₂, 3) calculation from measured oxygen isotope compositions of pedogenic carbonate, and 4) measured isotope compositions of water collected under tension in a soil solution sampler. The oxygen isotope compositions of water in equilibrium with CO₂ and water from the solution sampler were indistinguishable at 140cm and were interpreted as mobile water in macropores. The vacuum distilled water (which includes water from a mixture of macropores and micropores) always had lower δ ¹⁸O values than the macropore water and the other sampling methods, implying the presence of water with low δ ¹⁸O values. These oxygen isotope compositions of soil water pools were compared with δ ¹⁸O values of local precipitation (GNIP data from nearby Waco, TX). Below ~100cm, total soil water δ ¹⁸O values converge to -6.3 ± 0.7 %₀ (1σ, n=20), which is isotopically lighter than the δ ¹⁸O of mean annual precipitation (MAP) of Waco, Texas (-3.8 ± 2.7 %₀, 1σ, n=96). This could result from recharge of isotopically light September precipitation (SEPT); (-5.9 ± 2.4 %₀, 1σ, n=8)) replenishing the soil after dry periods and/or the contribution of winter precipitation (WP) (-5.5 ± 2.4 %₀, 1σ, n=25). The δ ¹⁸O values of soil water in equilibrium with soil CO₂ ((-4.1 ± 0.8%₀) are isotopically similar to or heavier than the isotopic composition of MAP. The δ ¹⁸O values of soil water in equilibrium with pedogenic carbonate (-2.7 ± 0.9%₀) are also isotopically similar to the isotopic composition of summer precipitation (SP, including June, July, and August) (-2.0 ± 2.9 %₀, 1σ, n=8). This suggests that, despite the more complex hydrology of Vertisols compared with other soils orders, the δ ¹⁸O values of pedogenic carbonates formed in central Texas Vertisols record SP and/or mean annual precipitation that has been subjected to evaporation, just as they do in other soils. If this holds true for Vertisols formed in other climates, then this facilitates the comparison among δ ¹⁸O values of paleosol carbonates from various soil orders, which is common practice in vertical successions of paleosols. Furthermore, the observation that the σ ¹⁸O values of water in equilibrium with pedogenic carbonate are more similar to the σ ¹⁸O values of macropore than micropore water suggests that pedogenic carbonates in central Texas Vertisols may form in macropores. Formation in macropores is more consistent with CO₂ degassing and/or evaporation, rather than root water uptake, as a proximal driver of calcite precipitation.Item Linking fish migration and hypoxia exposure to trophic ecology using natural chemical tags(2015-08) Mohan, John Austin; Walther, Benjamin D.; McClelland, James W; Shank, G. Christopher; Thomas, Peter; Limburg, KarinNatural tags in fish reveal life history information that includes migration pathways and dietary sources. The chronological and geochemical properties of otoliths record changes in fish growth and reflect ambient salinity gradients, allowing reconstruction of coastal and estuarine movements. Stable isotopes of carbon and nitrogen in liver and muscle tissues provide dietary histories over short and long time scales, respectively. In order to apply natural tags, validation studies are needed that examine incorporation dynamics of elements and isotopes, such as the effects of physiology and growth on element uptake and isotope turnover, and describe chemical gradients of dissolved elements over time and space. Along the coast of Texas, dissolved Sr:Ca and Ba:Ca varied predictably with salinity over broad geographic and annual time scales. Tissue isotope turnover experiments revealed faster turnover of liver compared to muscle, offering short- and long-term dietary indicators. Using established estuarine ion gradients and tissue turnover rate estimates, estuarine ingress and habitat residence patterns were examined in Atlantic croaker collected from positive, metastable, and negative estuary types, encompassing the north-south Texas coast. Migration patterns varied depending on local salinity regimes, with strong regional gradients exhibited in both otolith elements and tissue isotopes. Fine scale between bay movements were detected using differences in liver-muscle δ¹³C values as indicators of tissue equilibrium status and time since habitat shift. In the northern Gulf of Mexico, the long-term sublethal effects of seasonal hypoxia on fish populations remain unclear, due