HR: 0800h
AN: B31A-0982    [Abstracts]
TI: Nitrogen sources and cycling in the San Francisco Bay Estuary: A nitrate dual isotopic composition approach
AU: * Wankel, S D
EM: sdwankel@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd., MS 434, Menlo Park, CA 94025 United States
AU: * Wankel, S D
EM: sdwankel@usgs.gov
AF: Stanford University, Dept. Geological and Environemental Science, 320 Braun Hall, Stanford, CA 94305 United States
AU: Carol, K
EM: ckendall@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd., MS 434, Menlo Park, CA 94025 United States
AU: Chris, F A
EM: caf@stanford.edu
AF: Stanford University, Dept. Geological and Environemental Science, 320 Braun Hall, Stanford, CA 94305 United States
AU: Paytan, A
EM: apaytan@stanford.edu
AF: Stanford University, Dept. Geological and Environemental Science, 320 Braun Hall, Stanford, CA 94305 United States
AB: The dual isotopic composition of nitrate (δ¹⁵N and δ¹⁸O) was utilized for the first time within the estuarine system of San Francisco (SF) Bay, CA, in order to explore the utility of this approach for tracing sources and cycling of nitrate. Surface water samples from 47 sites within the estuary were sampled during a cruise during July-August, 2004. Spatial variability in the isotopic composition suggests multiple sources of nitrate (NO3) to the Bay ecosystem including seawater, several distinct rivers and creeks, and sewage effluent. The spatial distribution of nitrate from these sources is heavily modulated by the hydrodynamics of the estuary. Mixing along the estuarine salinity gradient is the main control on the spatial variations in isotopic composition of nitrate within the northern arm of SF Bay. However, due mostly to the long residence time during the summer study period, the nitrate isotopic composition in the southern arm of SF Bay exhibited a combination of source mixing and phytoplankton drawdown. Very low δ¹⁸O-NO3 values (as low as -6.1%) at the Sacramento-San Joaquin River Delta region give rise to a wide range of δ¹⁸O-NO3 values in the SF Bay system. The range in δ¹⁸O-NO3 values is more than twice that of δ¹⁵N-NO3, suggesting that δ¹⁸O-NO3 is an even more sensitive tool for tracing nitrate sources and cycling than δ¹⁵N-NO3.
DE: 0442 Estuarine and nearshore processes (4235)
DE: 0469 Nitrogen cycling
DE: 0470 Nutrients and nutrient cycling (4845, 4850)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
SC: Biogeosciences [B]
MN: Fall Meeting 2005