GROUNDWATER EXPORTS OF DOC FROM SALT MARSHES: A SURVEY OF SOUTH CAROLINA TIDAL CREEKS

The construction of carbon budgets for coastal systems requires accurate estimates of multiple factors, including carbon fluxes associated with groundwater discharge from tidal wetlands. Estimates of groundwater DOC fluxes from salt marshes are particularly scarce. We estimated DOC fluxes via >120 new porewater DOC analyses and published groundwater fluxes. Groundwater samples were obtained at multiple sites from 15 South Carolina salt marshes. Samples were collected for a variety of marsh widths and from undeveloped and developed watersheds. Although variability was substantial, we obtained average porewater DOC concentrations of 500-700 umol/L adjacent to tidal creeks (~1 m below the sediment-water interface). The level of development in the surrounding watershed did not affect these averages. Although positive correlations emerged between marsh width and parameters such as salinity and nutrient concentrations, there was no statistical correlation between marsh width and DOC concentration. Previous estimates of groundwater discharge from South Carolina salt marshes are of the order of 10 L m^-2 per tidal cycle, which, with our average DOC concentrations, gives DOC fluxes of 3.5 – 5 mol m^-2 yr^-1. These fluxes are 10-45% of published TOC fluxes from the region. We found slight but significant seasonal variations in porewater DOC concentrations. Groundwater fluxes also vary seasonally, suggesting larger DOC fluxes from salt marshes in the winter. However, prior studies of DOC export from North Inlet, SC, suggest that these seasonal variations are likely small compared to variations in input from high-DOC freshwater creeks and fresh groundwater in coastal South Carolina. Additional surveys are strongly recommended, but results suggest that groundwater exports must be considered in constructing salt marsh carbon budgets.