Full report - Conservation Gateway

Chapter 2 - Coastal Ecosystemsnumerous animals feed directly on seagrasses, includingfishes, geese, swans, sea turtles, and crabs. Seagrass providesstructure for benthic (seabed) communities and canslow down currents, thereby increasing sediment trapping.The seabed is stabilized by seagrass roots and rhizomes.Seagrass provides oxygen to the water column and shallowbenthos, and takes up nutrients (e.g. nitrogen andphosphorus) during its growing season (spring to fall),re-releasing the nutrients through organic decay.Seagrasses support a diverse epiphyte (plants that grow onthe surface of another plant) community, includingbenthic diatoms and other algae, and free-floating macroand microalgae. Other organisms living on blades of eelgrassinclude protozoans (ciliates, flagellates, and foraminifera),nematodes, and copepods (Perry 1985). Sessile(attached) animals living on the blades and at the base ofeelgrass shoots include bay scallops, crustaceans, sponges,anemones, bryozoans, tube worms, polychaetes, barnacles,and other arthropods and tunicates (Perry 1985).Seagrass beds can occur in association with a variety ofnatural shoreline types. Ecological factors contributingto the distribution and continued health of seagrassesinclude water quality, depth, substrate type, light andnutrient regime, existing meadow size, germination andgrowth, water temperature, pore water chemistry, salinity,sediment dynamics, and wave energy. Many of theseattributes are site specific. Although in many parts of theregion seagrass beds have significantly declined, computermodels (Short and Burdick 2005) have recently becomeavailable to help determine the most suitable places foreelgrass within some estuaries.Nearshore Shellfish AssemblagesDense beds of oysters, clams, scallops, and mussels oncepopulated the bays and estuaries of the Atlantic coast,providing a wide array of ecological services. For instance,oysters develop vertical reef structures that provide fishhabitat, filter the water and modify patterns of estuarinecirculation, sediment transport, and wave energy. Theviability of nearshore shellfish populations is highly dependenton sustainable harvest levels and presence of highquality settlement substrate, as well as estuarine waterquality and salinity regimes. Although many shellfishspecies are found in abundance in the region, populationsof some formerly dominant bivalve species are dwindling.Prominence as a food source often overshadows thecritical roles that shellfish assemblages play in ecosystemfunction (Grabowski and Peterson 2007). Bivalves aresuspension feeders that, in abundant colonies, have thecapacity to filter volumes of water equivalent to entire baysin a matter of days (Newell 2004). Filter feeders exertcontrols on harmful algal blooms and may facilitate eelgrassproductivity (Peterson and Heck 1999; Wall et al.2008). Reefs formed by oysters and blue mussels providerefuge and structure for many marine plants, animals, andinvertebrates (ASMFC 2007), including economicallyvaluable fish (Peterson et al. 2003). Once established,shellfish form dense colonies that provide many services,especially water filtration that directly benefits other speciesand habitats like eelgrass. In intertidal areas, shellfishbeds trap sediments and stabilize shorelines against waveand storm erosion (Piazza et al. 2005; Meyer et al. 1997).The loss of shellfish habitat therefore has wide-rangingand serious implications for human and marine communitiesalike.Larval forms of bivalves are preyed upon by many fish andmarine invertebrates. As juveniles and adults, bivalves aremajor forage for all forms of fish, invertebrates (especiallycrabs, whelks, and starfish), shorebirds, seabirds, and evenmammals.Eastern Oyster (Crassostrea virginica)Also known as the American oyster, this species is arguablythe most historically dominant and commerciallyvaluable shellfish species found throughout the region.Reefs occur in both subtidal and intertidal locations, withcommercial activities focused on subtidal beds. Oystersare widely recognized as “ecosystem engineers” that createessential fish habitat, augment water quality, and provideservices fundamental to the ecological health of estuariesand nearshore areas. The Eastern oyster occurs naturally2-16Northwest Atlantic Marine Ecoregional Assessment • Phase 1 Report