HR: 15:05h
AN: H52C-06    [PDF]
TI: New Stable Isotopic Techniques of Nitrate Applied to Investigation of Estuarine Nitrate Sources in Elkhorn Slough, CA
AU: * Wankel, S D
EM: sdwankel@stanford.edu
AF: Stanford University, 320 Braun Hall Dept of Geol and Env Sciences, Stanford, CA 94305 United States
AU: * Wankel, S D
EM: sdwankel@stanford.edu
AF: US Geological Survey, 345 Middlefield Rd. MS 434, Menlo Park, CA 94025 United States
AU: Paytan, A
EM: apaytan@stanford.edu
AF: Stanford University, 320 Braun Hall Dept of Geol and Env Sciences, Stanford, CA 94305 United States
AU: Kendall, C
EM: ckendall@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd. MS 434, Menlo Park, CA 94025 United States
AB: Elkhorn Slough is a tidally flushed estuary with exceptionally high amounts of N loading within the watershed. The main sources of N are thought to be in the form of nitrate from surface water runoff, agricultural fields, dairy farms, and urban runoff. More poorly quantified sources of nitrate may also include significant groundwater discharge as well as tidal inputs from Monterey Bay. Traditional studies, involving natural abundance stable isotopes within the N cycle, have been limited to $\delta^{15}$N and thus to a 'one-dimensional' perspective on N cycling. The use of a multiple isotope ($\delta^{15}$N and $\delta^{18}$O) model allows disentanglement of superimposed transformation processes. Furthermore, a dual isotope approach permits a more discrete characterization of these sources, with the goal of estimating their roles in the N budget of Elkhorn Slough. The isotopic values of nitrate from transects, conducted in the main channel of the slough during both flood and ebb tides, suggest mixing of a marine end member ($\delta^{15}$N = +11.6%; $\delta^{18}$O = +10%) with several other nitrate sources having discernable isotopic compositions. The Moss Landing Harbor, near the mouth of Elkhorn Slough, has very high nitrate concentrations ($>$200uM) and is likely a significant source of nitrate to the slough ($\delta^{15}$N = +15.4%; $\delta^{18}$O = +9.9%). With each rising tide, there is a distinct tidal bore from Monterey Bay also delivering nitrate (approx. 30uM) of a unique nitrate isotopic composition ($\delta^{15}$N = +5.3%; $\delta^{18}$O = +9.9%). During certain sample collections, nitrate from the head of the slough, with an isotopic composition of +5.2% ($\delta^{15}$N ) and +22.0% ($\delta^{18}$O), is suggestive of nitrification ($\delta^{15}$N = +5.2%; $\delta^{18}$O = +22.3%). Each of these estuarine members reflects both the composition of the original source of nitrate within the sub-watershed as well as the degree of transformation of these sources. The $\delta^{18}$O value for marine nitrate in this system is markedly higher than has been reported in open ocean systems - implying a coupling of the coastal and estuarine system. These initial data suggest cycling of nitrate within the main channel may be non-conservative.
DE: 1040 Isotopic composition/chemistry
DE: 1615 Biogeochemical processes (4805)
DE: 1803 Anthropogenic effects
DE: 4235 Estuarine processes
DE: 4805 Biogeochemical cycles (1615)
SC: Hydrology [H]
MN: 2003 Fall Meeting