Seagrasses are a valuable wetland habitat comprised of flowering marine plants that typically grow in wide continuous expanses, or meadows, in shallow, protected estuarine waters (Figure 1). Two species of seagrass are found in Massachusetts, Zostera marina (eelgrass) and Ruppia maritime (widgeon grass) although eelgrass is more common. Seagrass beds serve as spawning, nursery, refuge, and foraging habitat for a large number of fish and invertebrates (Chandler et al., 1996; Heck et al., 1989; Stoner, 1980) including recreationally and commercially important species such as scallops, lobster (Short et al, 2001) and striped bass. Seagrasses are important primary producers (organisms that generate biomass through photosynthesis) generating large quantities of carbon, which helps drive the coastal food web (Duarte and Christano, 1999). Seagrass beds also perform valuable erosion control functions, stabilizing sediment with their extensive roots and rhizomes (Ward et al. 1984). Their leaves facilitate the deposition of particles and absorb pollutants, thus improving water quality (Short and Short, 1984).
|Figure 1: Expansive eelgrass meadows (darker areas) in Manchester Harbor, Massachusetts|
Factors contributing to the loss of eelgrass are typically anthropogenic (manmade) in nature. Seagrasses are highly sensitive to changes in water quality, water clarity in particular (Lee et al, 2007), which is often degraded by the addition of nutrients from wastewater and stormwater runoff. Relatively small changes in water clarity can result in large changes in seagrass distribution (Dennison and Alberte, 1986). Other anthropogenic stressors include dredging, expansion of mooring fields, dock, pier and other coastal construction projects (Short and Wyllie-Echeverria, 1995).
The Boston Harbor Habitat Coalition has listed seagrass as a priority habitat type due to the many ecosystem services it provides related to habitat, pollution mitigation, and erosion control, as well as the sensitivity of this habitat type to anthropogenic impacts.
Seagrasses have specific habitat requirements for light, sediment type, nutrients, and wave energy. Seagrass beds are typically found where light can penetrate to the seafloor due to high water clarity. Eelgrass needs a minimum of 20% ambient light levels to survive (Dennison et al, 1993) and likely much more to thrive. As outlined above, excess nitrogen enrichment (eutrophication) from wastewater and stormwater can stimulate phytoplankton blooms, resulting in reduced water clarity and light penetration. The reduction in available light can result in seagrass meadows thinning out (lower shoot density) and the deep edge of the meadow retreating into shallower water. Seagrasses are more commonly found in softer sediments where the roots and rhizomes can penetrate the substrate. Seagrass beds are found along the coast of Massachusetts Bay in generally protected, estuarine waters, where exposure to storms and large waves is limited.
The abundance and distribution of seagrass beds in the coastal waters of Massachusetts Bay have fluctuated dramatically in the last 90 years, but the overall trend has been a dramatic loss of seagrass habitat. In the 1930s, a widespread disease outbreak resulted in the loss of approximately 90% of all eelgrass meadows on the Atlantic seaboard (Rasmussen, 1977). Surviving populations tended to be in lower salinity waters. Many areas in Massachusetts Bay recovered within 10 years, though eelgrass never returned to some areas of Massachusetts such as Essex Bay and Plum Island Sound on the Upper North Shore.
Historically, Boston Harbor supported a large amount of seagrass. Nautical charts from the 1800s show parts of the harbor covered with a bottom type described as “grass.” The location and depth range of these delineations are consistent with the habitat requirements of eelgrass. From the bathymetry on historic nautical charts, the Environmental Protection Agency estimated that eelgrass may have covered up to 16,000 acres (over 6000 hectares) of Boston Harbor (Colarusso, 2010).
Figure 2: Eelgrass in Boston Harbor
DEP surveys of Boston Harbor show a large meadow occurring behind Nahant, in Broad Sound (Figure 2). This meadow has remained relatively stable since the beginning of the study, though a slight decline over the 10 year period is evident (Table 1). Another bed is located off of the southwestern tip of Logan International Airport. Since the relocation of the Boston Harbor outfall pipe, this meadow has approximately doubled in size to 50 acres. However, recent construction of a runway safety area is expected to result in a reduction of this meadow by two acres. Some eelgrass recolonization of Deer Island and Governor’s Island flats has also been noted (but not mapped) since the outfall relocation.
