Continuous Atmospheric and Hydrologic Exchange Data Show Resilience of California Tidal Marsh Carbon Fluxes to July 2022 Heatwaves

Heatwaves can have disparate influences on ecosystem carbon dynamics depending on antecedent conditions. Lower rates of Net Ecosystem Exchange (NEE) are common due to enhanced ecosystem respiration and reduced photosynthetic efficiency where water is limited. The regularity of tidal flooding may offset impacts of extremely dry air, thus promoting resilience of tidal marsh carbon sequestration. Rush Ranch, a Northern California brackish tidal marsh in the San Francisco Bay National Estuarine Research Reserve with continuous measurements of atmospheric exchange (Ameriflux US-Srr) and hydrologic conditions (USGS NWIS 381142122015801), experienced extremes in both air temperature and tidal range during July 2022, providing a targeted opportunity to assess the relative impact of these different forcings at an event-level scale. Prolonged air temperatures over 40 °C depressed gross primary productivity (GPP) and enhanced nighttime respiration (Reco) to record high rates in early July, until an early morning "king" tide immediately and dramatically suppressed Reco (from 14 to 4 µmol CO2 m-2 s-1). The "cooling" effects of this spring tide continued for days, as NEE recovered to pre-heatwave rates. Concomitant water quality monitoring indicated tidal "cooling" of exported porewater during July spring tides, with enhanced aquatic pCO2 fluxes on ebbing tides, and sediment deposition on the flooding tides. Because methane (CH4) emissions remained low and invariable across July air temperatures, the coupled datasets illustrate significant resilience of this historic marsh to pulsed heatwaves, whereby CO2 sequestration, soil accretion, and alkalinity export were maintained or enhanced, and CH4 emissions were unaffected. Previous work at Rush Ranch has illustrated the multi-temporal responses of NEE to surface flooding and implied the role of tidal subsidies on marsh C resilience. Continuous long-term data are essential to incorporating these competing feedbacks into models of tidal wetland carbon sequestration, as the ocean-influence reduces sensitivity of C stocks to heatwave intensity and frequency.