EFFECT OF DITCH CONSTRUCTION ON CARBON STORAGE AND SALT MARSH RESILIENCY IN COASTAL NORTH CAROLINA

Continued upward accumulation of organic-rich coastal marsh sediment can effectively keep pace with sea level rise and sequester atmospheric carbon dioxide. In addition to shoreline protection and their role as nursery areas for marine life, salt marshes can serve as naturally resilient coastal buffers and important carbon sinks. However, cutting mosquito ditches in salt marshes is hypothesized to reduce vertical accretion rate and lower marsh sediment carbon content. To test this hypothesis, we analyzed cores from two salt marshes with similar geomorphology, salinity, wave exposure and vegetation. One marsh was extensively ditched in the 20^th century, and the other has remained unditched. Both sites border Core Sound, near Davis, NC.

We measured sediment bulk density and organic carbon content at 2-cm depth intervals in cores ranging from 50 to 150 cm depending on marsh sediment thickness. In addition to providing information on changing conditions through time (e.g., before and after ditch construction), our data allow determination of buried carbon stock per unit marsh area, termed “blue carbon stock.” Analyses of stable carbon isotopic composition of organic matter provide information on shifting carbon sources over time, e.g., from C3 Juncus high marsh vegetation to C4 Spartina alterniflora low marsh in response to increased tidal inundation. We find the highest organic carbon concentrations (>25%) in cores from unditched sites dominated by Juncus. On average, ditched marshes have higher bulk density and lower carbon content (5 – 15%) than similar depth intervals in cores from unditched sites (10 – 20%). The blue carbon stock in the upper 100 cm of the sediment column varies from as much as 430 MgC/hectare in unditched sites to less than 280 MgC/hectare at ditched sites. Analyzing carbon density by depth shows a general pattern toward lower carbon stocks in response to increased inundation due both to faster sea-level rise, and to ditch construction. Depending on minerogenic sediment delivery to ditched marshes, it appears that the widespread ditching of East Coast salt marshes in the 20^th century was detrimental in two ways: shorelines are more vulnerable to sea level rise, and marshes are less effective at reducing atmospheric carbon dioxide levels.