Relatively flat and sparsely vegetated areas of unconsolidated sand and mud that are exposed at low tide and submerged at high tide are known as intertidal flats (also known as "mud flats" or "sand flats"). These flats, which provide a critical link between the terrestrial and marine systems, are typically found in areas sheltered from wave action where fine-grained sediments settle (Donovan, et al., 2005). Conditions in intertidal flats are variable, given the unconsolidated nature of the sediment, and changes in temperature and presence or absence of water related to the tides. Despite the variability, these areas support a high degree of biodiversity.
|Figure 1: Intertidal flats with salt marsh (UHI)|
Primary production (the creation of new organic matter, which serves as the base of the food chain) by benthic algae occurs at high rates on many tidal flats, as does nutrient recycling through decomposition in the sediments. Many burrowing organisms such as soft-shell clams and bamboo worms dig into intertidal flats, creating burrows that help to oxygenate the mud and sand and release nutrients trapped in the sediment (Donovan, et al., 2005), all of which make intertidal flats an excellent place to find food.
As tides rise to cover the flats, juvenile fish often swim in from the nearby shallow subtidal zone to feed. High densities of crustaceans and shellfish thrive in sheltered tidal flats, making this habitat type an excellent foraging ground for migrating and wading shorebirds. Additionally, the presence of shellfish and bait worms for harvest, and the important habitat services provided to finfish and horseshoe crabs make tidal flats an important part of the marine economy.
The biotic activity in intertidal flats is critical to the base of the food web. Intertidal flats are home to benthic algae, which, as primary producers, not only create a food source, but also produce oxygen to support life in the tidal flats. The bacteria in these flats are also very important to the base of the food chain. In addition to serving as a source of food themselves, bacteria also consume dead plant and animal matter, and in the process release nutrients back into the system (Wohlgemuth, 1991).
Many animals rely on intertidal flats for foraging grounds, including the endangered Piping Plover, and the Common and Least Terns (both listed by Massachusetts as species of "special concern"), the horseshoe crab, and a variety of juvenile fish. Additionally, intertidal flats support many shellfish beds, including those of soft-shell clams, which are commercially harvested in Boston Harbor. Intertidal flats also help to mitigate the impacts from storm damage. More specifically, the gradual slope of these areas helps to slow the advances of flood waters and lessen the impacts of waves.
|Figure 2: Map showing intertidal flats (orange) in the Boston Harbor Region(UHI)|
Historically, intertidal flats in the Boston area were filled in as the City sought to create new land for its growing population and industry. Many areas of Boston – such as Back Bay and Logan Airport – were built on filled intertidal flats. Despite the great loss of intertidal flats since historic times, the habitat is still found in large swaths and small pockets within the Boston Harbor region. Some of the larger areas of flats are near Logan Airport, along the south side of Winthrop, at the mouth of Chelsea Creek, in Dorchester Bay, near the mouth of the Neponset River, off Wollaston Beach, in the Fore and Back Rivers, and in Hingham Harbor. These larger areas of intertidal flats coincide with regions of the Boston Harbor region which are less urban. According to 2006 MassDEP data, there are approximately 2,024 acres of tidal flats in the Boston Harbor region, with almost half of the flats (by area) found in Weymouth and Hingham (DEP, 2009).
While the sheltered nature of intertidal flats enables the fine sand and mud to settle and create the flats, these areas also have low flushing rates which makes them highly vulnerable to pollution from events such as oil spills and rain storms which wash pollutants into the water. In fact, the Environmental Sensitivity Index (found in Annex K of the US Coast Guard's Plymouth to Salisbury, Massachusetts Area Contingency Plan) rates exposed tidal flats as a 7 and sheltered tidal flats as a 9 on its scale of environmentally sensitive habitats for oil spill contamination (with 1 being least sensitive, and 10 being most sensitive) (Michel, et al., 1994).
|Figure 3: Degraded intertidal flat (UHI)|
Contaminants deposited in tidal flats often remain in the area for a long time, easily adhering to the small grains of sand and mud. Contamination is made worse by culverts and other types of development that reduce the flow of water over flats. The impacts of combined sewer overflows (CSOs) on tidal flats in Boston Harbor (specifically bacterial contamination) have been documented in Dorchester Bay's Savin Hill Cove (Shiaris, et al., 1987), and future monitoring should show whether or not sediment conditions improved as a result of efforts to replace the area's CSO with a large holding tank.
Furthermore, flats are subject to erosion and accretion due to coastal development activities such as the construction of jetties, wharves, and seawalls that disrupt water flow in the area. Coastal development activities and certain bottom-fishing activities can also negatively impact tidal flats by stirring up sediment which can then release contaminants into the water column and/or smother organisms.
Although many historical intertidal flats in Boston were filled in during the City's expansion, there has been relatively little loss of flats during the last decade. Additionally, with the Deer Island treatment facility and outfall pipe fully functioning, water quality has generally improved throughout the Harbor. Improved water quality will likely lead to the re-colonization of intertidal flats, and has supported the success of projects such as DMF's soft-shell clam seeding efforts. Despite these improvements, there may be loss or migration of flats in the next century, since sea level rise poses the risk of submerging some of these areas. Such losses will also pose a risk the organisms supported by intertidal flats (Galbraith, et al., 2005).
While a great deal is known about intertidal flats, there are several pieces of information that would be of use in terms of planning protection and restoration activities:
- Historical extent: While the current extent of tidal flats is available for the Boston Harbor region, spatial information about the extent of historic tidal flats is not readily available, making it difficult to know exactly how many acres of flats have been lost over the past several hundred years. Additionally, some report that the spatial extent of tidal flats currently available does not represent all intertidal flats in the Boston Harbor region. Specifically, it was noted that all soft-shell clam habitat should also be mapped as intertidal flats. Obtaining and updating information on the current and historical extent would provide the means to identify areas suitable for tidal flat restoration. LIDAR data may help with mapping the current extent of this habitat.
