Browsing by Subject "isotopes"
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Item Demographic and trophic dynamics of fishes in relation to hydrologic variation in channel and floodplain habitats of the Brazos River, Texas.(2009-06-02) Zeug, Steven ChristopherLarge rivers in North America have been subjected to a variety of hydrologic alterations that have negatively impacted aquatic fauna. These impacts have triggered restoration efforts, including management of flows, to restore or maintain ecological integrity. Ecological data relevant to flow management and habitat restoration is scarce, and conceptual models of ecosystem function have been widely applied to large rivers despite a lack of quantitative evaluation of these models. Here, I examine demographic and trophic dynamics of fishes with divergent life histories and trophic guilds in relation to habitat heterogeneity and flow variability in a relatively unaltered floodplain system: the Brazos River, Texas. Reproductive activity of fishes with three divergent life history strategies was positively associated with long-term river hydrology, although species with alternate strategies exploited different portions of the hydrograph (peak flow versus increasing flow). Despite the positive association with hydrology, low-flow periods were favorable for recruitment, and food resources for larvae and juveniles were denser during these periods. Some species used both oxbow and channel habitats during some point in their life cycle, whereas other species appeared to be almost entirely restricted to one habitat type. Terrestrial C3 macrophytes accounted for a significant fraction of the biomass of most consumer species examined. Small-bodied species in oxbow lakes assimilated large fractions of biomass from benthic algae, whereas this pattern was not observed in the river channel. Frequent flow variations in the river channel may reduce algal standing stocks, and significant contributions from autochthonous algal sources may only occur during low-flow periods. Trophic positions of detritivores indicated that terrestrial carbon sources were assimilated, for the most part, via invertebrates rather than by direct consumption. My results indicate that current conceptual models are too vague to provide accurate predictions for restoration strategies in rivers with variable flow regimes. Flow and habitat management strategies that focus on reproducing key features of historical fluvial dynamics are likely to be more successful than strategies that focus on single indicator species or flow dynamics that differ from the historical hydrograph.Item Modern Calibrations of Temperature and Nutrient Proxies for Paleoenvironmental Reconstructions in Tropical Mollusks(2014-12-15) Graniero, Lauren EMolluscan shell O- and C-isotope values have been shown to be useful indicators of upwelling and freshwater input (and thus nutrient status) in nearshore marine environments, but few studies have had the accompanying long term measurements of water ?^18O (?w) and dissolved inorganic carbon (DIC) ?^13C needed to best test the fidelity of shell isotope values as environmental indicators. Previously measured seawater ?w and ?^13CDIC values collected biweekly from 2011-2012 allow us to determine if shell ?^18O and ?^13C values record temperature, ?w, ?^13CDIC, and ultimately nutrient status in tropical waters. Conus, Vasum, and Strombus shell ?^18O values reflect predicted seasonal upwelling and freshening signals in the Pacific and seasonal freshening with minimal upwelling in the Caribbean. Both Pacific and Caribbean shell ?^13C profiles show cyclicity, but only Conus samples from Veracruz Beach (Pacific) record seasonal changes in ?13CDIC. This observation likely results from: (1) more distinct seasonal ?^13CDIC variation in Pacific waters compared with Caribbean waters and (2) greater availability of metabolic CO2 for shell growth in Strombus, which we hypothesize is related to greater activity associated with an herbivorous feeding habit. To examine N-isotope ratios (^15N/^14N) in tissues and shell organic matrix of bivalves as a proxy for natural and anthropogenic nutrient fluxes in coastal environments, Pinctada imbricata, Isognomon alatus, and Brachidontes exustus bivalves were live-collected and analyzed from eight sites in Bocas del Toro, Panama. Sites include a variety of coastal environments, including more urbanized, uninhabited, riverine, and oceanic sites. At all sites there is no single dominant source of organic matter contributing to bivalve ?^15N and ?^13C values. Bivalve ?^15N and ?^13C values likely represent a mixture of mangrove and seagrass N and C, although terrestrial sources cannot be ruled out. Despite hydrographic differences between end-members, we see minimal ?^15N and ?^13C difference between bivalves from the river-influenced Rio Guarumo site and those from the oceanic Escudo de Veraguas site, with no evidence for N from open-ocean phytoplankton in the latter. Lastly, ?