Watershed export events and ecosystem responses in the Mission-Aransas National Estuarine Research Reserve

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Title: Watershed export events and ecosystem responses in the Mission-Aransas National Estuarine Research Reserve
Author: Mooney, Rae Frances, 1982-
Abstract: River export has a strong influence on the productivity of coastal waters . During storm events , rivers deliver disproportionate amounts of nutrients and organic matter to estuaries . Anthropogenic changes to the land use /cover (LULC ) and water use also have a strong influence on the export of nutrients and organic matter to estuaries . This study specifically addressed the following questions : 1 ) How does river water chemistry vary across LULC patterns in the Mission and Aransas river watersheds ? 2 ) How do fluxes of water , nutrients , and organic matter in the rivers vary between base flow and storm flow ? 3 ) How do variations in nutrient /organic matter concentrations and stable isotope ratios of particulate organic matter (POM ) in Copano Bay relate to river inputs ? Water was collected from the Mission and Aransas rivers and Copano Bay from July , 2007 through November , 2008 and analyzed for concentrations of nitrate , ammonium , soluble reactive phosphorus (SRP ) , dissolved organic nitrogen , dissolved organic carbon , particulate organic nitrogen , particulate organic carbon (POC ) , and the stable C and N isotope ratios of the POM . The first half of the study period captured relatively wet conditions and the second half was relatively dry compared to long term climatology . Riverine export was calculated using the USGS LOADEST model . The percentage of annual constituent export during storms in 2007 was much greater than in 2008 . Concentration -discharge relationships for inorganic nutrients varied between rivers , but concentrations were much higher in the Aransas River due to waste water contributions . Organic matter concentrations increased with flow in both rivers , but POM concentrations in the Aransas River were two fold higher due to large percentages of cultivated crop land . Values of [delta]¹³C -POC show a shift from autochthonous to allochthonous organic matter during storm events . Following storm events in Copano Bay , increases and quick draw down of nitrate and ammonium concentrations coupled with increases and slow draw down of SRP illustrate nitrogen limitation . Organic matter concentrations remained elevated for ~9 months following storm events . The [delta]¹³C -POC data show that increased concentrations were specifically related to increased autochthonous production . Linkages between LULC and nutrient loading to coastal waters are widely recognized , but patterns of nutrient delivery (i .e . timing , duration , and magnitude of watershed export ) are often not considered . This study demonstrates the importance of sampling during storm events and defining system -specific discharge -concentration relationships for accurate watershed export estimation . This study also shows that storm inputs can support increased production for extended periods after events . Consideration of nutrient delivery patterns in addition to more traditional studies of LULC effects would support more effective management of coastal ecosystems in the future .
URI: http : / /hdl .handle .net /2152 /ETD -UT -2009 -08 -294
Date: 2011-02-16

Citation

Watershed export events and ecosystem responses in the Mission-Aransas National Estuarine Research Reserve. Master's thesis, The University of Texas at Austin. Available electronically from http : / /hdl .handle .net /2152 /ETD -UT -2009 -08 -294 .

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