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From Dune to Shining Sea: The Coastal and Marine Habitats of Massachusetts
By Anne Donovan and Dr. Megan Tyrrell, CZM
The Commonwealth's coastal zone encompasses 78 communities, 1,500 miles of shoreline, 2,500 square miles of ocean and bay (an area roughly half the size of Rhode Island), and dozens of habitats. From open water to salt marsh to sandy dune, these habitats provide the plant and animal species of this region with their requirements for life, including food, shelter, and the basic conditions for survival (appropriate ranges of temperature, salinity, oxygen, etc.)
Massachusetts is blessed with a wide variety of marine habitats—largely due to its unique geographic and geologic position. During the last major ice age 35,000 years ago, three great glaciers expanded from the north, slowly moving southwestward, scouring the landforms below and collecting and carrying debris of all kinds. When the weather warmed and the ice floes melted 14,000-18,000 years ago, this accumulated debris was left behind, forming a distinct boundary between the lands scoured by this tremendous force of nature and those where the collected materials were deposited. The flexed "arm" of Cape Cod marks the boundary between the two distinct regions created, the Acadian province to the north and the Virginian province to the south. By scouring soft sediments from the surface, the glaciers left northern New England with its characteristic rocky coast. In contrast, the sediments deposited from the Cape southward created the sandy beaches, mudflats, and other soft-sediment habitats common to this region.
While the geologic raw materials of a rocky coast, sandy beach, or mud flat are vitally important in determining habitat type, the vegetation present is often of equal importance, particularly in the cases of seagrass beds, salt marsh, and kelp forests. Some animals also serve as habitat for other organisms. For example, shellfish beds found on soft sediments provide hard surfaces for a diverse community of plants and animals that need this kind of surface for anchoring. These geologic and biologic features, along with a host of chemical and physical characteristics, create the diversity of habitat types found along the Commonwealth's coast and under the ocean's surface. This article will cover the most prominent of these habitats, which include seagrass beds, salt marshes, kelp beds, shellfish beds, sandy sediments, rock, mud, and the water column.
Importance: One of the most productive marine habitat types, seagrasses rival the productivity of intensively managed farmland (Thayer et al. 1984). Seagrasses produce oxygen, which benefits the animals that live in the beds, and improve water quality by absorbing nutrients. The plants also provide physical structure that would not otherwise be available in the sand and mud, sheltering small fish, crustaceans (such as shrimp, crabs, and other hard-shelled aquatic species), and other animals, and serving as attachment surfaces for invertebrates (animals without backbones) and epiphytic algae (algae that are specialized to live on other plants). This structure raises local species diversity, and studies in New England have documented 40 species of fish living in eelgrass beds. Seagrass beds are particularly important nursery areas for commercially valuable species such as bay scallops (Argopecten irradians), blue mussels (Mytilus edulis), and winter flounder (Pseudopleuronectes americanus). The "wasting disease" outbreak along the Atlantic coast in the 1930s, which killed an estimated 90 percent of the eelgrass in the region, underscores the importance of eelgrass beds. Massive erosion of sediments and changes in water quality followed the eelgrass dieoff, with animals (including waterfowl and shellfish) that depend on eelgrass beds for both food and shelter suffering large mortalities.
Threats: These submerged plants need sunlight to survive. Decreased water clarity due to dredging, pollution, and boating activity are therefore a direct threat, as is shading from docks and piers. Physical damage from fishing, boating, and dredging can also harm seagrass beds. The most widespread current threat, however, comes from the excess input of nitrogen into estuarine and marine systems. Runoff from agricultural lands, fertilized lawns, septic systems, and other sources carries nitrogen into seagrass beds, stimulating the growth of phytoplankton and other vegetation that cloud the water and shade out the seagrass.
Importance: One of the most productive environments on earth, salt marshes serve as important nursery grounds and wildlife habitat. Commercial species that use salt marsh as breeding or nursery habitat include menhaden (Brevoortia tyrannus), winter flounder, and striped bass. Animals and plants living beyond salt marsh borders also benefit from their productivity as tides carry nutrients and decayed plant materials from the marsh to surrounding areas, fueling other marine food webs. When fish and crustaceans feed in salt marshes and then move offshore and become prey, they also transfer energy from the marsh to these other environments and their resident animals. These dynamic ecosystems provide tremendous additional benefits for humans including flood and erosion control, water quality improvements, and opportunities for recreation.
