Rapid Variability in Subsurface Dissolved Oxygen along the Terrestrial-Aquatic-Interface Driven by Tidal Inundation
Ruby N Ghosh1, Dean D Shooltz2, Michael J Freeman1, Reza Loloee3, Terry Ball3, Charles McIntire1, Eric Mollon1, Nicholas D Ward4, Gary A Gill5, Allison Myers-Pigg6 and Li-Jung Kuo7, (1)Opti O2, LLC, Okemos, MI, United States, (2)Opti O2, Okemos, MI, United States, (3)Opti O2, LLC, Okemos, United States, (4)Pacific Northwest National Laboratory, Richland, WA, United States, (5)Pacific Northwest National Laboratory, Sequim, United States, (6)Pacific Northwest National Laboratory, Marine Sciences Laboratory, Richland, United States, (7)Pacific Northwest National Laboratory, Marine Sciences Laboratory, Richland, WA, United States
Abstract:
Dynamic biogeochemical variability in both terrestrial and aquatic coastal environments is often attributed to the presence of “hot moments” and “hot spots” that are not mechanistically understood. Here we examine a potentially critical driver for such hot moments that has large implications on how we interpret the fate of organic matter transported and/or stored along the coastal interface. This study examines dissolved oxygen (DO) dynamics across the terrestrial-aquatic-interface (TAI) of a coastal watershed in the Pacific Northwest, Beaver Creek. Opti O
2’s novel O
2 sensor technology provided continuous (24/7) high resolution data (Dt = 5min) DO data across discreet TAI components: (i) the tidally-influenced river (salinity of 0 – 30 psu), (ii) low lying marsh with variable salinity and water level and (iii) a “seep” which represents the connection point across the TAI. DO in groundwater plays a central role in determining the redox state of soil and sediment profiles, related chemical reactions, and the reactivity or stability of organic matter.
Throughout the spring/summer months we observed that groundwater DO was anoxic and static over time, indicating minimal communication with the river. However, in May and July 2019 we observed abrupt pulses in elevated groundwater DO that persisted 25 hours before returning to anoxia. These DO pulses were consistently observed over 3-6 day periods and were associated with high tides that inundated the landscape with saline, O2-rich seawater. The DO pulses are coincident with a stepwise increase in salinity, a spike in temperature and a large change in ground water level. Our data suggest that DO is not conservatively mixed in the ecosystem as surface water flows into the ground. The temporally resolved DO signal also enables a direct measure of soil respiration in response to a pulse disturbance. Rapid shifts in soil redox state have large implications on organic matter reactivity and related GHG emissions.