- Sediment condition: Information about the sediment condition of existing intertidal flats would also help to identify sites in need of restoration. The Massachusetts Water Resource Authority conducts sediment sampling in Boston Harbor, but sampling sites do not include intertidal flats
- Potential loss due to sea level rise: As communities plan for sea level rise, it would be useful to identify those tidal flats at greatest risk of complete submersion
Protection and Restoration Potential
Intertidal flats are afforded some level of protection at the municipal level through town wetland protection by-laws, and at the state level under the Wetlands Protection Act (MGL Ch 131 §40), which states, "No person shall remove, fill, dredge or alter any bank, riverfront area, fresh water wetland, coastal wetland, beach, dune, flat, marsh, meadow or swamp bordering on the ocean or on any estuary, creek, river, stream, pond, or lake, or any land under said waters or any land subject to tidal action, coastal storm flowage, or flooding, other than in the course of maintaining, repairing or replacing, but not substantially changing or enlarging, an existing and lawfully located structure or facility used in the service of the public … without filing written notice of his intention … and without receiving and complying with an order of conditions … ."
Despite the regulatory protections, restoration of intertidal flats in the Boston Harbor area is certainly possible. Restoration of tidal flats is often conducted as part of a salt marsh restoration or shellfish bed restoration project. Even if restored for its own reasons, however, there is benefit to restoring intertidal flats adjacent to other protected and/or significant habitats to maximize the positive impacts. Some example criteria for selecting restoration sites (Goals Project, 1999) include:
- Restoring flats between subtidal and tidal marsh habitats.
- Using sediments that do not contain toxins and can support organisms.
- Avoiding or minimize the presence of pilings and other artificial structures which can disturb sediment and alter habitat.
- Taking steps to minimize and assess levels of human disturbances, including the removal of debris such as that found in Savin Hill Cove.
Mapping the true location, condition, and vulnerability (to contamination and to sea level rise) of existing intertidal flats would be a useful first step in helping to successfully evaluate the significance of any restoration or protection effort.
Department of Environmental Protection. (2009). DEP Wetlands (1:12,000). MassGIS. Available online at: http://www.mass.gov/mgis/wetdep.htm (last viewed 11/14/2011).
Donovan, A. and Tyrrell, M. 2005. From Dune to Shining Sea: The Coastal and Marine Habitats of Massachusetts. Coastlines: Winter 2004-2005. Massachusetts Office of Coastal Zone Management. Available online at: http://www.mass.gov/czm/coastlines/2004-2005/habitat/lead_hab.htm (last viewed 11/14/2011).
Goals Project. 1999. Baylands Ecosystem Habitat Goals. A report of habitat recommendations prepared by the San Francisco Bay Area Wetlands Ecosystem Goals Project. U.S. Environmental Protection Agency, San Francisco, Calif./S.F. Bay Regional Water Quality Control Board, Oakland, Calif. Pages 141-174. Available online at: http://www.sfestuary.org/userfiles/ddocs/Habitat_Goals.pdf (last viewed 11/21/2011).
Galbraith, H., Jones, R., Park, R., Clough, J., Herrod-Julius, S., Harrington, B., and Page, G. (2005). Global Climate Change and Sea Level Rise: Potential Losses of Intertidal Habitat for Shorebirds. USDA Forest Service General Technical Report PSW-GTR-191. Available online at: http://www.fs.fed.us/psw/publications/documents/psw_gtr191/psw_gtr191_1119-1122_galbraith.pdf (last viewed 11/21/2011).
Massachusetts General Laws Ch 131 §40. Wetlands Protection Act. Available online at : http://www.mass.gov/dep/water/laws/ch131s40.pdf (Last viewed 11/21/2011).
Michel, J., Christopherson, S., and Whipple, F. (1994). Mechanical Protection Guidelines. Annex K in the United State's Coast Guard's 2011 Plymouth to Salisbury, Massachusetts Area Contingency Plan.
Patrick-Murray Administration Announces Reopening of Hull Shellfish Bad: Bed reopens after three-year project, part of a Boston Harbor shellfish restoration effort. Oct. 13, 1010. Available online at: http://www.mass.gov/eea/patrick-murray-administration-announces-6.html (last visited 11/21/2011).
Shiaris, M.P., Rex, A.C., Pettibone, G.W., Keay, K., McManus, P., Rex, M.A., Ebersole, J. and Gallagher, E. (1987). Distribution of Indicator Bacteria and Vibro parahaemolyticus in Sewage-Polluted Intertidal Sediments. Applied and Environmental Microbiology. 53(8).
Shields, T. Shellfish Stock Enhancement Project. Available online at: http://www.mass.gov/dfwele/dmf/programsandprojects/hubline/hubline_5yr_shellfish_stock_
enhancement.pdf (Last visited 11/21/2011).
Swain, P.C. & J.B. Kearsley. (2001.) Classification of the Natural Communities of Massachusetts. Version 1.3. Natural Heritage & Endangered Species Program, Division of Fisheries & Wildlife. Westborough, MA. Available online at: http://www.mass.gov/dfwele/dfw/nhesp/natural_communities/pdf/marine_intertidal_flats.pdf (last visited 11/21/2011).
Wohlgemuth, M. (1991). Primary Producers and Decomposers of Intertidal Flats. Wetlands Program Technical Report. Available online at: http://ccrm.vims.edu/publications/wetlands_technical_reports/91-4-primary-producers.pdf (Last viewed 11/21/2011).