Table 1: Eelgrass Trends in Boston Harbor
|Seagrass Location||Area 1996 (ha)||Area 2000 (ha)||Area 2005-2006 (ha)|
|Broad Sound (Lynn Harbor)||289.42||259.20||272.01|
|Boston Harbor (including World's End)||81.48||26.97||47.04|
In the southern part of the harbor, a low density, patchy meadow has persisted near Point Allerton in Hull. Larger, denser meadows were historically present by Grape Island and World’s End in Hingham (Figure 1). In the mid 1990s, the meadow at World’s End was estimated to be about 70 acres. Between July and September of 1995 it declined from about 70 acres to approximately 1 acre. The rapidity of this decline and lack of evidence of any physical disturbance suggests that the wasting disease may have been the cause. Recent data shows this meadow slowly gaining in size again (Colarusso, pers. comm.).
Quantitative data on seagrass condition or habitat quality for meadows in Massachusetts has been collected in a number of studies by a variety of investigators. The Massachusetts Division of Marine Fisheries (MarineFisheries) has established a long term monitoring site in Beverly that is tracking changes in a variety of plant parameters that shed light on habitat quality. The United State Geological Survey has done the same with a site in Pleasant Bay, on Cape Cod. The various meadows in Nahant have been sampled by Lent et al (1998), Colarusso (2006), and most recently by EPA as part of a coastal nitrogen study (2011).
In Boston Harbor, the two most important factors controlling seagrass condition and distribution are water and sediment quality. Since the elimination of sludge dumping, upgrading of sewage treatment capabilities, and the relocation of the Boston Harbor outfall, water quality, particularly in the northern third of the harbor, has measurably improved (Taylor, 2006; Massachusetts Bays Program, 2010). However, water quality and clarity remain a limiting factor in the recovery of seagrass meadows in the watershed. In addition to limiting water clarity, nutrient rich, eutrophic waters can support the growth of epiphytic algae on the surface of seagrass blades, impeding the absorption of light by seagrasses and reducing overall productivity. A heavy growth of algae will result in plant death. Introduced tunicates (sea-squirts), such as Botrylloides violaceus and Didemnum vexillum can also grow on eelgrass leaves, both shading them from light and also growing over and killing whole plants. Qualitative observations by seagrass biologists indicate that both the natural and transplanted grass in Boston are subject to a small to moderate degree of epiphytic algae, invasive tunicates, and sedimentation. The degree of epiphytes on the plants changes seasonally and spatially throughout the harbor.
The quality and characteristics of sediment within Boston Harbor are also important factors in controlling the distribution and condition of seagrass. Sediments in Boston Harbor are dominated by silt, clay, and sandy silt with some areas of gravel and cobble, particularly around the outer harbor islands. The history of sanitary waste disposal to the harbor also contributed a significant amount of organic material. Areas where sediment is fine-grained with a high organic content do not currently support seagrass. While sediment within an existing bed may contain a large fraction of silt/clay due to the filtering and trapping action of seagrass, MarineFisheries found that restoration efforts were not successful in areas that had >35% silt/clay, due to the increased stress on newly transplanted shoots including both higher turbidity from resuspension as well as the potential for toxicity to plants in anoxic sediments (Leschen et al 2010). Sediment quality has been slower to respond to improvements in wastewater treatment in comparison to water quality and clarity, but there have also been positive changes. Deer Island Flats has transitioned with time from a silty ooze, rich with organic matter, to a sandy silt that now supports eelgrass.
Protection and Restoration Potential
Due to the ecological importance of seagrasses, there is an extensive, international body of research on restoration techniques. Several different methods can be used to transplant or establish seagrass meadows, including seeding, hand planting, and planting sods (Evans and Leschen, 2010). In general, the vast majority of restoration attempts have failed to produce self-sustaining viable meadows. However, as outlined below, there are also examples of successful restoration efforts resulting from the application of site suitability models and transplanting methods tailored to the specific site conditions. Boston Harbor represents an opportunity for restoration due to improvements in its water quality as a result of the Deer Island wastewater treatment improvement and outfall project. As conditions continue to improve in the harbor, the opportunities for restoration and the natural expansion of seagrasses will also likely increase.
