04 WEB MFR 71(3).Indd

The Status of Eelgrass, Zostera marina, as Bay Scallop Habitat: Consequences for the Fishery in the Western Atlantic Item Type article Authors Fonseca, Mark S.; Uhrin, Amy V. Download date 28/09/2021 11:17:50 Link to Item http://hdl.handle.net/1834/26298 The Status of Eelgrass, Zostera marina, as Bay Scallop Habitat: Consequences for the Fishery in the Western Atlantic MARK S. FONSECA and AMY V. UHRIN Description of the Plant The plant is almost always submerged Relatively thin, flattened, blade-like or partially floating at low tide. In the leaves up to ~ 1 cm in width, dark Zostera marina L. is one of a small western Atlantic it is only occasionally green in color; genus of widely distributed seagrasses, intertidal (Fig. 2). all commonly called eelgrass (Fig. 1). However, eelgrass is actually not a Leaves usually 20–50 cm but up to This genus contains twelve species grass—it is in the same Class grouping 2 m in length, 4–10 mm wide, with worldwide but only three species are as other monocotyledonous plants, but it 5–11 veins and rounded leaf tips, found in North America (Z. asiatica then branches into strictly aquatic plant sometimes with a very small, sharp and Z. japonica on the west coast) with groups at lower taxonomic levels: point; Z. marina as the only confirmed native species. Eelgrass is found on sandy Phylum: Anthophyta (flowering Leaf sheath forms an envelope around substrates or in estuaries, and rarely plants), the aboveground stem; on the open ocean coastline and then, Class: Liliopsida (monocots), usually, in the shelter of boulders or Order: Potamogetonales, Reproductive shoot, terminal, other similarly immobile structures. Family: Zosteraceae (Greek ‘zoster,’ branched, and substantially longer meaning ‘belt’), than vegetative shoots; Genus/species: Zostera marina. Authority: Linnaeus, 1758. Seeds ovoid or ellipsoid, ~2–3 mm Mark S. Fonseca and Amy V. Uhrin are with the U.S. Department of Commerce, NOAA, National long with 16–25 distinct ribs; Ocean Service, Center for Coastal and Fisheries A summary of the key identification Habitat Research, 101 Pivers Island Road, Beau- features are as follows: Rhizome color (when living) is fort, NC 28516 (email: Mark.Fonseca@noaa. gov). dark brown and has a polished appearance; At each rhizome node, there are typi- ABSTRACT—Zostera marina is a mem- Today, increased turbidity arising from cally two root bundles; ber of a widely distributed genus of sea- point and non-point source nutrient load- grasses, all commonly called eelgrass. ing and sediment runoff are the primary Branching is alternate along the The reported distribution of eelgrass along threats to eelgrass along the Atlantic coast the east coast of the United States is from and, along with recruitment limitation, are rhizome and frequently irregular; Maine to North Carolina. Eelgrass inhabits likely reasons for the lack of recovery by each branch becomes an independent a variety of coastal habitats, due in part to eelgrass to pre-1930’s levels. Eelgrass is shoot. its ability to tolerate a wide range of envi- at a historical low for most of the western ronmental parameters. Eelgrass meadows Atlantic with uncertain prospects for sys- provide habitat, nurseries, and feeding tematic improvement. However, of all the To the casual observer there is little grounds for a number of commercially and North American seagrasses, eelgrass has morphological difference between the ecologically important species, including a growth rate and strategy that makes it two seagrass species that co-occur with the bay scallop, Argopecten irradians. In especially conducive to restoration and eelgrass in the western Atlantic, shoal the early 1930’s, a marine event, termed several states maintain ongoing mapping, grass, Halodule wrightii Aschers, and the “wasting disease,” was responsible for monitoring, and restoration programs to catastrophic declines in eelgrass beds of enhance and improve this critical resource. widgeon grass, Ruppia maritima L. the coastal waters of North America and The lack of eelgrass recovery in some However, the three species can be distin- Europe, with the virtual elimination of Z. areas, coupled with increasing anthropo- guished particularly by their blade tips marina meadows in the Atlantic basin. Fol- genic impacts to seagrasses over the last and rhizomes (Fig. 3). The leaf tip of lowing eelgrass declines, disastrous losses century and heavy fishing pressure on scal- were documented for bay scallop popula- lops which naturally have erratic annual eelgrass is round, sometimes with a very tions, evidence of the importance of eel- quantities, all point to a fishery with pro- small apical point, whereas H. wrightii grass in supporting healthy scallop stocks. found challenges for survival. has a bicuspidate (crowned) appearance 20 Marine Fisheries Review Figure 3.—Sketches of blade tips for the three cogeners. Reproduced from Thayer et al., 1984. left behind, rooted in place as with the other seagrass species—instead the terminal shoot actually migrates across the seafloor leaving a trail of rhizome rooted in place behind it which gives the plant unusual pattern development Figure 1.