to unknown exposure histories. Hypoxic redox conditions promote release of dissolved Mn²⁺ from the sediment, thus otolith Mn:Ca profiles can reflect lifetime hypoxia exposure. Lab experiments demonstrated a minimal influence of physiology on element uptake in response to hypoxia. Field studies in the northern Gulf of Mexico quantified estuarine habitat use using otolith Ba:Ca and coastal hypoxia exposure using otolith Mn:Ca, and assessed isotope trophic niche area using muscle tissue δ¹⁵N and δ¹³C values. Isotope niche area was similar between coastal and hypoxia exposed fish, suggesting trophic resilience of Atlantic croaker to seasonal hypoxia. This research provides a novel multi-proxy approach; using lab and field validated natural tags for linking migration and environmental exposure histories to trophic ecology in a marine fish.Item Natal origin of atlantic bluefin tuna (thunnus thynnus) from the gulf of st. lawrence using δ13c and δ18o in otoliths(2009-05-15) Schloesser, Ryan WalterIncreased knowledge of stock mixing and migration patterns of Atlantic bluefin tuna (Thunnus thynnus) is required to appropriately manage and conserve declining populations. The nursery origin of giant bluefin tuna present in the Gulf of St. Lawrence was identified using stable carbon (?13C) and oxygen (?18O) isotopes in sagittal otoliths. Anthropogenic and natural processes are capable of impacting atmospheric and oceanic concentrations of ?13C and ?18O, affecting otolith concentrations. Therefore, inter-decadal variation of ?13C and ?18O in the otolith cores (corresponding to the first year of life) of bluefin tuna was examined prior to stock predictions and temporal variability was detected in both isotope ratios. Significant changes in both ?13C and ?18O were recorded in the otolith cores of individuals with birthdates between 1947 and 2003. Both ?13C and ?18O varied significantly as a function of year of birth, with ?13C decreasing and ?18O increasing over the time period investigated (-2.39?10-2 and 5.78?10-3 per year, respectively). The rate of change in otolith ?13C was nearly identical to the reported rates of atmospheric ?13C depletion, recently attributed to the burning of fossil fuels (referred to as the Suess effect). Observed shifts in otolith ?18O were less pronounced and likely linked to changing physicochemical conditions (i.e. salinity) in oceanic reservoirs over the time period investigated. The results show that otolith cores of bluefin tuna effectively track inter-decadal trends and record past oceanic ?13C and ?18O levels. After adjusting for inter-decadal trends, the isotopic composition of milled otolith cores of giants from three decades (1970s, 1980s, 2000s) and three regions were compared to otolith ?13C and ?18O values of yearling bluefin tuna collected from eastern and western nurseries. Maximum likelihood estimates indicated that 99% of bluefin tuna caught in the Gulf of St. Lawrence fishery originated from the western nursery, with no significant differences among the decades and regions examined. Results suggest that little to no mixing of eastern and western populations of adult bluefin tuna occurs in the Gulf of St. Lawrence, making it important for the management and conservation of the declining western population.Item Natural fracture cementation in the Marcellus Formation(2013-05) Pommer, Laura Elizabeth; Gale, Julia F. W.; Eichhubl, PeterIn order to test the hypothesis that fractures in outcrops are equivalent to subsurface fracture systems I compare fracture cement morphology, texture, mineralogy and geochemistry from a suite of outcrop samples from Union Springs, NY, with fractures in four cores from a currently producing reservoir in southwest Pennsylvania. Transmitted light-microscope petrography and cold cathodoluminescence of calcite of outcrop and core samples reveals a variety of cement morphologies including crack-seal and blocky fracture cement textures that are interpreted as a record multiple repeated stages of fracture opening and sealing, as well as fibrous calcite fill and other mineral phases. The stable isotopic composition of calcite fracture cements from different fracture types in cores and outcrop range from -21.5 to +4.4‰ δ13C PDB and -8.0 to -12.0 ‰ δ18O PDB and indicate calcite precipitation temperatures between 46 and 89°C. Fluid inclusion microthermometry from secondary fluid inclusions