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Seasonal and Tidal Variations in Hydrologic Inputs Drive Salt Marsh Porewater Nitrate Dynamics
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  • Emilio Grande,
  • Erin Cedar Seybold,
  • Corianne Tatariw,
  • Ate Visser,
  • Anna Elizabeth Braswell,
  • Bhavna Arora,
  • François Birgand,
  • John Haskins,
  • Margaret Ann Zimmer
Emilio Grande
University of California Santa Cruz

Corresponding Author:emgrande@ucsc.edu

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Erin Cedar Seybold
Kansas Geological Survey, University of Kansas
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Corianne Tatariw
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Ate Visser
Lawrence Livermore National Laboratory (DOE)
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Anna Elizabeth Braswell
University of Florida
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Bhavna Arora
Lawrence Berkeley National Laboratory (DOE)
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François Birgand
North Carolina State University
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John Haskins
Elkhorn Slough National Estuarine Research Reserve
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Margaret Ann Zimmer
University of California - Santa Cruz
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Salt marshes remove terrestrially derived nutrients en route to coasts. While these systems play a critical role in improving water quality, we still have a limited understanding of the spatiotemporal variability of biogeochemically reactive solutes and processes within salt marshes, particularly nitrogen species. To investigate this knowledge gap, we implemented a high-frequency sampling system to monitor sub-hourly nitrate (NO3) concentrations in salt marsh porewater at Elkhorn Slough in central California, USA. We instrumented three marsh positions along an elevation gradient subjected to different extents of tidal inundation, which we hypothesized would lead to varied biogeochemical characteristics and hydrological interactions. At each marsh position, we continuously monitored NO3 concentrations at depths of 10, 30, and 50 cm with subsurface water levels measured from 70 cm wells over seven deployments of ~10 days each. We quantified tidal event hysteresis between NO3 and water level to understand how NO3 concentrations and sources fluctuate across tidal cycles. There was significant differences in the NO3-subsurface water level hysteresis patterns across seasonal wet/dry periods common to Mediterranean climates. In dry periods, the NO3-subsurface water level relationship indicated that the source was likely estuarine surface water that flooded the transect during high tides. In wet periods, the NO3-subsurface water level relationship suggested the salt marsh was a source of NO3. These findings suggest that tidal and seasonal hydrologic fluxes control NO3 porewater dynamics and influence ecological processes in coastal environments.