The number of tidal restriction projects is growing but the environmental analysis of their effects is inadequate. It is recognized that tidal restrictions reduce soil salinity and change soil chemistry as well as encourage peat subsidence. This leads to the death of native halophytes and the invasion of freshwater plants and exotic species like Phragmites. Moreover, it is increasingly difficult to quantify the effects of tidal restrictions on salt marsh landscapes because evidence suggests that community responses to tidal restoration projects vary immensely depending on the landscape. Smith et al. (2009) investigated the plant composition and cover of a restricted salt marsh after 7 years of incremental tidal restoration. They established 124 randomly assigned transect lines, which were surveyed every two years for the length of the project. After 7 years of restored tidal interaction, the restricted site still had a species composition different than the unrestricted site, but native grass coverage increased by two-thirds and the soil salinity was restored along the natural gradient. —Acadia Tucker
Smith, S., Roman, C., James-Pirri, M., Chapman, K., Portnoy, J., Gwilliam, E. 2009. Responses of Plant Communities to Incremental Hydrologic Restoration of a Tide-Restricted Salt Marsh in Southern New England (Massachusetts, U.S.A.). Restoration Ecology 17, 606–618.
Hatches Harbor Marsh in Northern Cape Cod, Massachusetts has a long history of human disturbance. Over 1400 ha of marshland have been tidally restricted by dikes during the last 350 years. The biggest dike project was established in 1930 to reduce the mosquito population and later to protect an airport from storm surges by restricting half of the tidal flow into the marsh. It severely impacted the community structure and function of the salt marsh by a serious decline in soil salinity and the expansion of invasive and freshwater species. Freshwater species expansion was so pervasive in the high marsh that shrubs and tress colonized the area creating a distinctly different habitat than the unrestricted section of the marsh. In 1998, the original 0.6 m culverts protecting the airport were replaced with four 2.1 m culverts complete with adjustable floodgates to control the amount of seawater flowing into the marsh. Seawater was introduced incrementally over a period of seven years by opening the floodgates gradually over time instead of rapidly restoring the complete flow of seawater, because authorities wanted to ensure the safety of the airport runway from tidal inundation.
There were 98 transect lines monitored in the restricted marsh and 26 transect lines monitored in the unrestricted area of the marsh. Incremental increased amounts of seawater were introduced into the restricted salt marsh each year for seven years as the floodgates were opened progressively wider every year. Smith et al. sampled the water level, soil salinity, and chemistry as well as plant composition and cover in 1997, 2002, 2004, and 2006. They used statistical tests to analyze the differences in species composition for the restricted and unrestricted sites between the different years and elevations of the marsh.
After 7 years of restoration, the restricted salt marsh had regained many of the native plant species that inhabited the unrestricted marsh. Both areas were similiar in terms of plant composition along the same salinity and elevation gradients. The tidal height of the restricted section of marsh was 0.26 m in 1997 before restoration efforts (39% lower than the unrestricted area) and increased by 50% in 2006 after the floodgates were fully opened, reaching nearly 63% of the total tidal activity of the unrestricted site. There was no change in soil salinity for the unrestricted site (average salinity was 32 ppt) but there was a dramatic jump in soil salinity for the restricted site each year after 1997. The biggest increase in salinity occurred between 1997 and 2002 during the first reinstatement of tidal activity into the salt marsh and the most significant increase occurred within 90 m of the tidal creek. Sulfide concentrations, from the decomposition of peat, remained lower than the average concentration for New England salt marshes and therefore results were considered inconclusive. However the low concentration of sulfide in the soil did indicate the high permeability and rate of leaching of the sandy peat. Of the vegetation profile recorded in 1997, roughly two-thirds of the freshwater and high marsh shrubs declined after tidal activity was fully restored in 2006.
S. alterniflora expanded much more rapidly than other native grasses. The total coverage of S. alterniflora tripled from 1997 records while S. patens and other low elevation native grasses did not expand as successfully. S. alterniflora is extremely salt tolerant and was able to survive in the lower elevations flooded by tidal seawater and colonize new areas upland. Smith et al. speculated that the low expansion rate of the other native plants suggests that they could not migrate upland fast enough to escape the increase in water level and soil salinity.
The overall cover of Phragmites remained unchanged after tidal restoration but its distribution throughout the salt marsh adjusted to the new hydrology. This invasive plant disappeared from all areas with a higher salinity of 25 ppt by shifting upland and away from the tidal creek. The authors believe that the incremental restoration of tidal activity into the salt marsh allowed for the gradual expansion of Phragmites into areas of salt-killed vegetation. Phragmites was quickly able to invade areas of the marsh that were vacant after salt intolerant plants died off from the increase in soil salinity. The decline of salt intolerant plants is rapid after restoring tidal flow but the growth of native grasses is slower than that of Phragmites. Therefore if the regulation of Phragmites is a priority for restoration projects, the most efficient way to restore a hydraulically isolated salt marsh is to reinstate the maximum tidal flow all at once. This will limit the competitive edge of Phragmites to colonize before the native grasses.