Browsing by Subject "mollusks"
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Item Community structure of deep-sea bivalve mollusks from the northern Gulf of Mexico(Texas A&M University, 2004-09-30) Chen, MinDensity, species diversity, species richness, and evenness of bivalve mollusks were measured in the deep (0.2km to 3.7km) northern Gulf of Mexico to describe the community structure of benthic bivalve mollusks. Density decreased gradually from shallow continental slope depths, with remarkably high values in the Mississippi canyon, to the deepest sites. Diversity of bivalve mollusks increased from shallow continental slope depths, with low values in the Mississippi canyon, to a maximum at intermediate depths (1-2km), followed by a decrease down to the deepest locations (3.7km). Nine distinct groups were formed on the basis of the similarity in species composition. The pattern varied more abruptly on the slope compared to the deeper depths, possibly due to steeper gradients in physical variables. ANOVA indicated that the density of bivalve mollusks was not significantly different at different depths, was not significantly different on different transects, was not significantly different between basin and non-basin, but was significantly different in canyon and non-canyon locations. Similar distinctions were observed in diversity, except that basins were lower than non-basins. The patterns observed reflect the intense elevated input of terrigenous sediments accompanied by high surface-water plankton production from the Mississippi River to the north central gulf.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 Seasonal isotope and trace-metal profiles of serially-sampled Conus gastropods: proxies for paleoenvironmental change(Texas A&M University, 2006-08-16) Gentry, David KeithWe test the fidelity of shallow-water gastropod skeletons as multi-proxy archives of seasonal paleo-environmental change by performing isotopic and trace-metal analyses on specimens of Conus ermineus from the Gulf of Mexico. Four adult specimens were collected from Stetson Bank in the Flower Garden Banks National Marine Sanctuary during the summer of 2002. Shell samples were milled along axes of growth to produce time-series profiles spanning up to eight years. We corrected the profiles for growth rate effects and compared the tuned results with in situ temperature and salinity records at the reef surface and temperature profiles from nearby surface buoys. Examination of sample densities in δ18O cycles shows that shell growth is faster during summers and slower during winters. Tuning the profiles versus time yields δ18O values that co-vary closely with seasonal temperatures to a high degree of coherency (R2 = 0.84). The δ13C profiles show cyclic variation modified by ontogenetic decreases in δ13C. These ontogenetic trends are attributable to decreasing metabolic efficiency, while seasonal cycles reflect hydrographic changes in the gastropods?? habitat. Salinity and δ13C of dissolved inorganic carbon show a strong correlation at Stetson Bank (R2 = 0.80), and early summer shell δ13C minima coincide with local salinity minima during times of peak river discharge. The terminations of these δ13C minima occur during annual upcoast reversals of shelf currents in this area. These effects are augmented by summer stratification and productivity minima that further decrease seawater δ13C. Sr/Ca ratios increase through ontogeny, most likely due to decreasing metabolic efficiency. However, seasonal variations in Sr/Ca profiles show strong similarity with δ18O profiles, confirming the temperature dependence of Sr/Ca and minimal influence of salinity on shell δ18O at Stetson Bank. The results of this study show that tuned δ18O and Sr/Ca profiles can be used to reconstruct seasonal paleotemperatures. Carbon isotope profiles and environmental data also demonstrate the utility of Conus δ13C as a proxy for freshwater flux and shelf circulation.