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EELGRASS
2.4.2 Functional Assessment
Adamus et al.’s (1987) evaluation methodology for wetlands attributed a number of functions and values to submerged aquatic beds (e.g. eelgrass):
• sediment stabilization - reduction of sediment erosion and export;
• sedimentltoxicant retention - increasing deposition, accumulation, especially of fine-grained particles;
• nutrient removal/transformation - uptake of nutrients from water column, and conversion to plant material;
• production export - annual dieback of above ground growth allowing transport out of local area;
• aquatic diversity/abundance (fish and invertebrates) - increase in habitat complexity, food availability for other species;
• wildlife diversity/abundance for migration - increase in food resources for migrating avifauna.
Investigations of eelgrass beds in the mid-Atlantic region (i.e., Chesapeake Bay and North Carolina) have indicated that eelgrass habitat provides a nursery function for finfish as well (Thayer et al. 1984). In this role, it provides both refuge and forage for juvenile fish.
Historic information available on eelgrass distribution in the vicinity of Sears Island is incomplete. It appears that there have been periods when it was virtually absent and periods, including 1992, when it has been widespread. A single-year study cannot project future conditions. Therefore, the relative densitites of eelgrass observed in 1992 do not necessarily indicate whether these eelgrass beds have the potential to provide the functions attributed to this resource. Because even low shoot densities increase habitat complexity, providing substrate for other organisms, it is assumed that the beds near the proposed project area can provide the functions typically attributed to eelgrass.
Sediment Stabilization; Sediment/Toxicant Retention
As a general rule, eelgrass beds could be assumed to function well in stabilizing sediments since the root-rhizome standing crop can exceed the above-ground standing crop (Thayer et al. 1984), even during the active growing season. Although above-ground shoots die back during the fall in New England, roots and rhizomes, which bind the sediments, are perennial, allowing year round stabilization of the, sediments. When above-ground growth is present, the bed can attenuate waves and currents, further reducing erosive forces. These characteristics also contribute to the ability to retain sediments (and associated toxicants, if present). Density of shoots and rhizomes, as indicated by pattern of coverage of the eelgrass bed, affect the ability of an eelgrass bed to stabilize or retain sediments. Sparsely distributed plants are less likely to perform this function well than a dense bed could. Eelgrass beds in high energy areas are characterized by varying topography - mounds and scour areas (Thayer et al. 1984). The relatively low energy environment in the study area off Sears Island results in a uniform topography, suggesting that shoot density in the area could be uniform. Although this was not observed to be true in 1992, distribution could have been controlled by other factors, particularly if the bed has been in a period of resurgence.
Nutrient Removal/Transformation
Eelgrass has been demonstrated to absorb nutrients both through leaves and roots (Thayer et aI. 1984). However, in the presence of high concentrations of dissolved nutrients, eelgrass does not compete well with phytoplankton, epiphytes or macroalgae (Short 1991). Penobscot Bay has been identified as having a medium susceptibility to nutrient inputs because of the relatively low flushing rate and upstream loading of nutrients (NOAAIEPA 1988); it would require more than an 800% increase in either nitrogen or phosphorus loading to reclassify the bay as having a high susceptibility to eutrophication. The Sears Island eelgrass beds contribute to nutrient removal and retention.
Production Export
Senescent leaves are usually transported from the bed into the wrack line, where they serve as temporary habitat for amphipods and other crustaceans. Storm events could transport the wrack into deeper waters, but most of the biomass probably remains onshore until it is decomposed. Decomposed shoots would be more readily available to enter the marine detritus-based food web. Since leaves are shed throughout the growing season, as well as during the more pronounced die off in the fall, the eelgrass beds off Sears Island contribute throughout the year to the detrital load of the local system, supplementing the salt marsh, plankton and upriver sources. Although, on a unit basis (g C/m2/yr), eelgrass production is lower than benthic macroalgae and saltmarsh species (Diaz et al. 1982), available information on Sears Island, Long Cove and Stockton Harbor (Timson 1976; USFW 1992; Short 1994) indicates that eelgrass covers more surface area than saltmarsh and may occur at a similar level to macroalgae. Thus, eelgrass is presently a relatively important contributor to the local detrital load.
Aquatic Diversity/Abundance
Incidental observations during this survey indicated that the eelgrass beds along the Sears Island shoreline provided habitat for several species of invertebrates and one species of fish. A detailed survey would likely result in a larger list of species, associated with the eelgrass beds. The variety of functions being provided to these organisms is exemplified by the types of organisms observed. Mussels attach to the blades using the plant for substrate. Snails could be grazing directly on the blades or on microbiota on the blades. Winter flounder and crabs could use the eelgrass bed for refuge from predators or for feeding. The eelgrass beds near Sears Island could be used by other species for similar functions.
