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Spotlight on Eelgrass A Species and Habitat at Risk By Anthony R. Wilbur, CZM Charles Darwin, the prominent naturalist who wrote On the Origin of Species in 1859, articulated the complexities of natural history, including evolution, species interactions, and community structure. Darwin explained that the abundance of every species is influenced by that of other species, which serve as its food, create its habitat, and/or influence it through a web of relationships. Consequently, the removal of one species or change in its abundance impacts the existence of other species. Since Darwin's original writings, ecologists have validated the concepts of evolution, species interaction, and coevolution, and have described species that make an unusually strong contribution to community structure or processes as keystone species. Sea otters on the Pacific coast of North America, for example, are a keystone species. Without sea otters and their appetite for sea urchins, the unchecked urchin population can explode. Urchins then heavily graze on kelp, devastating kelp habitat, with impacts that cascade further through this marine community. The keystone species concept can mean different things to different people (as is the case with many ecological concepts)—nevertheless, eelgrass (Zostera marina) clearly merits this label. Eelgrass is a prolific primary producer (i.e., converts sunlight to energy), supports diverse and distinct groups of species, provides critical nourishment to herbivores and detritivores (animals that eat partly decomposed organic material), and affects chemical and physical processes in coastal waters. In short, coastal waters with eelgrass are much different than those without it. The outbreak of wasting disease (caused by a slime mold) that devastated populations of eelgrass throughout waters of North America and Europe in the 1930s emphasized the ecological importance of eelgrass and justified its label as a keystone species. Populations of species dependent on eelgrass, as well as those indirectly associated with eelgrass, were substantially reduced. For example, the eelgrass limpet (Lottia alveus) went extinct, Atlantic Brant (Branta bernicla hrota) populations nearly vanished from North America, and bay scallop (Argopecten irradians) stocks crashed. In addition, current patterns and sediment movement changed because eelgrass no longer anchored seafloor sediments. Eelgrass populations generally recovered from this epidemic. However, because of the environmental consequences of eelgrass degradation of the 1930s, contemporary loss of eelgrass warrants further attention to identify and address the causes of decline. Geographic Range and Environmental Requirements of Eelgrass Eelgrass is common to estuarine and marine waters of the north Atlantic and Pacific Oceans. In the northwest Atlantic, eelgrass ranges from New Brunswick to North Carolina, including large areas in Massachusetts. Eelgrass is part of a group of marine plants collectively known as seagrass, which, unlike seaweeds or algae, have a vascular system to carry liquid and nutrients through the plant. There are over 60 species of seagrass worldwide, and two inhabit the coastal waters of Massachusetts—eelgrass and widgeon grass (Ruppia maritima). Eelgrass is the most abundant seagrass species in Massachusetts, forming extensive meadows and patchy beds, and sparsely covering vast coastal areas. Eelgrass is a living habitat, and its location and abundance change through time. (See Eelgrass: Its Habits and Habitats for details of eelgrass anatomy and habitat requirements.) Eelgrass grows in a range of environmental conditions, with hydrodynamic factors (such as tidal current speed, water depth, and exposure to waves) determining the character of eelgrass habitat. Waves and currents in high-energy, shallow waters move seafloor sediments, and the eelgrass in these environments grows and migrates with the moving sand, forming linear bands. Sheltered, low-energy embayments, however, can be completely covered by a contiguous eelgrass bed. Light penetration in the water column is also important to eelgrass growth, with eelgrass generally growing denser and deeper in clear water. Bed size and density may influence ecological functions of eelgrass, but whether in a dense meadow or sporadic individual stands, eelgrass is an essential part of coastal ecosystems and its presence indicates the overall environmental quality of coastal waters. Ecological Functions Eelgrass forms a complex underwater landscape, with long, narrow leaves floating and swaying in the water column, and tangled roots anchoring the plant to the seafloor and rhizomes connecting one plant to the next. The habitat provided not only depends on whether the eelgrass grows in a dense meadow, patches, or individual strands, but on the species and its life history stage when living in eelgrass habitat. (See Moving on Up (and Down) .) The bright green leaves of eelgrass are frequently covered by an assemblage of algae and invertebrates whose habitat needs are satisfied by an individual leaf. Bay scallops and mussels are examples of species whose habitat, at least for a time during their lifecycle, is a blade of eelgrass. The size of a pinhead, scallop and mussel larvae settle from the water column onto eelgrass, which provides refuge from predators that are unable to swim or climb eelgrass blades, and a stable supply of food flowing by the leaves. When these bivalves grow, they let go of the blade and move to the seafloor. Fishes and crabs, on the other hand, are associated with eelgrass habitat on a broader scale. The community of fishes and crabs inhabiting eelgrass is dramatically different from areas devoid of the plant, with eelgrass habitat supporting a higher diversity and abundance of life. Certain fishes, such as pipefish (Syngnathus fuscus), inhabit eelgrass habitat for the majority of their lives. Other fish and crab species, while capable of surviving in other habitats, use eelgrass during parts of their life cycle. Atlantic cod (Gadus