Runnels Reverse Mega-pool Expansion and Improve Marsh Resiliency in the Great Marsh, Massachusetts (USA)

Abstract

One of the main mechanisms for salt marsh decline across the United States is the inability of the marsh surface to keep pace with sea level rise. The interior platform is especially vulnerable, leading to the encroachment of short form Spartina alterniflora pannes, pool formation, and ultimately runaway pool expansion if recovery is not possible. Coastal ecologists in New England have been implementing a restoration strategy of runnels, or shallow ditches, to enhance drainage of oversaturated and ponding interior marshes. In 2015, runnels were constructed to drain two large and expanding pools in the Great Marsh System of Massachusetts, USA. Vegetation, elevation, and hydrology were monitored using field sampling and remote sensing analysis pre- and post-restoration over seven growing seasons to document the restoration trajectory of the pools and adjacent salt marsh platforms. Pool drainage improved reflecting tidal cycles after three growing seasons. Substantial colonization of S. alterniflora and S. patens into the previously unvegetated pools required three growing seasons. In the adjacent marsh platform, S. patens and Distichlis spicata increased in abundance with substantial declines in S. alterniflora. The runnel for one pool became partially blocked by vegetation after 2018 and inhibited drainage and recovery of the vegetation community in the pool yet not the platform. Runnels may be a viable solution for restoring interior marshes following vegetation loss yet substantial improvements in vegetation and hydrology may require 3–5 years and complete recovery of the vegetation community in the pool system at least a decade or more.