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Abstract:
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Changes 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 . |