Distance from Tidal Creek is an Important Factor Affecting the Ecology and Biogeochemistry in Ditched and Natural Salt Marshes

Abstract:

Ninety percent of New England salt marshes underwent hydrologic alteration through the installation of parallel grid ditches in the 1920’s – 1940s. Ditching increases marsh drainage, thereby reducing standing water and mosquito habitat, but the long-term effects of ditching on marsh biogeochemistry and sustainability are not well understood. We characterized plant community, soil, and porewater properties along 100 meter transects, extending from creekbank edges to the marsh interior, in adjacent ditched and unditched sections of Great Barnstable Marsh (MA, USA). Elevation was similar at creek bank edges in both marsh sections but diverged toward the interior, increasing in the unditched section and decreasing in the ditched. The drop in elevation triggered a response in the dominant marsh grass, Spartina alterniflora, which grew taller in the ditched section. Despite this, we found stronger patterns in plant and soil properties with distance into the marsh than between ditched and unditched sections. In both sections, S. alterniflora cover increased but height decreased with distance into the marsh. Plant responses were mirrored by soil properties and composition. Soil organic matter was higher in surface horizons (0-10cm) and increased with distance into the marsh while bulk density decreased. Correspondingly, soil carbon : sulfur ratios decreased in surface horizons and porewater sulfide concentrations increased in all horizons with distance into the marsh. This suggests that sulfate reduction increased, or sulfide oxidation decreased, as organic matter accumulated and S. alterniflora coverage increased. While ditching lowered the elevation in one section of Great Barnstable Marsh, the change in elevation with distance from the creeks in both the ditched and unditched marshes appears to generate a stronger response in both the plant community and the biogeochemical environment.