Surveys conducted by Heck et al. (1980) and Colarusso (USEPA, 3/28/94, pers. comm.) provide an indication of the diversity of fish and invertebrates that inhabit eelgrass beds in New England. Heck et al. (1989) observed significantly higher (t-test) abundances of fish in eelgrass beds than over unvegetated sand during daytime collections in the Cape Cod National Seashore, MA. Although there were higher abundances of decapod crustaceans in vegetated areas, mean abundances were not statistically different (Heck et al. 1989). Although nighttime sampling in vegetated areas yielded higher abundances of crustaceans than daytime sampling, distribution of fish exhibited no consistent diurnal pattern. Unvegetated areas were not sampled at night.
A number of the fish species Heck et al. (1989) collected in their study were common in the upper Penobscot Bay (CMP 1974). Three-spine stickleback (Gasterosteus aculeatus), winter flounder (Pleuronectes americanus), longhorn sculpin (Myoxocephalus octodecemspinosus) and white hake (Urophycis tenuis) exhibited a tendency to utilize the Cape Cod eelgrass habitat during the daylight (Heck et al. 1989) and, so, would be likely to do so off Sears Island. American sand lance (Ammodytes americanus) occurred in higher abundances over unvegetated substrate (Heck et al. 1989), thus would be unlikely to use the Sears Island eelgrass beds but may use other habitats in the project area. Several other species common in the vicinity of Sears Island, Atlantic herring (Clupea harengus), Atlantic silver-sides (Menidia menidia) and windowpane flounder (Scophthalmus aquosus) were caught in numbers too low to discern a distinct preference in Cape Cod (Heck et aI. 1989).
Colarusso (unpublished data) reported that, using several types of sampling gear, a survey of eelgrass beds in Boston Harbor and Broad Sound (MA) in 1993 yielded 24 species of finfish (Table 2-1) and 28 species of invertebrates (Table 2-2) (P. Colarusso, U.S. EPA, pers. comm. 3/28/94). He observed that pelagic finfish were collected in similar numbers in vegetated and unvegetated areas but that stomach analysis indicated that these species fed in the eelgrass beds. Of the finfish species collected in Colarusso’s study, 16 species were recorded in the vicinity of Sears Island during the mid-1970s (CMP, unpublished data, reported in USDOT and MDOT 1987). It is likely that these species would use eelgrass beds near Sears Island.
The three most abundant crustaceans collected by Heck et al. (1989), sand shrimp (Crangon .septemspinosus), green crab (Carcinus maenas) and rock crab (Cancer irroratus) are commonly found in upper Penobscot Bay (CMP 1982d). Short (U7NH, pers. comm. 3/17/94) has found green crabs to be more common outside of eelgrass beds. He has reported the use of eelgrass beds as nursery areas for juvenile lobsters (Short et al., in prep.). Colarusso (unpublished data) collected 28 species of benthic and epibenthic invertebrates (primarily motile species) in eelgrass beds in Boston Harbor and Broad Sound (MA). Of these species, 15 have been recorded from hard or soft substrates or plankton in the vicinity, of Sears Island (CMP 1982d), and could occur in the eelgrass beds adjacent to the island.
stage; larvae of the other species were uncommon (NAI 1975). Eelgrass could provide cover and food resources for juveniles of these species, particularly rainbow smelt. These species would be likely to utilize the eelgrass beds along Sears Island, as well as other places in the bay. For example, the extensive eelgrass beds in the Bagaduce River and near Islesboro may also function as nursery areas for Penobscot Bay finfish.
Wildlife Diversity/Abundance
Migrating and summering waterfowl could feed on eelgrass and associated fauna as well. Nearshore beds would be visible and accessible to wading, dabbling and diving species (e.g. great blue heron, red-breasted merganser, black duck, common goldeneye, bufflehead, common eider) that are common in the region. The beds would be available during fall migration but would be less likely to provide feeding habitat during winter because of ice or spring migration due to annual dieback of the above-ground shoots.
The western shoreline of Sears Island was categorized as Class B - of regional significance to marine wildlife, compared to the Class A (national and/or state significance) designation of Stockton Harbor (Woodward et al. 1987). The classification was based primarily on the relative abundances of birds observed in each area.
The entire study area included “all islands, exposed ledges, tidal waters and adjacent shorelines of the area bounded by the Veazie Dam on the north, Matinicus Rock on the south, Graffam Island (Muscle Ridge) on the west, and Long Island (Frenchboro) and Naskeag Point (Brooklin) on the east” (Woodward et al. 1987), and includes more than 1000 square miles (2.6 million ha) and 600 miles (9600 kin) of shoreline. NAI (1993a) concluded that the southwestern shoreline of Sears Island, the causeway area, and inner Stockton Harbor had the highest concentrations and species richness of coastal wildlife, while the proposed terminal area scored among the lowest for these same elements. The August 1992 survey for eelgrass was limited to the northwestern shoreline of Sears Island so it is not possible to evaluate the local relationship between coastal wildlife and eelgrass fully. p>