indicates trapping temperatures between 110 and 120°C. Microprobe analysis of fracture calcite cement indicates a range in Fe, Mn, and Mg composition, with subsurface and outcrop cement of similar composition. Assuming burial history predicts thermal history, isotopic compositions together with fluid inclusions suggest calcite precipitated in vertical fractures during prograde burial, during the Acadian to early Alleghanian orogenies. These findings indicate that fractures in outcrops of the Marcellus Formation can be used as a proxy for those in the subsurface.Item Oxygen isotope evidence for interaction of Franciscan high-grade blocks in the mantle wedge with sediment derived fluids, Ring Mountain (Tiburon) and Jenner Beach, California(2012-08) Errico, Jessica Cori; Barnes, Jaime D.; Cloos, Mark; Catlos, ElizabethOxygen isotopes and major and trace element geochemistry have been used to evaluate the geochemical and tectonic history of a Franciscan hornblende-amphibolite and a eclogite block from Ring Mountain, Tiburon and three eclogite/blueschist blocks from Jenner Beach, California, all blocks have experienced varying amounts of retrogression. Relative to the presumed basaltic protolith, enrichments in large ion lithophile elements (LILEs) indicate interaction with sediment derived fluids in the retrograde eclogite and retrograde blueschist samples and high Mg, Cr, and Ni in actinolite rind indicate interaction with ultramafic rock. The [delta]¹⁸O values of chlorite from the Ring Mountain hornblende-amphibolite and the eclogite block have a narrow range of [delta]¹⁸O values (+7.7-8.2%₀, n=8) and actinolite from actinolite rind on the eclogite block from Ring Mountain and the blueschist/eclogite blocks from Jenner Beach are (+7.8-8.5%₀, n=5). Chlorite-actinolite geothermometry yields temperatures of 200-280°C for actinolite rind formation. Additionally, the [delta]¹⁸O values of both chlorite and actinolite at these temperatures indicates equilibrium with the measured value of Tiburon serpentinites, (7.6 to 8.1%₀, n = 3 Wenner and Taylor, 1974). Oxygen isotope analyses of garnet mineral separates from the eclogite and hornblende-amphibolite from Ring Mountain have [delta]¹⁸O values of +6.8±0.3%₀ (n=7), and +8.2±0.2%₀ (n=7), respectively. Garnets from the three eclogite/blueschist blocks at Jenner Beach have a [delta]¹⁸O value of +9.8±0.7%₀, (n=23). The difference in [delta]¹⁸O values of garnets between the high-grade blocks is likely due to in situ hydrothermal alteration of the seafloor basalt prior to subduction. The geochemical trends can be explained by a model in which during the early stages of subduction pieces of altered oceanic crust are detached from the downgoing slab and incorporated into the mantle wedge soon after reaching peak eclogite or amphibolite facies conditions. As subduction continues, the hanging wall cools and fluids released from subducted sediments infiltrate the overlying mantle wedge. As the blocks cool they develop a retrograde blueschist facies overprint under relatively static conditions. With cooling of the hanging wall and infiltration of sedimentary fluids, serpentinization induces reaction between the blocks and surrounding mantle wedge and Mg-rich actinolite rind is formed. The blocks are then plucked from the mantle wedge and incorporated into the subduction channel where they flow back to the surface via corner flow.Item Patterns in seagrass coverage and community composition along the Texas coast : a three-year trend analysis(2015-05) Wilson, Sara Susan; Dunton, Kenneth H.; Buskey, Edward J.; Maidment, David R.Seagrasses are extremely productive coastal plant communities that serve as habitat for various types of marine and estuarine fauna and provide numerous ecosystem services. Seagrass meadows around the world have become threatened by environmental and anthropogenic pressures such as altered hydrologic regimes, physical disturbances, and eutrophication. Monitoring programs that provide high-resolution information and document changes in cover, morphometric characteristics, species composition, and tissue nutrient content across large spatial scales are critical in global conservation and management efforts. In an attempt to address the uncertainties regarding the current distribution and condition of seagrasses in the southwest Gulf of Mexico, I conducted annual sampling from 2011-2013 