Understanding the mechanisms behind surface elevation loss in ditched marshes

Abstract

Loss of surface elevation makes salt marshes more susceptible to impacts from accelerated sea level rise, such as vegetation drowning, die-off, and conversion of marsh to open water. The ultimate degradation of the salt marsh system is disastrous with ramifications ranging from loss of critical habitat to loss of an important buffer for coastal communities from storm surges. Effectively, a more comprehensive understanding of the mechanisms driving surface elevation loss in anthropogenically altered and degraded marshes is key to engineering successful marsh restoration projects, in an effort to reverse this trend. This study aims to achieve that goal in an area of a northern Massachusetts salt marsh with high man-made ditch density, through comparison of the hydrologic, sedimentary, and vegetative conditions to a non-ditched, reference portion of salt marsh. It was hypothesized that a decrease in subsurface hydroperiod through increased drainage, characteristic of areas of high ditch density, would allow for increased oxygen diffusion into the subsurface causing belowground decomposition rates to increase. This ultimately would lead to a reduction in organic matter, and without compensation from an inorganic sediment supply, marsh subsidence would occur. Water table levels, belowground biomass, bulk density data, and percent organic content data all supported this hypothesis, but direct analysis of the belowground litterbag component of this study did not demonstrate significant differences in decomposition rates between the ditched and non-ditched sites. Further study of belowground conditions, resulted in a live root turnover rate about twenty percent slower in the ditched marsh than in the non-ditched marsh. This suggests that turnover rates, not decomposition rates, may ultimately be the mechanism behind surface elevation loss in ditched marshes.