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Submitted in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY in COASTAL MARINE SYSTEM SCIENCE
The objectives of the present study were to identify the effects of freshwater inflow (FWI) on sediment transport and nutrients supply to estuaries, and the effects of sediments on nutrient supply at sediment-water interface. The Nueces River and its estuary, the Guadalupe River and its estuary, and the Lavaca-Colorado Estuary were selected for field studies. Laboratory experiments were conducted to test the study hypotheses, and data was analyzed using multivariate modeling and statistical approaches. In the Guadalupe Estuary, variability of suspended solids and nutrients were correlated with FWI, whereas in the Nueces and Lavaca-Colorado Estuary they were related with seasonal differences. Suspended solids in the estuaries increased during frontal events and during windy days. In some of the field samplings, stations closer to the Gulf of Mexico had higher suspended sediments than the stations closer to the river. The increase in suspended sediments increased ammonia concentration in the Guadalupe Estuary, phosphorus and silicate concentrations in the Nueces Estuary, and silicate concentration in the Lavaca-Colorado Estuary. The presence of silicate minerals in the estuaries maintains silicate concentration as well. Organic matter and calcium carbonate shells in sediments of the estuaries can bind phosphate, thus, may have played role in decreasing phosphate concentration in the water. Guadalupe River sediments, when transported to the estuary, can release ammonia at higher concentrations the river water concentration. Salinity in the estuary, thus, has a significant role in controlling nutrient concentrations. The release of ammonia by organic matter decomposition was lower in the Guadalupe Estuary than in the Nueces Estuary. The high inflow volumes in the Guadalupe Estuary may have washed away organic matter from the sediments and may have disturbed bacterial community resulting in the lower release of ammonia from Guadalupe Estuary sediments. The research performed here demonstrates the importance of sediments, organic matter, and inflow in maintaining nutrient concentrations in estuaries. Fluctuation of these nutrients can affect water quality, and hence, may influence the ecology of the estuary.
Physical and Environmental Sciences
College of Science and Engineering