Rocky intertidal habitats are dynamic, high-stress areas that provide an important link between marine and terrestrial systems. Conditions including wave action, changes in salinity, thermal stress, alternating periods of exposure and submersion, and exposure to winter ice have necessitated that organisms develop strategies to survive in this stressful environment. For example, mobile organisms hide under algae and rocks or move within the rocky intertidal habitat to protect themselves from predation and desiccation as the tides retreat. Less mobile organisms, such as mussels and snails, seal their shells to retain moisture until the tides rise again.
As the tides rise, animals can be exposed to pounding waves and swift currents. To avoid being washed away, some organisms use an anchoring mechanism to secure themselves to the rocky surface (e.g. byssal threads of mussels, muscular feet of limpets, etc.). Other organisms take refuge under rocks or in rocky crevices. Having flexibility, a low profile, and a streamlined shape all help to minimize the impacts of waves on certain organisms.
|Figure 1: Rocky intertidal habitat (Urban Harbors Institute)|
Rocky intertidal areas exposed to wave action show a distinct zonation pattern related to the gradient of exposure/submergence along the vertical slope (Donovan, et al., 2005). Just above the high tide line is the "splash zone," which is subject to sea spray as waves crash against the rocks. The splash zone is commonly covered in encrusting algae and dark lichens that are tolerant of sea spray. Few other marine organisms can tolerate the almost terrestrial conditions found in the splash zone.
Below the splash zone is the high intertidal zone, which is exposed to air for a large part of the day. The twice-daily submersion brings necessary food, nutrients, and dissolved oxygen to support a wider variety of organisms than in the splash zone, however, this area still provides little refuge from the hot sun and cold air, and is typically dominated by barnacles.
Similar to the high intertidal zone, the mid-intertidal zone is submerged twice a day, but the longer submersion and resulting access to increased nutrients and dissolved oxygen create a more hospitable habitat for marine organisms. These areas are typically dominated by mussels, gastropods, and algae which provide shelter for mobile organisms (Donovan, et al., 2005). Predation, competition for space, larval settlement, and grazing opportunities in the high and mid-intertidal zones play significant roles in colonization, as compared to the splash zone, where colonization is driven largely by the ability of an organism to survive the harsh conditions (Senechal-Brown, 1996).
The lower intertidal zone is submerged for the majority of the day and supports more diverse communities than the other zones within rocky intertidal areas. Predation by organisms such as sea stars prevents colonization by mussels and barnacles, causing this area to be dominated by algae, which provide food and cover for many additional species.
In addition to variations between the zones of intertidal areas, variations also exist depending on whether or not an area is exposed to heavy wave action (Senechal-Brown, 1996). Areas experiencing heavy wave action favor those organisms with the capability to withstand waves (e.g. has anchoring mechanisms or the ability to retreat into a different zone or a crevice or under an overhang).
|Figure 2: Grey seals on Boston Harbor Islands (National Park Service)|
Some rocky intertidal habitats also have tidepools - areas of standing water that are subject to some tidal inundation. These tidepools often host communities different from those on the surrounding rock. The nature of these communities varies depending on where the pools are located with respect to tidal height (e.g. in the lower intertidal zone as opposed to the high intertidal zone) (Senechal-Brown, 1996). The biotic community of any given tidepool varies with the frequency of tidal influence, the distribution of potentially colonizing species, and the associated physical conditions such as water temperature and salinity, with tidepools in the lower areas accommodating a larger variety of organisms than those in the higher areas (Senechal-Brown, 1996).
Rocky intertidal areas do occur naturally in Boston Harbor, but to a limited extent, and mostly on the Boston Harbor Islands. Manmade rocky intertidal areas also provide many of the same services as natural rocky intertidal habitats, in addition to fulfilling their roles as wharves, shoreline hardening/retaining structures, bridge foundations, etc. While these manmade habitats certainly have value, they are also more susceptible to colonization by non-native species which can out-compete native species (Tyrrell, et al., 2007).
Rocky intertidal habitats provide many ecosystem services such as stabilizing shorelines and protecting upland areas against the eroding action of waves and the impacts of storm surges and sea level rise; creating dramatic vistas; offering easily accessible coast for recreation, education, and research; providing haul-out areas for seals; supporting the diets of foraging birds; and supporting diverse organisms vital to the base of the food web.