—Zostera marina (eelgrass) showing the whole plant structure. capabilities and high spatial recoloniza- tion rates. The life history of eelgrass has been well-described for almost a century (Setchell, 1929). The plant typically follows a 2-year (perennial) life history (Fig. 4). For most of the range in the western Atlantic, eelgrass seeds germinate in the late winter and grow vegetatively through the summer, cre- ating daughter shoots (ramets) almost continuously every 2–4 weeks. These clones then over-winter in a slow growth phase. In the second year of their existence, shoots of that age undergo a dramatic alternation of generation and transform into luxurious flowering structures that produce dozens of seeds. After setting seed, the shoot dies. Seeds tend to stay very near the parent plant yet the role of seeding in eelgrass bed maintenance remains somewhat of a mystery. Like terrestrial plants, there appears to be “mast years” where extraordinary Figure 2.—Eelgrass bed in Long Island Sound (Photograph by C. Pickerell). numbers of seedlings germinate which can result in significant new bed forma- tion in locations otherwise long devoid and R. maritima is lancelate (pointed). Eelgrass is unlike all the other native of cover. Flowering stalks can break off Also, the living rhizome of eelgrass is North American seagrass species in and float for many miles (Phillips and brown while the rhizomes of the other that each seagrass shoot is a “terminal Meñez, 1988; Harwell and Orth, 2002), species are much lighter, almost white shoot”; that is, it is always located at the providing a means for colonization at far depending on sediment type. end of the rhizome. There are no shoots distant locations. 71(3) 21 Figure 4.—Stylized life history of perennial eelgrass. Redrawn from Setchell, 1929. Limiting Factors its upper limit. The emersion tolerance irradiance (Dennison, 1987; Gallegos, of eelgrass is not well quantified, but 1994) but these values are undergoing Eelgrass inhabits a wide range of observations indicate that it has a low re-evaluation (Kenworthy2). Depth dis- coastal habitats, due in part to its ability desiccation tolerance and thus cannot tribution of eelgrass varies with water to tolerate a wide range of environmen- withstand prolonged exposure at low quality on a local scale. For example, tal parameters. These parameters are tide unless the environment is cool (typi- in both Chesapeake Bay and the North discussed in more detail below. cally below 20°C) and a film of water Carolina coastal zone, where waters can persists to keep the plant wetted. When be turbid, eelgrass is usually limited Substrate exposed to truly dry conditions and a to depths of 2 m or less (Dennison et Eelgrass is limited to unconsolidated mild breeze, eelgrass blades can desic- al., 1993; Ferguson and Korfmacher, sediments, and thus, comparatively cate beyond recovery in minutes while 1997; Fonseca et al., 2002; Kemp et quiescent environments. However, luxu- the sheath bundle, which contains the al., 2004). In contrast, further north, rious eelgrass beds may be found cling- meristems, may withstand much longer estuaries become less turbid and light ing to cobble sediments behind highly periods of true desiccation (Fonseca1). is able to penetrate to greater depths, exposed islands along the New England Eelgrass is generally limited to depths with eelgrass growing in excess of 10 coast. where light is at least 15–25% of surface m in some areas (Maquoit Bay, ME: Light / Depth 1Fonseca, M. Unpubl. data. NOAA, Center for 2Kenworthy, J. NOAA, Center for Coastal Fish- Eelgrass is limited in its depth distri- Coastal Fisheries and Habitat Research, Beau- eries and Habitat Research, Beaufort, N.C. Per- bution by light at depth and emersion at fort, N.C. sonal commun., 2009. 22 Marine Fisheries Review Short and Short, 2003; Fort Weatherall, Jamestown, R.I.: Fonseca1). Moreover, the periodicity of light-reducing events (acute versus chronic diminishment of light) may play a significant yet difficult-to-detect role in the distribution of eelgrass. Moore et al. (1997) found that a month-long elevated turbidity event at a site in Chesapeake Bay caused eelgrass to die off, an event that was otherwise very difficult to detect using data aver- aged over longer periods of time. Temperature Due to its widespread distribution, eelgrass can experience water temperature fluctuations from less than 0°C to greater than 30°C. Although eelgrass may grow at temperature extremes, physiological processes within the plant (i.e. photosynthesis, respiration) require a more limited range for optimum performance (Fig. 5; Penhale, 1977; Evans et al., 1986, Marsh et al., 1986). Although there are a number of factors to consider, it is widely considered that Figure 5.—Graphic illustration of Setchell’s topology describing the relationship a sustained temperature approaching between temperature and eelgrass phenology.

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