Mitigating the Legacy Effects of Ditching in a New England Salt Marsh

Abstract

The legacy effects of mosquito ditching have made salt marshes more vulnerable to flooding impacts from climate change, presenting management challenges in New England where the majority of salt marshes have been ditched and greater rates of sea level rise and storm events are being observed. One legacy effect of mosquito ditching appears to be subsidence of the marsh, with greater effects near the ditches and extending into the marsh plain. We found an average of 9 cm subsidence midway between ditches that averaged 14 m apart. Ditch Remediation is a new approach to filling ditches that uses existing hydrology and vegetation to mend ditches from the bottom up to restore marsh plain elevations. Smaller, auxiliary ditches are selected for treatment. Hay is mown, allowed to dry, and rolled into the treatment ditch where it is held using twine. Nine ditches in four areas were treated in fall 2014 and 2015. Depth of treated ditches decreased an average of 18 cm by fall 2017, and Spartina alterniflora colonized the ditch centers (plant cover and stem density increased). It is unknown whether the trajectory of filling and revegetation of the ditches will continue on their own or if the reduction in drainage depth will stimulate marsh plain building. Ditch remediation could remove excessive drainage effects of half the ditches, thereby approaching an optimal drainage density that may allow tidal marshes to rebuild elevation. This simple and inexpensive technique to reduce ditch impacts should be considered by partnerships that include mosquito control agencies.