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Biogeochemical processes in land-ocean interfaces, surface estuaries, subterranean estuaries and sediment-water interface
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Spatial-temporal variations of water column nitrogen in the tidal freshwater zone of the Aransas River, South Texas
Monday 7th @ 1530-1550, Conference Room 1
Hengchen Wei* , University of Texas Marine Science Institute, Port Aransas, TX
Xin Xu, University of Texas Marine Science Institute, Port Aransas, TX
Allan E Jones, Department of Geological Sciences, University of Texas at Austin, Austin, TX, USA
Kevan B. Moffett, School of the Environment, Washington State University Vancouver, Vancouver, WA, USA
Amber K Hardison, University of Texas Marine Science Institute, Port Aransas, TX
James W McClelland, University of Texas Marine Science Institute, Port Aransas, TX
Presenter Email: hwei@utexas.edu |
| Riverine nitrogen (N) loadings are responsible for eutrophication in many coastal and marine waters. However, estimates for riverine N loadings generally do not account for the biogeochemical processing in tidal freshwater zones (TFZs), which may alter amounts and forms of N at the river/estuary interface. In this research, we studied the potential role of TFZs as hotspots for N losses and transformations by measuring concentrations and building budgets for N in waters above (non-tidal) and within the TFZ of the Aransas River in South Texas. Sampling was conducted during two winters and two summers in 2015-2017. Dissolved inorganic nitrogen (DIN) concentrations were higher in winter than summer, whereas particulate organic nitrogen (PON) concentrations were not significantly different between seasons. PON concentrations increased from non-tidal to TFZ waters during both seasons. These increases in PON were accompanied by large decreases in DIN. Dissolved organic nitrogen (DON) decreased from non-tidal to TFZ, but remained stable within the TFZ. Mass balances were built for the TFZ with regards to different forms of N. On average, during baseflow conditions (i.e. excluding storm events) the Aransas TFZ received 12,263 kg TN (8,724 kg DIN, 2,906 kg DON, 633 kg PON) from upstream, and delivered 5,518 kg TN (2,860 kg DIN, 1,453 kg DON, 1,205 kg PON) to estuarine waters downstream of the TFZ, which represented 55% removal for TN (68% DIN, 50% DON, -90% PON). Overall, these results suggest that the TFZ removed more than half of the TN and transformed DIN into PON at the river/estuary interface. Continuing to overlook TFZ systems could lead to large errors in N concentrations and forms when calculating riverine N export to estuaries. |
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