morhua), tomcod (Microgadus tomcod), cunner (Tautogolabrus adspersus), rock crab (Cancer irroratus), and American lobster (Homarus americanus), for example, move around the leaves and stems and scurry along the bottom among the roots, using the eelgrass for protection from predation and to ambush prey. These mobile species and many others also use eelgrass as nursery habitat. Many benthic invertebrates live in and around the root system of eelgrass. Newly settled lobster burrow between the roots, and a diversity of lesser-known animals from worms to snails inhabit the sediments among the roots and rhizomes. Not only do these creatures find refuge and prey in eelgrass beds, many creatures directly forage on leaves and the accumulated partly decomposed leaves (detritus). Species also frequently live around, not necessarily within, eelgrass beds, periodically moving into beds for protection or to feed. Winter flounder (Pseudopleuronectes americanus), for example, are observed in bare areas between eelgrass patches. Above the surface, waterfowl peer through the water looking for a meal of eelgrass or the creatures harbored there. The biological community associated with eelgrass is well studied in temperate waters, but the ecology of eelgrass in southern New England and the Gulf of Maine is not completely understood and more associated species and species interactions are sure to be discovered in the future. Eelgrass leaves naturally break away from the root system every autumn, similar to trees losing their leaves. These leaves accumulate on the shore and provide important ecological services. Piles of beach wrack are found all along the coast and may be the only aspect of the eelgrass lifecycle that beachcombers notice. The beach wrack, predominately composed of eelgrass in many areas, shelters abundant insect and amphipod communities that are critical prey for shorebirds, including the threatened Piping Plover (Charadrius melodus). Not only is wrack important foraging habitat, it captures and holds sand and other sediment, helping to reduce beach erosion. Most leaves that become detached do not reach the shore, however, and sink to the seafloor, creating a detritus soup that is fundamental to the ocean's food web and is used by deposit feeders and exported to other biological communities. Eelgrass also contributes to chemical and physical processes of coastal waters. Eelgrass produces oxygen (through photosynthesis), absorbs nutrients and pollutants, and improves water quality. Leaves slow water movement and roots stabilize sediments, promoting sediment deposition and minimizing shoreline erosion. Overall, the presence and condition of eelgrass is a strong indication of the environmental quality of coastal waters. Threats to Eelgrass Habitat Physical disturbances, such scarring from boat propellers, anchors, and mooring chains, and activities that alter intertidal and subtidal environments (e.g., dredging, shellfishing, and aquaculture activities) can degrade and reduce eelgrass populations. Poor water quality is also a significant threat, since eelgrass has the highest light requirement of any marine plant. Minor changes in light availability, which is synonymous with water clarity, substantially influence the quality of eelgrass habitat. Light available to eelgrass is dictated by phytoplankton abundance, algal abundance and cover (e.g., epiphytic algae and benthic algae), and sediment suspension (turbidity). Eutrophication (i.e., nutrient over-enrichment), which is typically linked to development in coastal watersheds and the associated runoff from ground-water, lawns, parking lots, and septic systems, promotes growth of algal epiphytes (algae species that grow on other plants) and phytoplankton (microscopic plants in the water column). Both of these plant types absorb light in the water column and decrease its availability to eelgrass. Low water clarity and high nutrient levels stimulate the proliferation of benthic and drift algae. These algae require less light and can smother or out-compete eelgrass for space. Other pollutants degrade and kill eelgrass, such as herbicides used on lawns and larger landscapes (e.g., golf courses). Currently, eelgrass loss in Massachusetts is more widespread on southern Cape Cod and Buzzards Bay than in the waters to the north. Although historic losses in Massachusetts Bay were dramatic and contemporary disruption of eelgrass habitat remains, existing beds in waters of western Massachusetts Bays generally appear stable. The watershed and coastal development that occurred in the past several decades on Cape Cod and in southeastern Massachusetts has resulted in increased nitrogen loads and extensive eelgrass habitat degradation. In addition, sea level rise and global warming are also considerable future threats to eelgrass. With rising seas and increasing water temperature, seagrass habitats, including eelgrass, may be drastically diminished along the Massachusetts coast and throughout coastal waters of the world. Eelgrass Management
For Further Information Chesapeake Bay Program - http://www.chesapeakebay.net/. Fonseca, M., P.E. Whitfield, N.M. Kelly, and S.S. Bell. 2002. Modeling seagrass landscape pattern and associated ecological attributes. Ecological Applications 12(1):218-237. Futuyma, D.J. 1994. The evolution and importance of species interactions. In (G.K. Meffe and C.R. Carrol, editors), Principles of Conservation Biology. Sinauer Associates, Sunderland, MA. Meffe, G.K. and C.R. Carrol. 1994. The Principle of Conservation Biology. Sinauer Associates, Sunderland, MA. Orth, R.J., R.A. Batiuk, P.W. Bergstrom, and K.A. Moore. 2002. A perspective on two decades of policies and regulations influencing the protection and restoration of submerged aquatic vegetation in Chesapeake Bay, USA. Bulletin of Marine Science 71(3): 1391-1403. Thayer, G.W., W.J. Kenworthy, and M.S. Fonseca. 1984. The ecology of eelgrass meadows of the Atlantic coast: a community profile. U.S. Fish and Wildlife Service. FWS/OBS-84/02. 147 pp. Eelgrass Schematic
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