Evaluating Alkalinity Export From Intertidal Salt Marshes

Intertidal salt marshes are highly productive coastal ecosystems that export carbon and other biogeochemical species to the coastal ocean. Recent carbon budgets for the coastal ocean suggest organic carbon, nutrient, dissolved inorganic carbon (DIC) and total alkalinity (TA) exports may be important fluxes. However, despite their importance, published literature on salt marshes contains few studies with sufficient spatial and temporal resolution to accurately quantify these fluxes. In particular, marsh TA export flux and its effects on carbonate chemistry and carbon cycling of adjacent coastal systems is unknown. Alkalinity is produced in salt marsh sediments due to anaerobic respiration processes such as denitrification and sulfate reduction. In this study, a combination of high-resolution, in situ measurements, and bottle sampling is used to quantify high-resolution total alkalinity flux. We confirmed that during ebb tides the exported water has high concentrations of DIC and low pH due to input from sediment porewaters. However, while DIC export occurs throughout the year, most of the alkalinity export occurs in the summer, concurrent with higher rates of marsh productivity and anaerobic respiration in marsh sediments. An accurate estimate of TA export fluxes allows for a better assessment of salt marshes as sources of buffering capacity to coastal oceans. Export of high TA and low pH water from salt marshes can act simultaneously as buffering and acidifying sources to the coastal ocean and our time series data show how this delicate balance changes tidally and seasonally. These results will help reduce uncertainty in current estimates of marsh DIC and TA export and establish a baseline export flux in order to evaluate the importance of intertidal salt marshes in coastal carbon cycling. This is particularly timely as marshes continue to be destroyed by anthropogenic perturbations such as land development and nutrient enrichment.