Changes in lateral connectivity between a tidal creek and its floodplain alter oxygen dynamics

As sea levels rise, coastal systems are increasingly impacted by inundation and salinization. Removal of barriers to saltwater intrusion provide the opportunity to study the impacts of sea level rise on an accelerated timescale. At our study site in the Pacific Northwest, a barrier removal in 2014 abruptly restored tidal influence on a small creek and its floodplains. We utilized a network of in-situ hydrology and water chemistry sensors, located along lateral and longitudinal transects of a tidal creek-floodplain ecosystem, to capture how tidally driven changes in hydrologic connectivity across the terrestrial-aquatic interface influences coastal biogeochemistry. Using wavelet analysis, we identified short episodes of strong lateral connectivity followed by extended periods where the floodplain remained largely disconnected from tidal inputs. During the dry season, when tidal influence was strongest, these periods of disconnection were accompanied by discharge of saline, anoxic groundwater to the creek on the ebb tide. As we only observed this behavior at one of two creek sites, and only for a portion of the tidal cycle under a specific combination of environmental conditions, these events appear to represent `cold' moments/spots of biogeochemical activity, where depleted oxygen limits aerobic activity. However, we suggest concurrent increased export of carbon dioxide may represent a hot spot/moment in carbon biogeochemistry. This apparent dichotomy epitomizes the complexity of coastal biogeochemistry, and the importance of both high-frequency monitoring at sufficient spatial resolution and sufficiently powerful statistical and modeling techniques to synthesize stronger data-driven predictions of how coastal system biogeochemistry will respond to rising seas.