Tidal frequencies and quasiperiodic subsurface water level variations dominate redox dynamics in a salt marsh system

Abstract

Salt marshes are hotspots of nutrient processing en route to sensitive coastal environments. Whilst our understanding of these systems has improved over the years, we still have limited knowledge of the spatiotemporal variability of critical biogeochemical drivers within salt marshes. Sea‐level rise will continue to force change on salt marsh functioning, highlighting the urgency of filling this knowledge gap. Our study was conducted in a central California estuary experiencing extensive marsh drowning and relative sea‐level rise, making it a model system for such an investigation. Here we instrumented three marsh positions subjected to different degrees of tidal inundation (6.7%, 8.9%, and 11.2% of the time for the upper, middle, and lower marsh positions, respectively), providing locations with varied biogeochemical characteristics and hydrological interactions at the site. We continuously monitored redox potential (Eh) at depths of 0.1, 0.3, and 0.5 m, subsurface water levels (WL), and temperature at 0.7 m depth at each marsh position. To understand how drivers of subsurface biogeochemical processes fluctuate across tidal cycles, we used wavelet analyses to explain the interactions between Eh and WL. We found that tidal forcing significantly affects key drivers of biogeochemical processes by imparting controls on Eh variability, likely driving subsurface hydro‐biogeochemistry of the salt marsh. Wavelet coherence showed that the Eh‐WL relationship is nonlinear, and their lead–lag relationship is variable. We found that precipitation events perturb Eh at depth over timescales of hours, even though WL shows relatively minimal change during events. This work highlights the importance of high frequency in situ measurements, such as Eh, to help explain factors that govern subsurface biogeochemistry and hydrological processes in salt marshes.

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

“…The terrestrial water level responds to precipitation with relative increases during precipitation events (Figure 2). Marsh porewater levels were subject to daily, biweekly and seasonal tidal cycle inundation dynamics, resulting in multiple water level fluctuation frequencies (Grande, Arora, Visser, et al, 2022). Porewater levels at the lower marsh position were consistently lower than at the upper and middle marsh positions during low tides, draining below the marsh surface elevation in each tidal cycle.…”

“…Therefore, we have historically understudied short‐term and fine spatial scale porewater $$$ \mathrm{N}{\mathrm{O}}_3^{-} $$$ dynamics over more than a few tidal cycles in these ecosystems. For example, impacts of short‐lived, episodic events such as storms and tidal inundation can drive biogeochemical parameters like redox potential (Grande, Arora, Visser, et al, 2022), but may be missed by coarse resolution sampling.…”

Cold winters drive consistent and spatially synchronous 8‐year population cycles of cabbage stem flea beetle

“…red noise) and fitting a wavelet transform to the AR1 time series (Towler & McCreight, 2021). A Chi 2 test with an α = 0.05 is used to assess if the wavelet power spectrum is different from that of the red noise model (Grande et al, 2022). Though this method of significance testing is known to be lenient, the wavelet transform analysis in this package is preferred as it corrects a bias for finding greater power at longer time‐scales (Liu et al, 2007; Schulte, 2019).…”

“…test with an α = 0.05 is used to assess if the wavelet power spectrum is different from that of the red noise model (Grande et al, 2022).…”

Corrigendum

“…In the referenced article, Grande et al . (2022), an additional affiliation for co‐author Braswell, A. has been added:‐.…”