Salt Marsh Restoration through Tidal Restriction Removal

Restoration of degraded salt marshes has become increasingly important given their vital ecological and economic value, particularly in light of climate change mitigation and coastal resilience. For decades, salt marshes were dredged, diked and filled for development and agriculture but as recognition of their immense importance has risen, so has the need to comprehensive research projects documenting restoration.

In 2018, the Foundation published the results of a ten year research project following the removal of a tidal restriction at the Medouie Creek wetland complex. This project is one of the most highly monitored salt marsh restoration projects and provides insights to restoration around New England.

The Medouie Creek wetland complex is located on the northeastern shore of Polpis Harbor, a sub embayment of Nantucket Harbor. Originally one large salt marsh; channel dredging and the creation of dike roads within the marsh sometime prior to 1938, severely altered water flows and restricted tidal salt water inputs, creating an impounded freshwater marsh. As this freshwater marsh became colonized by the non-native, invasive common reed (Phragmites australis), the restoration of salt marsh ecology and function while incorporating extensive restoration monitoring, became a priority of the Foundation.

Restoration at the site consisted of draining out impounded freshwater, restoring tidal saltwater flow and facilitating saltwater 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 bank.

Restoration monitoring included documenting water level fluctuations as tidal influence increased, soil pore water salinity as soil chemistry evolved and vegetation changes in response to hydrologic and soil chemistry alteration. As a side research project, we used a common garden study to look at what level of salinity Phragmites would be negatively impacted. Phragmites plants collected from Medouie Creek saw significant health impacts when watered with salinity at 30ppt and highest (as a reference, ocean water tends to be 32ppt while freshwater is 0-5ppt).

In the first year post-restoration, the impounded freshwater marsh rapidly drained and hydrology converted to daily tidal patterns throughout the marsh. By two years post-restoration, soil pore water salinity rapidly increased throughout the marsh, altering soil chemistry. In the first two years, freshwater marsh plants experienced a dramatic die-off due to the changes in salinity and hydrology. After 4 years though, salt marsh plants began to colonize from the seedbank and brought in with tides. The first colonizing plant was Salicornia ssp (glasswort) followed quickly by Spartina ssp (salt marsh cordgrasses) and various grasses and flowering plants adapted to tidal saltwater hydrology. The non-native Phragmites experienced rapid death and health impacts as soil salinity approached 25-30ppt within Medouie Creek.

10+ years after restoration actions, the restored areas of Medouie Creek have been successfully colonized by native salt marsh plants with restored tidal hydrology bringing daily pulses of salt water into the marsh. The simple restoration act of restoring hydrology has proven remarkably successful at Medouie Creek with salt marsh function and ecology restored. In 2016 and 2021, researchers working with SHARP - Saltmarsh Habitat & Avian Research Program - documented a successful nesting population of the threatened saltmarsh sparrow thriving within Medouie Creek! 

In 2018, this project transitioned into a long-term monitoring project which will allow the Foundation to collect information about the long-term success of this restoration as well trends in adaptation of this salt marsh to the changing climate.