RESPONSE OF NORTH CAROLINA SALT MARSHES TO HYDRAULIC MODIFICATIONS, AGRICULTURAL RUNOFF AND ACCELERATED SEA LEVEL RISE

The accumulation of organic-rich sediment beneath coastal marshes mitigates atmospheric CO₂ levels by sequestering carbon, potentially for thousands of years. For net carbon burial to continue, the areal extent and upward growth of salt marshes must be maintained. Over the last century, however, the vegetation communities and sediment properties of many coastal marshes have changed in response to human coastal modifications, accelerated sea-level rise and other ecological perturbations.

We measured bulk density, organic carbon content, and stable carbon isotopic composition in shallow (50-75 cm) cores from Juncus roemarianus and Spartina alterniflora marshes near Core Sound, North Carolina. In addition to ongoing sea level rise, local human modifications at our three study sites include mosquito ditching, diking to create freshwater impoundments, and increased runoff as adjacent upland swamp forest was drained for agriculture. These human impacts affected the three study sites to differing degrees, whereas all sites experienced the same sea level history.

Down-core isotopic data illustrate that most of the sites have already shifted from Juncus- to Spartina-dominated vegetation in response to increased tidal inundation. The timing and extent of the observed vegetation transition correspond to site elevation and exposure to the higher salinities of Core Sound; i.e., more saline and lower elevation sites switched from Juncus to Spartina earlier. At all of the inundation-impacted core sites, sediment carbon concentrations have dropped by ~15% coincident with the vegetation shift. In contrast to the sites noted above, a 10-15% increase in carbon content is observed at the site most heavily impacted by agricultural runoff despite the tidal inundation effects of ditching and diking. The healthy Juncus monoculture at this higher elevation site indicates it has been buffered thus far from the effects of sea-level rise and other perturbations that have driven shifts at our other sites. Nevertheless, the decrease in below-ground organic carbon storage observed at most of our sites implies reduced CO₂ uptake by these coastal wetlands, potentially driving feedback that may ultimately hasten sea-level rise to rates above the limit of upward marsh accretion.