Threats: Polluted runoff from upland development is a major threat to salt marshes, contributing contaminants that harm the plants and excessive nutrients that stimulate the growth of algae. Sea level rise is also an increasingly prominent issue, potentially drowning salt marsh plants. In addition, although the Massachusetts Wetlands Protection Act has prevented the outright destruction of salt marshes and other wetlands since 1963, the legacy of detrimental wetland impacts remains visible in the undersized culverts below roads and railways that prevent adequate salt-water flow into these environments. (See Joint Ventures and Adventures in Coastal Wetlands Restoration for details on how CZM's Wetlands Restoration Program is addressing this issue.) The reduced salinity alters the plant community and facilitates the spread of the invasive reed Phragmites australis (note, this particular species does not have a consistent common name in this region so will be referred to as Phragmites throughout this article), which out-competes other salt marsh vegetation including salt meadow hay. Because of its lower habitat value for many species, biodiversity is reduced in areas where Phragmites becomes dominant. Docks and piers that span the width of the salt marsh shade the vegetation and can cause reduced growth rates or death of the plants.
Importance: Like underwater forests, kelp beds provide the same type of physical structure as trees on land. This structure serves as a refuge for a diverse array of invertebrates and fish and provides shelter from physical stresses including ultraviolet radiation from the sun and strong currents. The holdfasts, or root-like structures, harbor their own mini-world, serving as habitat for a host of small invertebrates, including brittle stars and juvenile mussels. With one of the highest primary productivity rates in the world, kelp beds also cycle nutrients (i.e., they use the energy of the sun and nutrients in the water column to produce plant material, which is then eaten by a variety of underwater animals). Extensive kelp beds reduce current speeds and buffer upland areas from erosion and storm damage.
Threats: Population explosions of herbivorous green sea urchins (Strongylocentrotus droebachiensis) led to destruction of kelp beds in many parts of the Gulf of Maine in the late 1980s and 1990s. Some scientists attributed the drastic increase in sea urchins to overfishing of groundfish that consume urchins. (Although this problem continues, the overfishing of sea urchins has somewhat reduced the pressure on kelp.) Kelps are also particularly susceptible to overgrowth by several introduced species, including the lacy crust bryozoan (Membranipora membranacea), which slows kelp growth rates by reducing light penetration. These bumpy, crust-like animals also increase the friction of the kelp surface, making it more likely that the algae will be dislodged by storm waves. Reduced water quality, especially increased sedimentation, also slows kelp growth rates by blocking light penetration. Finally, in some areas in the Gulf of Maine, kelp beds are being replaced by the introduced green fleece algae (Codium fragile ssp. tomentosoides) causing a major change in the physical structure of the seafloor with the bushy growth of green fleece algae supporting a very different biological community than the tree-like structure of kelp.
Importance: Humans, crabs, lobsters, fish, and diving seabirds all consume large quantities of shellfish. For coastal residents and tourists, clamming is an important pastime, while for commercial fishermen in Massachusetts, shellfish beds support a significant fishery. Through filter-feeding, shellfish improve water quality by removing suspended material and particulate pollutants from the water column. Shellfish beds also provide an important link between benthic (bottom) and pelagic (open water) habitats by capturing small food particles from the water column and transferring them to the benthos.
Threats: Reduced water quality is the biggest threat to nearshore shellfish beds, with high levels of nutrients, excessive sedimentation, toxics, and increased water temperatures all factors that contribute to diminished water quality. Outbreaks of disease and parasites have been implicated in the severe declines of coastal oyster populations, and reduced water quality and increased salinity are thought to contribute to the success of these pathogens. Overfishing of shellfish can also diminish their filtering function, potentially leading to increased turbidity (cloudiness due to sediments or other substances in the water) and diminished light penetration to the seafloor. Shellfish beds can be destroyed if they are dredged or if dredged material is deposited nearby or in upstream locations. Bottom-tending fishing gear, such as trawls, also harm shellfish beds through direct physical damage and re-suspension of sediments, which can slow growth rates or even smother filter-feeding shellfish.
Importance: The coastal habitat type that is most intensively used by humans for recreation, sandy beaches and the frequently abutting dunes also provide habitat to many endangered and threatened species. Bird species that nest in sand dunes or upper sections of sandy beaches include the endangered Roseate Tern (Sterna dougallii); the threatened Northern Harrier hawk (Circus cyaneus) and Piping Plover (Charadrius melodus); and the Common Tern (Sterna hirundo) and Least Tern (S. antillarum), which are both listed in Massachusetts as species of special concern. The threatened diamondback terrapin (Melaclemys terrapin) also uses sand dunes for nesting. Dunes and beaches also protect inland areas from storm surge and wind. On sandy beaches, amphipods (commonly known as beach hoppers) consume the decaying plant and animal material left by the retreating tide, and are in turn consumed by birds. Although sand-dominated habitats have relatively low rates of plant growth, they are important as foraging grounds for shorebirds, fish, and crabs.
