Shoreline Stewardship

IV. B. 1. Eelgrass Meadows

Eelgrass (Zostera marina) is a marine seagrass (i.e., a rooted plant that produces flowers) that forms meadows, literally pastures of flowing grass, that range from patchy to contiguous and extensive (Figure IV-1) (Table IV-1). Eelgrass meadows are formed within the lower intertidal to shallow subtidal zones, from about +1 m to -5 m relative MLLW in the central Puget Sound area (Bulthuis 1994; Thom et al. 1998). Eelgrass shoot density is highly variable and range from 50 to 800 shoots per square meter in central Puget Sound (Thom et al. 1998).  According to recent data, eelgrass covers 10,500 ha (26,000 acres) in Puget Sound (Puget Sound Water Quality Action Team 2002).  An invading species of seagrass (Zostera japonica) occurs in Puget Sound also.  This species is smaller in size but can reach very high densities on the order of 5 times greater than eelgrass.  Although Z. japonica can grow intermixed with eelgrass, it generally forms meadows at somewhat higher elevations in the intertidal zone than does eelgrass.

Through photosynthesis, eelgrass is a major contributor to the detritus used in both nearshore and deep-water food webs.  Annual reported eelgrass production rates range from 200 to 806 g of carbon per square meter per year within Puget Sound (Williams et al. 2001). Detritivores (animals that feed upon dead plant and animal material), such as harpacticoid copepods, gammarid amphipods, and isopods, incorporate carbon energy directly from detritus formed by eelgrass dieback, and fish utilize carbon energy from the detritus indirectly by consuming these benthic organisms (Simenstad et al. 1979; Nightingale and Simenstad 2001b).  New evidence reveals that mats of detached eelgrass (and probably Z. japonica) are common at depths as great as 100 m in Puget Sound (Woodruff et al. 2000).  Through this process, very large amounts of organic matter reach the deeper parts of Puget Sound where the material can be used by animals far from areas where eelgrass was produced. 

Eelgrass grows to a height in excess of 2 m in some areas, and shoots can be as dense as 500 per square meter.  This dense and lush canopy provides a three-dimensional surface for the attachment of many species and an effective hiding place for small fish. By its position in the intertidal-shallow subtidal zone, it forms refuge habitat for a wide variety of nearshore fish and invertebrate species.  Many of these fish species show a strong affinity for eelgrass because it offers shelter from predators and abundant food resources.  Among these species are bay pipefish, crescent gunnel, kelp perch, lingcod, penpoint gunnel, shiner perch, snake prickleback, striped seaperch, and tubesnout (Simenstad et al. 1991).

Juvenile chum and chinook salmon are often found feeding and residing in eelgrass meadows and their edges.  Juvenile salmon feed on small crustacea found associated with the leaves of eelgrass and at the base of the eelgrass plants.  Many of the prey items include harpacticoid

copepods, which occur in high abundances in the epiphytes (small algae that attaches to the leaves of the seagrass) that attach to the oldest portions of the eelgrass leaves.  These prey taxa are most abundant during the spring when the juvenile salmon migrate along the nearshore region. Pacific herring (Clupea harengus pallasi) populations depend on eelgrass meadows where they often deposit eggs and rear as juveniles.  Herring, in turn, are important in the diet of many larger animals including, salmon, seals, and sea birds. 

Dungeness crab (Cancer magister) are also commonly associated with the protective cover of eelgrass habitats.  Other common invertebrates include bivalves, such as the cockle (Clinocardium nuttallii).  A number of unique species of invertebrates are found almost exclusively in eelgrass meadows, such as the brooding sea anemone (Epiactis prolifera), the chink shell (Lacuna variegata), the sea slug (Phyllaplysia taylorii), and the bubble shell (Haminoea virecens).  The large nudibranch gastropod (sea slug) Melibe leonine is often found in eelgrass meadows, and is considered one of the foremost curiosities in Puget Sound.  Several other bird species are often found feeding in eelgrass meadows including great blue heron, greater yellowlegs, least sandpiper, and spotted sandpiper.  Eelgrass is the preferred food of black brant geese.  The non-native seagrass Z. japonica has also been found in the stomachs of American widgeon (Baldwin and Lovvorn 1994).

Eelgrass grows in saline waters but can withstand periodic flushes with freshwater, as long as they do not last too long (the actual duration of time is not well studied).  Eelgrass can occur on river and stream deltas away from very high salinity waters. Eelgrass can grow in a wide variety of substrata types ranging from fine sands to gravel but grows best in medium-fine sand with some organic matter.  The organic matter is a source of nutrients to the plants through its roots.  Nutrients in the water column can also be taken up by the leaves of the plants.  Desiccation (drying) stress limits the upper boundary of eelgrass meadows, and the lower boundary is limited by light penetration in the water (Thom et al. 1998).  Competition for light and nutrients with macroalgae species can also affect eelgrass distribution. Eelgrass is harmed by any activity that reduces light or disturbs the sediment where it grows.  Hence, docks, boat wakes, and modification of shorelines all have resulted in loss of eelgrass.   Heavy shellfish harvesting can also impact eelgrass (Boese 2002). High inorganic nutrient levels can fuel seaweed or ulvoid blooms that have resulted in smothering of eelgrass.  The WDNR ShoreZone Inventory (Washington State Department of Natural Resources 2001) shows continuous or patchy green algae (Ulva ssp.) along a majority of Bainbridge Island shoreline.  The few areas that did not contain ulvoids include an area northwest of Yeomalt Point, and portions of Fletcher Bay and Hidden Cove (Algae Occurence Map, Appendix A).  Although there is concern that the invading non-native smooth cordgrass, Spartina alterniflora, may eventually take over areas now occupied by eelgrass, that has not occurred as yet (Simenstad and Thom 1995).  

Eelgrass occupies an estimated 18.7 miles of Bainbridge Island shoreline (Washington State Department of Natural Resources 2001).  Eelgrass is dominant along the northwestern, northern and eastern shorelines, and notably absent along the western shoreline south of Battle Point to Point White (Eelgrass and Kelp Occurrence Map, Appendix A).  In two separate studies conducted by Battelle Marine Sciences Laboratory during the summers of 2000 (Woodruff et al. 2000) and 2001 (Borde et al. 2001), several small beds of eelgrass were documented along the southern shoreline of Bainbridge Island.  These studies also confirmed the lack of eelgrass along the western shoreline just north of Point White.  The reasons eelgrass was lacking in this area are not evident.  The WDNR ShoreZone Inventory utilized helicopter flyovers to conduct surveys (i.e., quick visual survey of large areas of intertidal shoreline), whereas the Battelle MSL surveys focused on the specific areas near Rich Passage and utilized aerial photography and diver transects to examine subtidal beds.  Each survey result indicated eelgrass in the same general locations, with slight differences in the actual distribution, probably because the MSL surveys were heavily ground-truthed.  The ShoreZone surveys are meant to provide information on the general broad-scale distribution and may not be highly accurate at finer scales.  However, additional eelgrass data should become available in the future through the WDNR Submerged Vegetation Monitoring Project.  Approximately 60 sites throughout Puget Sound have been selected for long-term monitoring.  One site, Battle Point, is located on Bainbridge Island.  The project was initiated in 2001 and is designed to assess changes in eelgrass abundance and distribution, an indicator of estuarine health.