High-frequency variability of carbon dioxide fluxes in tidal water over a temperate salt marsh

Existing analyses of salt marsh carbon budgets rarely quantify carbon loss as CO₂ through the air–water interface in inundated marshes. This study estimates the variability of partial pressure of CO₂ (pCO₂) and air–water CO₂ fluxes over summer and fall of 2014 and 2015 using high-frequency measurements of tidal water pCO₂ in a salt marsh of the U.S. northeast region. Monthly mean CO₂ effluxes varied in the range of 5.4–25.6 mmol m⁻² marsh d⁻¹ (monthly median: 4.8–24.7 mmol m⁻² marsh d⁻¹) during July to November from the tidal creek and tidally-inundated vegetated platform. The source of CO₂ effluxes was partitioned between the marsh and estuary using a mixing model. The monthly mean marsh-contributed CO₂ effluxes accounted for a dominant portion (69%) of total CO₂ effluxes in the inundated marsh, which was 3–23% (mean 13%) of the corresponding lateral flux rate of dissolved inorganic carbon (DIC) from marsh to estuary. Photosynthesis in tidal water substantially reduced the CO₂ evasion, accounting for 1–86% (mean 31%) of potential CO₂ evasion and 2–26% (mean 11%) of corresponding lateral transport DIC fluxes, indicating the important role of photosynthesis in controlling the air–water CO₂ evasion in the inundated salt marsh. This study demonstrates that CO₂ evasion from inundated salt marshes is a significant loss term for carbon that is fixed within marshes.