On the seafloor, below the reaches of the tide, few organisms remain exposed in flat, sandy areas. Instead, they generally bury beneath the sand to avoid predators and currents. Some burying species include moon snails, whelks, sand dollars (Echinarachnius parma), and American sand lances (Ammodytes americanus). Another adaptation common among subtidal sandy bottom inhabitants is camouflage—flounder, gobies, skates, and shrimp are especially difficult to see against the sand. Other species, such as silver hake (Merluccius bilinearis), are commonly found within ripples in the sand, which provide protection from currents and cover for ambushing prey (Auster et al. 2003). A variety of shellfish and crustaceans inhabit subtidal sandy areas, including surf clams (Spisula solidissima), coquina clams (Donax variabilis), and hermit crabs. (See Beach Profiles for more on the residents of these sandy areas.)
Threats: Commercial and residential development on sand dunes is the most obvious human-induced threat to this habitat type. In addition, by developing just landward of dunes, humans have prevented the natural movement of these landforms away from the sea. Trampling of dune vegetation can also lead to dune demise. (Historically, trampling and overgrazing by livestock feeding on dune grass was a big problem.) Because humans have altered dunes to a large degree, extensive sand dune re-vegetation efforts and fencing (to avoid trampling of vegetation) have been undertaken on several popular sandy shores, especially on the Cape Cod National Seashore. Erosion can threaten sand beaches, especially when natural migration of sand is disrupted by jetties, groins, and seawalls. Off-road vehicles threaten sandy beach inhabitants by compacting the sand, making burying and burrowing more difficult. These vehicles can also crush organisms that live just below the surface, and disturb crabs and nesting birds.
Seaward of the intertidal zone (the area between low and high tide), sandy bottoms are generally less threatened by human activities because the organisms that inhabit these dynamic environments have adaptations and behaviors that allow them to handle strong currents, such as efficient swimming skills and a capacity to bury themselves. Nevertheless, trawling and other fishing gear can disrupt sandy bottom communities, especially if the disturbances are frequent. Sand mining for beach nourishment poses a threat to communities inhabiting sandy bottoms, especially if large quantities of sand are continually removed from one area.
Rocky subtidal habitats commonly harbor kelp (see the Kelp Beds section above), other fleshy algae, and crustose algae (algae that grow in sheet-like form over rocky surfaces). Mobile inhabitants of the rocky subtidal zone include lobsters, crabs, sea urchins, and a variety of fish species. Some of the organisms found attached to rock ledges and boulders include mussels, anemones, bryozoans, tunicates (sac-like animals with siphons—or seasquirts), and even soft corals. Finally, the biota of subtidal rocky habitats is distinct—many of the species found in these habitat types can only be found attached to rocky substrates.
Importance: The physical structure provided by both the rocks, and the plants and animals that adhere to them, provide valuable habitat for many other organisms, especially small invertebrates and juvenile fish. This structure is important for spawning and for providing protection from predation by larger organisms that cannot access the small spaces between rocks. For example, juvenile Atlantic cod (Gadus morhua) are known to congregate around rocky substrates in the subtidal zone. As previously described, kelp in the subtidal zone and the other algae in the intertidal zone (such as rockweed) are vitally important because they provide shelter and structure. Intertidal algae protect snails, mussels, barnacles, and crabs from exposure to sun, wind, rain, and predators when the tide is low. Because of their high productivity, algae in these rocky habitats also serve as important food source. The high abundance of animals that occur in subtidal rocky habitats also support larger species such as diving birds, large fish, lobsters, and humans that target these habitat types while fishing.
Threats: Coastal development can degrade rocky intertidal habitats, especially when nearshore currents are disrupted, so that sediments accumulate on rocky shores. Excessive human visitation can damage and kill rocky shore organisms that are trampled or exposed when rocks are moved. Harvesting of canopy-forming brown algae can have dramatic consequences for organisms that rely on its shelter as a buffer. Rocky intertidal shores have been the subject of scientific scrutiny for decades and recent shifts in species distributions (i.e., declines in cold-tolerant species and increases in the relative abundance of southern species), which are potentially linked to climate change, have been documented. Rock bottom habitats are also susceptible to damage by fishing gear, especially from trawls and anchors.
