Inter-annual Variations of Hot Moments in a Coastal Flood Plain: Insights from a Three-year Time-series

What controls the timing, frequency and magnitude of hot moments at the terrestrial-aquatic-interface (TAI) in a coastal flood plain? Dissolved oxygen plays a key role in biogeochemical processes at the land-sea interface. Oxygenation associated with floodplain inundation events influence oxygen-sensitive microbial activity and determines the cycling and mobility of nutrients as well as minerals in groundwater. Over the past three years, we have observed a series of "hot moments", oxygenation events followed by return to anoxia, in a coastal floodplain triggered by tidal inundation from an adjacent creek. These observations are made possible by high frequency (every 5min) data from in-situ oxygen sensors deployed within the flood plain from July 2019 to date. Additionally, the continuous time-series allows for both resolution of sub-diurnal events as well as observations of seasonal and inter-annual variabilities/trends.

Our study was conducted at Beaver Creek (Washington, USA) that is tidally connected to the Pacific Ocean. Between July 2019 to July 2022 we have recorded 31 periods of high water inundation of the flood plain. During each of these periods we observe between one to five individual oxygenation events. Co-located salinity, water level and temperature data show that the ground water is anoxic during most of the year. Inundation of the flood plain by saline high tides rapidly oxygenates the ground water resulting in a spike in dissolved oxygen (lasting 30-60min), the DO level then drops prior to arrival of the next tidal spike. Using the 5min temporal resolution of the DO data, we directly measure the total oxygen consumption during each individual "hot moment".

From this comprehensive three year record we find that the floods during summer months are the most impactful. However, there exists a great deal of inter-annual variations in both the timing and magnitude of the hot moments. Some years specific seasonal events fail to occur. We seek to determine what are the drivers of the hot moments. Hypotheses include are these biogeochemical events tied to(i) the long term, i.e multi-year, variations in the lunar cycle or (ii) annual precipitation. Our findings will shed light on the interplay between the multiple mediators of oxygen dynamics in coastal ecosystems.