Three recent studies evaluated potential eelgrass restoration sites in Boston Harbor. From 2004-2009, MarineFisheries, conducted an eelgrass restoration project in Boston Harbor supported by mitigation funds from the construction of the Hubline project, a natural gas pipeline in Massachusetts Bay. MarineFisheries adapted the model developed by Fred Short of the University of New Hampshire (Short and Davis, 2002) to identify suitable locations for transplant. The model includes parameters related to physical, chemical, and biological characteristics, as well as historic eelgrass distribution (Table 2). As a result of the site suitability analysis and transplant work, MarineFisheries successfully transplanted eelgrass to four sites on two different Boston Harbor islands. They established new meadows on Long Island and Peddocks Island (Leschen et al, 2009), which now have persisted for over 4 years.
Table 2: MarineFisheries Eelgrass Restoration Site Selection Model
|Depth||Percent silt/clay||Previously vegetated|
|Sediment||Percent silt/clay||<70% silt/clay to very coarse sand|
|Historical distribution||Historical eelgrass distribution||Previously vegetated|
|Exposure||Fetch from the northeast direction||NE fetch < 2724m|
|Current distribution||Current eelgrass distribution||Unvegetated|
|Water quality||Nutrient content||Meets or exceeds nutrient parameters|
|Bioturbation||Density of green crabs, skates, etc||< 1 crab/m2|
In 2009, additional mitigation was required of the same Hubline project. Battelle Memorial Institute was awarded a contract to conduct further site selection work. In the summer of 2009 Battelle conducted an additional site suitability analysis (Battelle, 2009). The Battelle study evaluated potential restoration sites from Boston Harbor North to Cape Anne and added additional parameters such as sediment quality, light availability measured as photosynthetically active radiation (PAR), and a disturbance gradient to further refine the MarineFisheries model (Table 3).
Table 3: Battelle Eelgrass Restoration Site Selection Model
|Light availability (percent)||0: 0-10%
|Dessication||0: at or above -0.3m MLW
1: below-0.3m MLW
|Wave energy||0: energies above 475 W/m
1: energies below 475 W/m
|Sediment type||Areas with anthropogenic influence, high relief or medium relief cobble were eliminated as potential restoration sites|
MarineFisheries carried out the mitigation work building on the results from Battelle's site selection model. In the summer of 2011, Marine Fisheries began the final phase of Hubline restoration at sites in Boston Harbor and in Salem Sound. In Boston Harbor, sites that ranked high in Battelle's site selection model, including Deer Island Flats and Governors Island Flats, were initially slated for restoration by MarineFisheries. MarineFisheries decided to postpone restoration at these sites due to potential impacts of the runway safety area work at Logan airport. Instead, MarineFisheries continued to search for other potential restoration sites in Boston Harbor focusing on augmenting and improving existing light data to improve the model and planting additional test plots at sites near Peddock's, Long Island, and Lovell's Island.
In the outer harbor, MarineFisheries planted test plots on Lovells Island despite its initial low model rating due to gravel and cobble substrate. The outer Islands provide better water quality and light availability for eelgrass but their rocky nature is challenging for eelgrass planting. MarineFisheries tested innovative methods developed by researchers on Long Island Sound to plant in these environments. So far test-plots have fared well and MarineFisheries plans to continue restoration at these sites in the summer of 2012.
Another recent eelgrass restoration project was undertaken as part of the mitigation for the Logan International Airport Runway Safety Area project by Anchor QEA (Anchor QEA, 2010). The detailed eelgrass restoration site selection model used a combination of the MarineFisheries and the Battelle model to re-evaluate potential restoration sites previously identified by MarineFisheries. The Anchor QEA model included a more detailed assessment of light availability (as photosynthetically active radiation), presence of anthropogenic influence, desiccation potential, and sediment type (Table 5). Light availability was assessed under a normal condition and a low light/high water condition to account for cloud cover, higher than normal tides, and large rainfall events.
