Salt marshes play an enormously important ecological role throughout New England as nurseries for commercially important marine species, habitat for shorebirds, protecting uplands from storm surges and floods, and as sinks for nutrient runoff from nitrogen and phosphorus. Unfortunately, for centuries salt marshes were dredged, diked and filled because they held no development value along the shoreline. Now that we understand the vital ecological and economic value of these ecosystems, particularly in light of climate change, restoration of degraded salt marshes has become increasing important. As the owner of the highest percentage of salt marshes on Nantucket, the Foundation has conducted various research projects exploring salt marsh function and has recently completed a ten year study on one of the most highly monitored salt marsh restoration projects in New England.
Our Medouie Creek property, located on the northern shore of east Polpis Harbor, became the focus of a large-scale restoration project in 2008. Originally one large salt marsh, channel dredging and the creation of dike roads sometime prior to 1938 severely altered water flows and restricted tidal salt water inputs, creating a freshwater marsh. As this freshwater marsh became colonized by the non-native, invasive common reed (Phragmites australis), salt marsh restoration became a priority.
Restoration at the site consisted of draining out impounded freshwater, restoring tidal salt water flow and facilitating salt water movement into the wetland – primarily through installation of a culvert and targeted channelization done in December 2008. Native salt marsh cordgrass (Spartina alterniflora) was planted along the altered channels, otherwise we allowed native vegetation to seed in from adjacent salt marshes and the seed back.
Monitoring water level fluctuations, soil pore water salinity and vegetation changes prior to and after the restoration construction has provided us with valuable information on the effectiveness of this restoration, which has been largely successful. With the installation of the culvert, the impounded freshwater marsh rapidly converted to daily tidal hydrology and soil pore salinity quickly became elevated. The freshwater adapted plant species experienced an initial rapid die off from salt water and have continued to die off as salt marsh vegetation has moved into and established in the previously freshwater marsh. The non-native Phragmites has been dramatically impacted in both population size and plant health. This restoration has been proven highly successful as salt marsh habitat now dominates a large portion of the wetland complex. We are continuing monitoring research as this site long term to document the continual shifts in marsh ecology resulting from the restoration.
Of growing concern at Medouie Creek and a number of other salt marsh properties on Nantucket is increasing evidence of salt marsh dieback. This phenomenon had been observed on Cape Cod salt marshes about 10 years prior to showing up on Nantucket. Essentially, salt marsh dieback is the loss of vegetation, particularly salt marsh cordgrass (Spartina alterniflora) which grows at the lowest elevation of the marsh with it’s roots flooded daily by tides. Loss of this vegetation exposes the remaining marsh soils to those daily tides, increasing the risk of marsh erosion and loss. Research has documented that the direct cause of salt marsh dieback is an increased population of the native purple marsh crab (Sesarma reticulata), which eats the salt marsh cordgrass (even the roots). As the crab population explodes, their grazing activities increase and salt marsh cordgrass is eliminated, leaving large swaths of bare soil.
In a survey of key Foundation-owned marshes across the island, we documented high numbers of the purple marsh crab in salt marshes with dieback and much lower numbers within healthy marshes systems. Scientists don’t yet know why the crab populations are increasing so precipitously, although potential reasons include increased fishing pressure and increased nutrient inputs. In 2019, we initiated a research project at Medouie Creek to explore the feasibility of mitigating salt marsh dieback by trapping and removing purple marsh crabs and non-native, invasive green crabs to reduce population numbers. So far, we have seen encouraging results with some reestablishment of native salt marsh cordgrass within impacted areas.
Tidal Hydrology and Salinity Drives Salt Marsh Vegetation Restoration and Phragmites australis Control in New England; J.M. Karberg, K.C. Beattie, D.I. O'Dell and K.A. Omand; Wetlands Volume 38, Issue 5, pp 993–1003. Journal copyright does not allow open website access to this publication- please email to request a copy.
Salinity Tolerance of Common Reed (Phragmites australis) at the Medouie Creek Restoration Site, Nantucket MA; J.M. Karberg, K.C. Beattie, D.I. O'Dell and K.A. Omand; Wetlands Science & Practice Vol. 32, No. 1: 19-23. 2015.