Oxic hot moments in a coastal floodplain highlight the bidirectional flow of surface water-groundwater exchanges at the terrestrial-marine interface

Due to their saturated conditions resulting from frequent inundations, coastal floodplains are crucial in sequestering atmospheric carbon and regulating nutrient cycling. Tidal inundations can mobilize dissolved oxygen (DO), a critical driver of biochemical function, towards the subsurface, stimulating microbial activity when DO-rich surface water mixes with anoxic groundwater (i.e., hydrodynamics of surface/groundwater exchange driven by periodic flooding). While our knowledge of these systems has improved over the years, we need a greater understanding of the spatiotemporal variability of essential biochemical drivers within tidal floodplains. Mainly, measuring DO at sufficient temporal resolution in such rapidly changing environments to resolve the relative influence of each factor is challenging, limiting our ability to predict how chronic sea-level rise will impact coastal floodplain functionality and spotlighting the urgency to fill this knowledge gap. We used a state-of-the-art optical DO probe (Opti O2) to continuously measure subsurface DO concentrations at 5 min resolution over ~4 years at Beaver Creek, a freshwater creek in Washington, USA. Following the removal of a barrier in 2014, the site floods during tidal events with water from Grays Harbor, located at the northwestern Pacific coast. This data set is the first measurement in a coastal environment with the requisite temporal resolution to obtainin-situ, subsurface oxygen consumption time series (see attached image). With our data, we investigate the evolution of processes following a controlled sea level rise experiment. Co-located instruments monitoring surface water and groundwater levels, salinity, and meteorological parameters (including rainfall, air temperature, barometric pressure, solar flux density) let us parse the critical drivers of coastal floodplain DO dynamics. To understand how drivers of subsurface biogeochemical processes fluctuate across tidal cycles, we used wavelet analyses to explain the interactions between DO and water levels. We observed multiple oxygenation events (34 clusters of hot moments from June 2019 to date) followed by subsequent returns to anoxia. We used information theory to explore DOs relationship with the hydro-meteorological data. This work highlights the importance of multi-year, high-frequencyin-situmeasurements, such as DO, to elucidate the non-linear coupling of climate, hydrology, and biochemistry in coastal floodplains.