to examine seagrass cover and condition at 558 permanent stations. Sampling occurred in three regions of the Texas coast: the Coastal Bend (CB), Upper Laguna Madre (ULM), and Lower Laguna Madre (LLM), which together comprise over 94% of the seagrasses in Texas. Significant trends in seagrass coverage and tissue elemental composition were highly location- and species-specific. In the CB, I did not observe significant changes in seagrass cover and no spatial patterns in tissue nitrogen (N) or phosphorus (P) were apparent. However, I observed a species shift in the northern ULM, where significant decreases in Syringodium filiforme cover were coupled with significant increases in Halodule wrightii cover. Long-term salinity records at four stations throughout the study area suggest that S. filiforme mortality in the ULM in 2013 was a product of an extended period of high salinity (> 55) that began in late 2012. In LLM, there were significant increases in H. wrightii cover in the north and significant decreases in T. testudinum cover in the south, which cannot be explained based on underwater light levels, salinity, or nutrient availability. Both H. wrightii and T. testudinum displayed lower C:N, C:P, and N:P ratios, along with enriched δ¹⁵N signatures nearest urban areas, particularly in the LLM. This study illustrates the value of integrating rapid-assessment field sampling and rigorous statistical and spatial analysis into a large-scale seagrass monitoring program to uncover patterns in seagrass community structure. I detected significant trends in seagrass coverage and condition across multiple spatial and temporal scales, including a massive species replacement that coincided with a prolonged period of hypersaline conditions.Item Trophic Ecology of Frugivorous Fishes in Floodplain forests of the Colombian Amazon(2012-10-19) Correa Valencia, Sandra BibianaDiverse fish species consume fruits and seeds in the Neotropics, in particular in the lowland reaches of large rivers, such as the Amazon, Orinoco, and Parana in South America. Floodplains of the Amazon River and its lowland tributaries are characterized by marked hydrological seasonality and diverse assemblages of frugivorous fishes, including closely related and morphologically similar species of several characiform families. Here, I investigated whether or not these fishes are capable of detecting fluctuations in food availability and if they are, how they adjust their feeding strategies. I tested predictions of optimal foraging, limiting similarity and resource partitioning theories with regard to expansion or compression of niche breadth and reduction in trophic niche overlap among species in relation with fluctuations in the availability of alternative food resources. I monitored fruiting phenology patterns to assess food availability and conducted intensive fishing during the high-, falling-, and low-water seasons in an oligotrophic river and an adjacent oxbow lake in the Colombian Amazon. I combined analysis of stomach contents and stable isotope ratios to evaluate dietary patterns, niche breadth, and niche overlap. Diets of six characiform fish species (Brycon falcatus, B. melanopterus, Myloplus asterias, M. rubripinnis, and M. torquatus) changed in a manner that indicated responses to fluctuations in food availability. Feeding strategies during the peak of the flood pulse were consistent with predictions of optimal foraging theory. During times of high fruit abundance, fish preferentially consumed items to which their phenotype is best adapted, maximizing net energy gain and enhancing fitness. As the flood pulse subsided and the availability of forest food resources was reduced in aquatic habitats, there was not a consistent pattern of diet breadth expansion or compression, even though diet shifts occurred, suggesting interspecific differences in foraging efficiencies. Analyses of diets and isotopic ratios revealed a general pattern of increased dietary segregation as the water level receded. Although there never was complete niche segregation among these fishes, these dietary changes effectively reduced interspecific niche overlap. Implications of these results and contribution of allochthonous food resources to diversity maintenance of floodplain fishes are discussed.Item Trophodynamics of the benthic food webs in the Chukchi and Beaufort Seas, Alaska(2013-12) McTigue, Nathan