Salt marshes provide food and shelter for a variety of invertebrates, fish, and birds. Striped bass, flounder, mummichogs, and sticklebacks use salt marshes for food, shelter, spawning, and nursery areas. Food webs of the marshes and mudflats in the upper Bay of Fundy are important to the summer feeding of American shad. Clams and ribbed mussels inhabit salt marshes and adjacent tidal flats. Birds rely on salt marshes for breeding habitat and migratory rest stops, and thrive on the rich abundance of food. Sharp-tailed sparrows, seaside sparrows, long-billed marsh wrens, and American bitterns all nest in salt marshes.
Ice scouring, which can retard the growth of a salt marsh, remains a natural threat to these habitats. From the time when cordgrass begins to take hold, it may take over 500 years for a salt marsh to reach maturity (Berrill and Berrill, 1981). The last glacial period wiped out the salt marshes that existed in the present-day Gulf of Maine.
Removing tidal restrictions
The natural hydrology of tidal wetlands is affected by tidal restrictions, dikes, and fill deposited on the wetland. Tidal restrictions occur where manmade structures block or restrict natural tidal flow to wetlands. They are usually caused by roads, causeways, dikes and filled areas. Tidal restrictions are common throughout the Gulf of Maine. Inventories of tidal restrictions are completed or underway in all jurisdictions surrounding the Gulf of Maine.
Ecosystems that once supported salt marsh vegetation often become freshwater-dominated environments when the tidal influence is restricted. Tidal restrictions prevent fish from entering the marshes, change the physical and chemical properties of tidal wetlands, and cause erosive scour pools on either side of the restriction because of increased water velocity.
Tidal restrictions can often be reduced or eliminated by enlarging existing culverts or installing bridges. Each project must measure the degree of restricted flow and estimate the tidal heights that would result from increased flows. In highly developed areas of the Gulf of Maine's coast, it may be necessary to regulate tidal flow to protect flooding of infrastructure that might be affected if flow were unrestricted.
Self-regulating tide gates (SRT) are devices that regulate the flow through culverts. SRTs are used in situations where there is risk of tidal flooding (and property damage) from unrestricted ocean water flow. The SRT usually remains open to allow free exchange of tides if the tides are not abnormally high. If the tide reaches the maximum setting of the SRT (during storm surges or spring tides), the gates will close and stop the inland flow of water until the water level recedes to the preset height. SRTs are an alternative in highly developed areas of the Gulf of Maine.
Removing dikes and fill
Many salt marshes have been filled and diked. Marshes often show evidence of sediment fill from nearby dredging for navigation purposes. Dumping dredged material on salt marshes was a common practice throughout the Gulf of Maine into the 1970s. This practice kills the native vegetation and raises the elevation of the marsh to an unnatural height. Because salt marshes depend on regular influxes of saltwater and natural peat accumulation, sediment placed on top of the marsh is detrimental to the marsh ecosystem. Removal of dredge sediments from salt marshes may restore a marsh to a natural condition. Grading the surface elevation of the marsh is critical to restoring the native vegetation since different plant species rely on varying salt regimes and saltwater inundation.
Diked marshes have berms designed to hold back salt water. The berms are often created to convert part of the marsh to agricultural land. This occurred throughout the Gulf but is particularly common practice in the upper Bay of Fundy in New Brunswick and Nova Scotia. Many of the largest salt marshes in the Gulf of Maine are in the Bay of Fundy region and large areas have been diked for agricultural use. Many of these areas remain diked, and in Canada the agricultural agencies at the Provincial level are obligated to repair the dikes to maintain farmland. In some instances, diked marshes have failed because dikes were not maintained, causing serendipitous habitat restoration by allowing salt water back onto the marsh. There is considerable opportunity for removal of dikes from salt marshes that are no longer used for agriculture.
Controlling invasive species
Numerous invasive species inhabit the Gulf of Maine and are often indicative of anthropogenic disturbance. The invasive common reed is an indicator of salt marsh disturbance. Common reed can create a monoculture environment that eventually will reduce the diversity of the native plant community by crowding out other species. Typically, this results in a reduced diversity of fish, birds, and other species that rely on salt marshes. Common reed can grow so densely that vertebrates have a difficult time utilizing the marsh. In addition, common reed can be a fire hazard because the dry stems can fuel large fires. Common reed can be managed by eliminating tidal restrictions and allowing adequate salt water to enter and exit the marsh system. Cutting and sometimes herbicide treatment can slow the growth of common reed and allow native vegetation to return.
Other threats
The threats identified above are the focus of salt marsh restoration efforts in the Gulf of Maine. Salt marshes face other threats, such as dredging and non-point source pollution, but they are beyond the scope of this document.