Browsing by Subject "Gulf coast"
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Item Freshwater inflows in the Nueces Delta, TX : impacts on porewater salinity and estimation of needs(2012-05) Stachelek, Joseph Jeremy; Dunton, Kenneth H.; Liu, Zhanfei; Ward, GeorgeEstuarine wetlands and salt marshes are fundamentally driven by variations in freshwater inflow. In semi-arid salt marshes, such as the Nueces River Delta, TX, the stochastic nature of freshwater inflow events exposes resident organisms to a wide range of environmental conditions. In this study, we investigate (1) the relative importance of environmental variables on porewater salinity and (2) determination of freshwater inflow needs based on the response of emergent plants to salinity variations. Porewater salinity variations were tracked on a continuous basis with deployed conductivity sensors and on a synoptic basis with soil water extracts. We found that spatial patterns of porewater salinity were characterized by a high degree of variability in creekbank areas (23.8 ± 7.68) relative to interior marsh areas (44.2 ± 3.4). Our observations were used to test a simple model capable of predicting porewater salinities based on environmental variables. Both empirical measurements and model simulations indicated that semiannual tides play a critical role in controlling porewater flushing from precipitation and freshwater inflow events. Estimation of freshwater inflow needs for the Nueces Delta proceeded in two steps. First, we examined the response of three common emergent plants species (Borrichia frutescens, Spartina alterniflora, and Salicornia virginica) to variations in salinity. The abundance of one species in particular (S. alterniflora) was tightly coupled to salinity variations whereby salinities exceeding 25 ± 5 resulted in dramatic declines in coverage. Next, the relationship between freshwater inflow and porewater salinity was examined with respect to the salinity “tolerance” of S. alterniflora. Estimated inflow needs based on maintenance of substantial (> 20%) S. alterniflora coverage was comparable to both previous inflow needs estimates and mean annual inflows observed over the course of the study. The results of this study suggest that S. alterniflora abundance provides a reliable indicator of overall estuarine hydrological condition in the Nueces Delta.Item Holocene depositional history of Lavaca Bay, central Texas Gulf coast(1975) Byrne, J. R.; Jonas, E. C. (Edward Charles), 1924-Sediments in Lavaca Bay represent a texturally fining upward marine transgressive sequence. The entire sediment sequence is 120 feet (36.4 m) thick at the mouth of Lavaca Bay. Gravel and sand from the Lavaca-Navidad fluvial-deltaic complex comprise the oldest Holocene sediments in the bay. Successively younger sediments are represented by up to 80 feet (24.2 m) of bay-estuarine mud. The bay-estuarine sediments reflect three major mud sources. These are: (1) suspended load transported by the Colorado-Brazos fluvial complex, which was discharged into marine and estuarine bodies over 50 miles (83 km) to the northeast and carried southwest by longshore currents, (2) erosion of Pleistocene deltaic sediments exposed along the bay margins, and (3) suspended load transported by the Lavaca-Navidad fluvial complex, discharging into the head of Lavaca Bay. Comparison of the volumes of mud presently filling this Pleistocene valley, with calculated volumes of mud that have been supplied from these potential sources has resulted in a rough argillaceous sediment budget for this coastal estuary. Sufficient mud had been made available from the Lavaca-Navidad fluvial complex and the bluffs exposed along the bay margins to completely fill Lavaca Bay since Recent stillstand of the Gulf of Mexico. However, the sediment accumulation rate in Lavaca Bay is slow due to the transport of significant quantities of sediment out of Lavaca Bay by waves and surface currents. In addition, lithologic characteristics and facies relationships of the respective muds indicate that discharge of suspended load by the Colorado-Brazos fluvial complex had been an important source as well. Progradation of the Lavaca-Navidad fluvial-deltaic complex since Recent stillstand has resulted in a regressive sediment sequence which overlies transgressive sediments at the head of Lavaca Bay. The clay mineral suite in sediments in Lavaca Bay reflects clay minerals in the source area. No changes in the clay minerals were detected as a result of increasing salinity, sediment depth and time. X-ray diffraction data indicate significant potassium fixation by montmorillonite after treatment with 1N potassium chloride. Sediment facies are the main control on trace element distribution in Lavaca Bay. In general, high trace element concentrations are correlative with clay sediment content and, to a lesser degree, organic carbon content. Data suggest that both "detrital" and "nondetrital" trace elements contribute to trace element accumulations in Lavaca BayItem Pelican Swarm - by The Reserve(2015-11-10) Mission-Aransas National Estuarine Research ReserveItem Texas Coastal Zone Issues, PRP 39(LBJ School of Public Affairs, 1980) Spurr, Stephen H.