^15N values of tissue and shell organic matrix correlate significantly for pterioideans P. imbricata and I. alatus. Thus for these species, N isotope studies of historical and fossil shells may provide records of ecology of past environments.Item Oxygen and Carbon Isotopes and Coral Growth in the Gulf of Mexico and Caribbean Sea as Environmental and Climate Indicators(2011-02-22) Wagner, Amy JoThe Gulf of Mexico and Caribbean Sea comprise a sensitive and important region, both oceanographically and climatically. A better understanding of the history of climate and marine environmental conditions in this region provides valuable insight into the processes that affect climate globally. This dissertation furthers our understanding of these factors via investigations of the isotopes of corals and seawater, as well as coral growth. Results improve our understanding of how the isotope and coral growth records from the Gulf of Mexico and Caribbean Sea reflect recent environmental conditions, enhancing our ability to reconstruct the history of climate in this important region. Analysis of the relationship between salinity and oxygen isotopic composition of seawater from the Texas/Louisiana continental shelf and Flower Garden Banks yield improved understanding of the relative contribution of the fresh water sources into the northern Gulf of Mexico, and also the oxygen isotopic composition of open-ocean seawater in this region. Variations in the growth of long-lived coral cores from the Flower Garden Banks are compared to local and regional climate conditions, particularly winter air temperatures. During the latter half of the twentieth century, a close correlation has existed between slow coral growth and cold wintertime air temperatures along the Gulf Coast, which are due to a meridional orientation of the North American jet stream (associated with the Pacific/North American climate pattern). Existing long coral growth records are too limited to assess this relationship during earlier years. Knowledge of the marine radiocarbon (14C) reservoir age is important for understanding carbon cycling and calibrating the radiocarbon ages of marine samples. Radiocarbon concentrations in corals from the Flower Garden Banks, Veracruz, and the Cariaco Basin are measured and used to determine the surface ocean 14C reservoir ages for the Gulf of Mexico and Caribbean Sea. Results also indicate that the post-nuclear weapons testing Delta 14-C values of the Gulf of Mexico and Caribbean Sea differ. This difference is attributed to the advection of 14C-depleted surface water from the Southern Hemisphere into the Caribbean Sea. The results reported in this dissertation provide valuable information for understanding the marine environment when using carbonate proxies to study and reconstruct past climate and marine conditions in the Gulf of Mexico and Caribbean.Item Sources and Fates of Dissolved Organic Carbon in Rural and Urban Watersheds in Brazos County, Texas(2012-10-19) Cioce, DanielleThe Bryan/College Station (B/CS) region has been reported to have elevated concentrations of dissolved organic carbon (DOC) in surface water. Increased DOC concentrations are worrisome as DOC has been shown to be an energy source for the recovery and regrowth of E. coli and many watersheds are impaired by high bacteria levels. To examine the sources and fates of DOC in rural and urban regions to better understand DOC movement though the environment, seven watersheds were studied. To investigate source, streams were analyzed using diffuse reflectance near infrared spectroscopy (DR-NIR) and carbon isotopes. Fate of DOC was determined through monthly streams samples, gathered between March 2011 and February 2012, which were incubated for biodegradable DOC (BDOC). Soil in the region was sampled based on land use categories. Soil was analyzed for DOC and BDOC as well as DOC adsorption, the other major fate of DOC. Above ground vegetation was sampled in conjunction with soil and analyzed for BDOC. Data indicated that fecal matter from cliff swallows provided considerable organic material to streams in the B/CS region as shown through DR-NIR. Carbon isotope values in streams ranged from -23.5 +/- 0.7% to -26.8 +/- 0.5%. Stream spectra may be able to predict carbon isotope values in streams (Adj. R2 = 0.88). Mean annual stream DOC concentrations ranged from 11 +/- 3 mg/L to 31 +/- 12 mg/L, which represents a significant decrease in DOC between 2007 and 2011. Concurrent increases in pH and conductivity were also recorded. The decrease in DOC and the increases in pH and conductivity may be due to impacts of high sodium irrigation tap water. Biodegradable DOC was low in streams, which is likely due to DOC being present in streams in refractory forms that are resistant to microbial breakdown. Soil chemistry, including soil adsorption, was greatly influenced by sodium. The elevated adsorption coefficients and release values seen in highly developed and urban open areas can be attributed to frequent exposure to high sodium irrigation water. The results indicate that sodium is a major driver of DOC in the system. Sound management decisions concerning irrigation water chemistry and urban development might eventually emerge to protect water quality as a result of this research.