The Massachusetts Department of Environmental Protection (MassDEP) wetlands GIS layer identifies approximately 105 acres of rocky intertidal habitats (both natural and manmade) in the Boston Harbor area, with much of the natural rocky intertidal habitat found on the Boston Harbor Islands. According to a 2004 study conducted by the National Park Service, rocky intertidal substrates (including rock, boulder, and cobble substrate types) occupy 27.4 hectares on the 20 Harbor Islands that were included in the survey, which is 8.23% of all intertidal substrates. Rocky intertidal substrates occur on all but one of the surveyed islands, ranging from none on Sheep Island in the inner harbor, to more than 95% on Little Brewster Island in the outer harbor. Overall, of the 20 surveyed islands, six islands have less than 5% rocky intertidal substrate, nine have 5-25%, five have between 31-42%, and two have >90%. Of the seven islands with more than 31% rocky intertidal substrate, five were in the outer harbor area and two (Raccoon and Langlee) were in Hingham Bay. Five additional outer harbor islands with substantial rocky intertidal habitat (Green, Little Calf, Middle Brewster, Shag Rocks, and The Graves) were not included in the survey.
|Figure 3: Boulders, rock, and cobble rocky intertidal habitat on Boston Harbor Islands (2011 NPS Survey)|
|This map shows four islands for which there is no data from the NPS study – Green, Little Calf, Middle Brewster, and Shag Rocks (from north to south). Additional data on rocky intertidal habitats is available in the DEP Wetlands datalayer; however the focus of that datalayer is wetlands, therefore the rocky intertidal area may not be fully represented.|
While little rocky intertidal habitat seems to have been lost in Boston Harbor, many man-made structures (wharves, bulkheads, rip rap, etc.) have replaced other types of habitats throughout Boston Harbor's history, increasing the amount of artificial structures which mimic rocky intertidal habitat.
One example of where rocky intertidal habitat has been modified in Boston Harbor is Nixes Mate, one of the Boston Harbor Islands. Legend has it that the once twelve-acre island, which was used to hang the bodies of pirates as a warning to other pirates, more-or-less disappeared in accordance with the prophecy of an innocent man who was accused of piracy and hanged at the Island. Another explanation, however, is that the Island was whittled away by exposure to wind and waves, as well as the extraction of shale and cobble for ballast (Klein, 2008). Portions of Winthrop were also excavated and used for ballast (Department of Conservation and Recreation, 2011).
One of the most common stressors to rocky intertidal communities is the colonization by non-native species. Non-native species have fundamentally altered coastal communities and ecosystems. On artificial structures non-native species, especially sea squirts may represent up to 40% of the biomass. Others, such as the green crab and periwinkle snail, are ecological engineers in rocky shore intertidal habitats and have changed the distribution of plants and animals. The challenge is to prevent new invasions because once a non-native species is established, it is virtually impossible to eradicate it.
|Figure 4: MIMIC 2010 monitoring sites in Boston Harbor|
Since 2000, taxonomic specialists have been surveying non-native species on floating pontoons and associated structures. Non-native species arrive as a result of activities such as shipping, hull fouling, marine recreational activities, aquaculture along with other human mediated activities such as building canals, releasing live organisms and seafood, and hull cleaning to name a few. Many of these activities take place in ports and harbors where organisms can easily transfer to the artificial surfaces of floats and docks on which they settle, become established and disperse. The surveys conducted by MIT Sea Grant identified approximately 25 non-native species on the floating structures alone, of which six were previously not reported.
Several additional efforts have monitored and/or are monitoring non-native species in Boston Harbor:
- The National Park Service 2004 intertidal survey of 20 Boston Harbor Islands included an inventory of biotic assemblages which showed relatively little cover of non-native species on rocky intertidal substrates.
- Additional monitoring, utilizing the Massachusetts Office of Coastal Zone Management's MIMIC protocol, has identified non-native species on docks and cobble beaches since 2009.
- A long term monitoring program to assess the health of the rocky intertidal biota of the Boston Harbor Islands has been established by the National Park Service. Permanent sampling plots have been established on Outer Brewster, Green, and Calf islands. This protocol, which is also being conducted at Acadia National Park, in Maine, and at Maine Coast Islands National Wildlife Refuge, will assess trends in more than 30 species along with environmental variables.
These monitoring programs are useful not only for identifying trends in non-native species, they also provide important baseline information and information about impacts to native communities. The results of these monitoring programs could be used to identify impacts of sea level rise, to assess the damages related to a contamination event (see Aquatic Exposure and Injury Report: Bouchard B-120 Oil Spill for a contamination assessment and valuation example), and to inform management decisions.
In addition to the concern for colonization by non-native species, rocky intertidal areas face a wide variety of pressures including:
- Contamination from oil spills – oils may impact each of the zones differently and can affect the habitat differently depending on the nature of the contamination, the location of the spill in proximity to the habitat, the duration of exposure to the contamination, and the biota affected (FWS, 2010).
