Seasonal and tidal variations in hydrologic inputs drive salt marsh porewater nitrate dynamics

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. 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 amounts of tidal inundation, which we predicted would lead to varied biogeochemical characteristics and hydrological interactions. At each marsh position, we continuously monitored porewater NO3- concentrations at depths of 10, 30, and 50 cm and porewater levels measured at 70 cm depth over seven deployments of similar to 10 days each that spanned seasonal wet/dry periods common to Mediterranean climates. We quantified tidal event hysteresis between NO3- and water level to understand how NO3- concentrations and sources fluctuate across tidal cycles. In dry periods, the NO3--porewater level relationship indicated that the NO3- source was likely estuarine surface water that flooded the transect during high tides and the salt marsh was a NO3- sink. In wet periods, the NO3--porewater level relationship suggested the salt marsh was a source of NO3-. These findings suggest that tidal and seasonal hydrologic fluxes together control NO3- porewater dynamics and export and influence ecological processes in coastal environments.