David; Dunton, Kenneth H.The Chukchi and Beaufort Sea shelves host diverse and productive seafloor ecosystems important for carbon and nitrogen cycling for the Arctic Ocean. The benthic food web transfers energy from primary producers to high trophic level organisms (e.g., birds, fish, and mammals), which are important for cultural practices and subsistence hunting by Native Alaskans. This work focuses on the trophic ecology of arctic food webs through use of several different approaches. First, variation in the natural abundance of stable carbon and nitrogen isotopes facilitated the identification of trophic pathways and, subsequently, allowed the comparison of trophic guilds and food webs from the Chukchi and Beaufort Seas. Compared to water column and sedimentary organic matter end-members, second trophic level grazers and suspension feeders were conspicuously ¹³C-enriched throughout the Chukchi Sea, which supports the hypothesis that microbial degradation of organic matter occurred prior to metazoan assimilation. Second, food web recovery from disturbances caused by exploratory oil drilling at the seafloor that had occurred approximately 20 years prior were assessed in both the Chukchi and Beaufort Seas. Based on isotopic trophic niche overlap between organisms common to drilled and reference sites in the Chukchi and Beaufort Seas, the oil drilling sites had similar food web structure, indicating recovery from the activity associated with the drilling process. Third, photosynthetic pigment biomarkers were used to better understand the diagenetic process, specifically focusing on how both microbial and metazoan grazing pathways degrade organic matter in relation to seasonal sea ice retreat in the Chukchi Sea. The benthic macrofaunal and microbial food web caused rapid degradation of organic matter upon the initial pulse of microalgal food sources to the seafloor. These diagenetic pathways are linked to the ¹³C-enrichment of residual organic matter, which corresponds to the stable isotope values measured in the benthic macrofauna. Lastly, high-precision liquid chromatography and spectrophotometry were compared for estimating sedimentary pigments in the marine environment. Substantial differences in pheopigment (chlorophyll degradation products) concentrations were observed between the two techniques, suggesting the need for revisions to the monochromatic spectrophotometric equation that relates absorbance to pigment concentrations. One pheopigment, pheophorbide, was found to interfere with the accuracy of the spectrophotometric equation and caused the overestimation of pheopigments.Item The use of δ]¹³C values of leporid teeth as indicators of past vegetation(2013-05) Wicks, Travis Zhi-Rong; Shanahan, Timothy M.; Bell, Christopher J., 1966-Records of change of [delta]13C values in vertebrate teeth offer an opportunity to gain insight into changes in past vegetation. Increasingly, teeth from small mammals are used for such purposes, but because their teeth grow very rapidly, seasonal changes in vegetation potentially provide a large source of variability in carbon isotope composition, complicating interpretations of small mammal tooth isotope data. To investigate the controls of seasonality on the stable isotope composition of fossil teeth, we constructed a Monte-Carlo-based model to simulate the effects of changes in the seasonal pattern of diet in leporid lagomorphs (rabbits and hares) on the distribution of [delta]¹³C values in random populations of leporid teeth from the Edwards Plateau in central Texas. Changes in mean-state, seasonal vegetation range, and relative season length manifest themselves in predictable ways in the median, standard deviation, and skewness of simulated tooth [delta]¹³C populations, provided sufficient numbers of teeth are analyzed. This Monte Carlo model was applied to the interpretation of a 20,000 year record of leporid tooth [delta]¹³C values from Hall's Cave on the Edwards Plateau in central Texas. Variations in the [delta]¹³C values of teeth deposited at the same time (standard deviation = 1.69%) are larger than changes in the mean vegetation composition reconstructed from bulk organic carbon [delta]¹³C, indicating the influence of short-term variability, making it difficult to assess changes in mean C3/C4 vegetation from the tooth [delta]¹³C data. However, populations of teeth from different