- Contamination from runoff – nutrients, sediments, pesticides and other runoff contaminants can impair water quality and an organism's ability to survive. Runoff likely has greater impact on rocky intertidal habitats on the mainland as compared to those habitats found on the Boston Harbor Islands simply due to their proximity to human development.
- Climate change – this may lead to warmer water species migrating north.
- Sea level rise – as sea levels rise, the zonation patterns may shift higher onto the shore or create new rocky intertidal areas out of areas that were not historically exposed to the influence of tides.
- Direct human disturbances – includes impacts from people visiting rocky intertidal areas for recreation, education, and research as well as impacts from algal harvesting and responses to oil spills (FWS, 2010).
- Construction projects – includes activities that cause sedimentation, increase turbidity, disrupt water circulation, alter water temperature, and otherwise affect water quality (Bliven, et al., 2003).
Several data gaps exist with regard to the extent and condition of/stressors to rocky intertidal habitats. These areas serve as opportunities to expand the existing knowledge of intertidal habitat with the Boston Harbor region to better restore and protect the critical and threatened habitat.
- Non-native species: In addition to the National Park Service's new monitoring protocol mentioned above, and the ongoing assessments utilizing MIMIC, steps should be taken to monitor additional rocky intertidal areas for non-native species. As part of this monitoring procedure, it would be helpful to identify whether or not the non-native species were found on natural or artificial habitats, and assess the impacts of non-native species.
- Artificial rocky intertidal habitats: Identify and map those man-made structures that mimic rocky intertidal habitats.
- Sea level rise: Identify those rocky intertidal areas that might be most severely impacted by sea level rise, and those areas which might eventually become rocky intertidal areas.
- Visitor impacts: Identify those areas most impacted by visitors.
- Oil spill vulnerability: Identify those rocky intertidal habitats that might be most vulnerable to oil spill contamination.
- Extent: Expand upon the work completed by the Department of Environmental Protection and the National Park Service to identify and map the full extent of natural rocky intertidal habitat in the Boston Harbor area.
Protection and Restoration Potential
While rocky intertidal areas are not commonly restored, the rarity of these naturally occurring habitats within the Boston Harbor region helps to elevate the importance of protecting them. Rocky intertidal shores receive some level of protection from development-related disruptions under the Massachusetts's Wetlands Protection Act. In addition, specific efforts should be made to prevent damage from construction projects. In Rocky intertidal communities: past environmental changes, present status and predictions for the next 25 years, the author suggests that tidal and wind energy projects could alter the wave patterns and adversely impacte rocky intertidal habitats. Should this become an issue in the future, guidelines for minimizing and mitigating (e.g. via reef balls) impacts to rocky intertidal habitats should be developed.
Development poses just one threat to rocky intertidal areas; however, humans can also harm rocky intertidal areas through excessive visitation and/or improper use. The harvesting and trampling of organisms are two common problems associated with visitation. While it is important to make these areas accessible for the educational, recreational, and research values, a "Best Management Practices" document should be developed and implemented to minimize visitor impacts. As part of this research, efforts should be made to set aside areas protected from visitor impacts. A specific section of this plan should focus on guidelines for low-impact educational use of rocky intertidal habitats.
|Figure 5: Common terns on Boston Harbor Islands (National Park Service)|
Additionally, based on initial oil spill response maps for Boston Harbor, generated through the development of the Massachusetts Geographic Response Plan (Nuka Research and Planning Group, 2010), rocky intertidal areas do not seem to be a priority habitat warranting special protection. According to the Environmental Sensitivity Index used in the Coast Guard's Plymouth to Salisbury Area Contingency Plan, sheltered rocky intertidal habitats are quite vulnerable (rated an eight on a scale of one to ten, with ten being most vulnerable), while exposed rocky intertidal habitat types are much less vulnerable, receiving rankings of one (for exposed rocky cliffs, and vertical sea walls) and two (for exposed wave-cut platforms in bedrock) on the scale Michel, et al., 1994). Especially given the vulnerability of sheltered rocky intertidal habitats, updates to that plan may need to consider these areas as deserving of special protection.
Non-native species monitoring and control programs are another strategy that can help protect rocky intertidal habitats and their biological integrity. Early detection and eradication of non-native species can limit competition with native species, and can minimize the spread of non-native species. Public education initiatives can also reduce the introduction of new species, and can help with monitoring and eradication activities. Education and early detection and eradication efforts are already underway in the Boston Harbor area, and should be enhanced as appropriate.
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