Assessment of geochemical cycling within tidal wetlands and measurement of fluxes of dissolved and particulate constituents between wetlands and coastal water bodies are critical to evaluating ecosystem function, service, and status. The U.S. Geological Survey and collaborators collected surface water and porewater geochemical data from a tidal wetland located on the eastern shore of Sage Lot Pond in Mashpee, Massachusetts, within the Waquoit Bay National Estuarine Research Reserve, between 2012 and 2019. Additional porewater geochemical and field data from a tidal wetland on the eastern shore of Great Pond in East Falmouth, MA are also included. These data can be used to evaluate biogeochemical conditions and cycling of carbon and other elements within the marsh platform and to calculate lateral tidal exchange fluxes of a suite of dissolved and particulate constituents between the wetland and estuary. Analytes include but are not limited to: dissolved oxygen, oxidation reduction potential, pH, salinity, dissolved and particulate organic and inorganic carbon, stable carbon isotopic ratios, nitrogen species, phosphate, silica, dissolved methane and nitrous oxide gas, trace elements, radium isotopes, alkalinity, and sulfide. Much of the surface water data at Sage Lot Pond was collected from the mouth of a tidal creek across full-tidal (12 to 14 hour) time series sampling events at 0.5 to 2-hour intervals at different points in the spring/ neap cycle and season. Porewater samples were collected at multiple depths (9 to 245 centimeters) in transects extending across the marsh platform at different times in the season between 2014 and 2019. Sage Lot Pond creek data are concurrent with extended time-series measurement of water quality and flow data measured with deployed sensors in the tidal creek (Mann and others, 2019), and with carbonate chemistry data measured at the site (Wang and others, 2019, 2020

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.