Importance: Noted for their high density of crustaceans and shellfish, mud flats provide an important food source for large numbers of migrating shorebirds. Wading birds also feed in shallow waters over mud bottoms, and juvenile fish commonly swim from the shallow subtidal zone to feed in mud flats that are submerged at high tide. Under the sea, mud bottoms provide habitat for a variety of benthic organisms—burrowers (clams, crustaceans, and worms) and those that remain above the mud (horseshoe crabs [Limulus polyphemus], mud snails [Ilyanassa obsoleta], skates, and fish). Undisturbed mud bottoms in the deep subtidal zone are characteristically home to tube-dwelling anemones (Cerianthis borealis), tube-dwelling amphipods, brittle stars (e.g. Ophuria sarsi), or sea pens (Pennatula aculeata). Intertidal mud flats also support recreational and commercial fisheries for soft-shell clams, razor clams (Siliqua costata), quahogs, and baitworms (the bloodworm, Glycera dibanchiata, and the sandworm, Nereis virens). Commercially important species that inhabit subtidal mud bottoms include northern shrimp (Pandulus borealis), cancer crabs, the American lobster (Homarus americanus), and winter flounder.
Threats: Historically, tidal flats were filled for development, and while this practice no longer occurs, many of these areas are permanently altered. In addition, because they are located in sheltered areas where sediments accumulate, muddy bottoms are especially vulnerable to pollution, as the contaminants deposited are unlikely to be flushed away and because pollutants easily adhere to the small grain size of muddy sediments. Contaminants from the discharge of sewage and stormwater to tidal flats, as well as use of these areas as disposal sites for dredged material, consequently have long-term consequences. Jetties and shoreline stabilization structures alter sediment flow in ways that can result in mud flat erosion or excessive sediment deposition. Bottom-tending fishing gear can stir up sediments and smother sedentary organisms. Introduced species, especially the European green crab, threaten commercially valuable mud flat species. Prior to the introduction of the European green crab, the soft-shell clam was an important fishery resource, but heavy predation by this crab is currently blamed for suppressing soft-shell clam populations.
In some areas, offshore winds blow surface waters away from shore, which results in the upwelling of bottom water to replace the surface water, creating areas of high productivity stimulated by the re-suspension of nutrients. The abundant phytoplankton that grow in these high nutrient conditions feed large quantities of zooplankton (small, floating animals), which feed dense aggregations of small fish such as herring, who in turn feed larger fish, birds, and marine mammals.
Threats: Nonpoint source pollution (runoff from the land that carries nutrients from fertilizer and septic systems; contaminants from car exhaust, pesticides, and numerous other sources; and sediments) is currently the greatest threat to coastal water quality. Harmful algal blooms, or red tides (which are caused by a superabundance of toxin-producing planktonic plants known as dinoflagellates) are also becoming increasingly prominent along the Atlantic coast. Red tides can lead to beach closures and blooms of the dinoflagellate Alexandrium sp. can lead to parasitic shellfish poisoning in humans. Overfishing may also strongly influence the species found in the water column. For example, the dramatic increases in the abundance of jellies in coastal waters has been linked to the depletion of fish stocks. Many jellies eat similar food items as fish, and food that was formerly consumed by fish is now available for jellies (Mills 2001). Global climate change, and the associated change in weather and current patterns, pose another threat to water column habitats. Resulting shifts in predominant winds could alter or halt upwelling and changes in the direction or strength of currents could affect the mixing of distinct water masses—both of which could reduce re-suspension of nutrients and lead to diminished productivity in the water column.
While such structures can improve conditions for certain species, they also diminish or destroy habitat value for many native inhabitants. In addition, the assemblages of algae and animals that attach to artificial materials are referred to by marine scientists as fouling communities, and, interestingly, many of the species found here are introduced from other regions. By placing structures in the water, humans may inadvertently be helping introduced species to become established in areas where hard substrates do not naturally occur. For more on the invasive species issue, see There Goes the Neighborhood: The 2003 Northeast Invasive Species Survey.
Finally, many species do make marine and coastal habitats altered by humans home. The habitats within ports and harbors support important populations of fish, shellfish, eelgrass, and other plants and animals. See Urban Marine Habitats for more on the habitat value of these areas.
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Thayer, G.W., W.J. Kenworthy, and M.S. Fonseca. 1984. The ecology of eelgrass meadows of the Atlantic coast: A community profile. US Fish Wildl. Serv. FWS/OBS-84/02. 147 pp.
Watling, L. 1998. Benthic fauna of soft substrates in the Gulf of Maine. In: E.M. Dorsey and J. Pederson (eds). Effects of fishing gear on the seafloor of New England. pp. 20-29. MIT Sea Grant Publication 98-4, Boston, MA.
Whitlatch, R.B. 1982. The ecology of New England tidal flats: a community profile. U.S. Fish and Wildlife Service, Biological Services Program, Washington, DC. FWS/OBS-81/01. 125 pp.
Witman, J.D. 1987. Subtidal coexistence: storms, grazing and mutualism and the zonation of kelps and mussels. Ecological Monographs 57:167-187.