Table 4: Logan Runway Safety Area/Anchor QEA Project Eelgrass Restoration Site Selection
|Light availability||Level of photosythetically active radiation||>20% or <50%|
|Dessiccation||Potential for eelgrass to dry out at low tide||Below -0.3m MLW|
|Potential stressors||Presence of anthropogenic influence, anchorage area, cable area, channel boundary area, pipeline, area of sewer line||Absence of stressor|
|Suitable bottom||Anthropogenic modifications, high relief bedrock and boulder, or medium relief boulder and cobble||High relief or medium relief cobble|
The Anchor QEA study identified sites as either a 'primary' restoration site or a 'backup' restoration site (Figure 3). Anchore QEA planted four acres of eelgrass at Whitehead Flats and Old Harbor and is now monitoring the success of these transplants. Backup sites included World's End, Grape Island, and Quantum Point.
|Figure 3: Results of Anchor QEA Site Suitability analysis|
Figure 4: Hazelett Marine conservation|
mooring system (block and helix anchors)
Mitigating Physical Disturbance
There are other seagrass restoration opportunities beyond establishing seagrass beds through transplanting. Conservation moorings use an innovative mooring technology that reduces or eliminates scour of the substrate beneath the mooring while providing the same functionality of traditional mooring systems. Critical to a conservation mooring design is the change from a traditional chain which drags on the substrate, to a suspended chain or floating, flexible rode that does not touch the bottom, eliminating scour and direct impacts to seagrass. Some conservation mooring designs also eliminate the need for a concrete block placed on the seafloor and instead use a helical anchor that screws into the substrate. The helical anchor limits direct loss of benthic habitat including eelgrass (Figure 4). Moorings that are currently located within existing or historic seagrass beds can be identified as potential restoration sites and as candidates for the installation of conservation moorings. Challenges related to the implementation of conservation moorings include expense, permitting logistics, and trepidation over new and unfamiliar technology.
Improving Water Quality
Another restoration opportunity for seagrass beds exists in advocating for and implementing water quality improvement projects. Recent restoration efforts have shown that Boston Harbor is a suitable location for eelgrass restoration because of large-scale wastewater treatment upgrades. However, water quality in sub-embayments may not meet the requirements for seagrass establishment and survival, and additional stormwater or wastewater remediation efforts may be warranted.
Due to the significant capital investment required, large scale seagrass restoration efforts that require transplanting, seeding, or other methods that directly establish sealgrass are very difficult to undertake. As outlined above, there are examples of success, but these types of initiatives may not be appropriate for a municipality, small non-profit, or other conservation organization to undertake. However, there are other examples of projects that could be undertaken by the Boston Harbor Habitat Coalition and its members.
- Installation of conservation moorings: As outlined above, conservation moorings are designed to mitigate the impacts of traditional block and chain moorings to seagrass beds. There is overlap between mooring fields and seagrass beds in Boston Harbor in two locations with evidence of scarring. The first location is Nahant Harbor (Figure 4) and the second is Hingham Harbor (Figure 5). The Boston Harbor Habitat Coalition will work with the Towns of Nahant and Hingham to indentify ownership of moorings located in seagrass beds, and explore interest and funding sources for replacing them with conservation moorings. All participants must be cognizant of the relevant regulations related to replacing moorings including federal (Army Corps of Engineers), state (Department of Environmental Protection), and local authority (Conservation Commission, Harbormaster).
Overlap between seagrass beds and traditional moorings in Nahant Harbor shows opportunities for conservation moorings.
Mass Bays Program: Eelgrass - Nahant
Figure 5: Mapped mooring fields and eelgrass beds in Nahant Harbor
Overlap between seagrass beds and traditional moorings in Hingham Harbor shows additional opportunities for conservation moorings.
Mass Bays Program: Eelgrass - Hingham
Figure 6: Mapped mooring fields and eelgrass beds in Hingham Harbor
- Local Water Quality Remediation Efforts: Through the recent seagrass site suitability modeling efforts, researchers have begun to identify areas where conditions are suitable for seagrass recolonization and restoration. In many cases, suitable areas may still be limited by poor water quality and clarity. While localized water quality remediation opportunities have been identified to date in relation to seagrass restoration, these models can be used to identify areas where stormwater remediation or wastewater improvements might open the door to seagrass restoration. The Boston Harbor Habitat coalition work to identify these opportunities and advance these water quality remediation efforts.
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Boston Harbor Habitat Atlas