climate intervals (e.g., the late Glacial, Younger Dryas, and the Holocene) display changes in the shape of the tooth [delta]¹³C distributions. Interpretation of these changes as shifts in seasonal vegetation patterns that are based upon results from our model are consistent with hypothesized climatic changes. An increase in the standard deviation of the tooth population between the late Glacial and the Younger Dryas -- Holocene is consistent with an increase in seasonality. Furthermore, a shift to more C3-dominated vegetation in the tooth [delta]¹³C distribution during the Younger Dryas is accompanied by a more skewed population -- indicative of not only wetter conditions but an increase in the duration in the C3 growing season. However, late Holocene changes in vegetation are not clear in the tooth data, despite the evidence from bulk organic carbon [delta]¹³C values for an increase in % C3 vegetation of 57%. Small mammal teeth can potentially provide unique insights into climate and vegetation on seasonal and longer timescales that complement other data, but should be interpreted with a careful consideration of local conditions, taxon ecology and physiology, and the dominant timescales of isotope variability.Item Watershed export events and ecosystem responses in the Mission-Aransas National Estuarine Research Reserve(2009-08) Mooney, Rae Frances, 1982-; McClelland, James W.; Dunton, Kenneth H.; Maidment, David R.River export has a strong influence on the productivity of coastal waters. During storm events, rivers deliver disproportionate amounts of nutrients and organic matter to estuaries. Anthropogenic changes to the land use/cover (LULC) and water use also have a strong influence on the export of nutrients and organic matter to estuaries. This study specifically addressed the following questions: 1) How does river water chemistry vary across LULC patterns in the Mission and Aransas river watersheds? 2) How do fluxes of water, nutrients, and organic matter in the rivers vary between base flow and storm flow? 3) How do variations in nutrient/organic matter concentrations and stable isotope ratios of particulate organic matter (POM) in Copano Bay relate to river inputs? Water was collected from the Mission and Aransas rivers and Copano Bay from July, 2007 through November, 2008 and analyzed for concentrations of nitrate, ammonium, soluble reactive phosphorus (SRP), dissolved organic nitrogen, dissolved organic carbon, particulate organic nitrogen, particulate organic carbon (POC), and the stable C and N isotope ratios of the POM. The first half of the study period captured relatively wet conditions and the second half was relatively dry compared to long term climatology. Riverine export was calculated using the USGS LOADEST model. The percentage of annual constituent export during storms in 2007 was much greater than in 2008. Concentration-discharge relationships for inorganic nutrients varied between rivers, but concentrations were much higher in the Aransas River due to waste water contributions. Organic matter concentrations increased with flow in both rivers, but POM concentrations in the Aransas River were two fold higher due to large percentages of cultivated crop land. Values of [delta]¹³C-POC show a shift from autochthonous to allochthonous organic matter during storm events. Following storm events in Copano Bay, increases and quick draw down of nitrate and ammonium concentrations coupled with increases and slow draw down of SRP illustrate nitrogen limitation. Organic matter concentrations remained elevated for ~9 months following storm events. The [delta]¹³C-POC data show that increased concentrations were specifically related to increased autochthonous production. Linkages between LULC and nutrient loading to coastal waters are widely recognized, but patterns of nutrient delivery (i.e. timing, duration, and magnitude of watershed export) are often not considered. This study demonstrates the importance of sampling during storm events and defining system-specific discharge-concentration relationships for accurate watershed export estimation. This study also shows that storm inputs can support increased production for extended periods after events. Consideration of nutrient delivery patterns in addition to more traditional studies of LULC effects would support more effective management of coastal ecosystems in the future.