No Eelgrass, No Scallops
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
There is probably no single plant more important to our harbor ecosystem than eelgrass. We have relatively healthy beds of eelgrass in both harbors and in shallow areas around the island and the islands of Tuckernuck and Muskeget. The protection of eelgrass habitat is critical in order to have a viable bay scallop fishery. Eelgrass (Zostera marina L.) is a subtidal marine angiosperm (flowering plant), or "seagrass," that grows in temperate waters, often forming extensive underwater meadows. It is not seaweed, but actually an underwater submerged grass that flowers and primarily spreads via rhizomes or roots.
Along with what seems to be half of the natural world, Zostera marina Linnaeus or Zostera marina L. is named by our old buddy Linnaeus (the world’s hardest working taxonomist—soon to be a new reality show for The Learning Channel) who first identified it in 1753. The marina part of the name should be obvious and refers to its strictly marine habitat. The Greek “zoster” means “belt” which refers to its ribbon-like leaves and creeping rhizomes http://seagrassli.org/ecology/eelgrass/taxonomy.html).
Zostera marina L. can be found on both coasts of the United States as well as throughout Europe and eastern Asia. This species prefers cold water, but it grows as far south as the Carolinas on the U.S. east coast and the Baha Peninsula on the west coast. This species is usually a perennial, but in Mexico, Z. marina grows as an annual, possibly as an adaptation to the heat of summer. Eelgrass is a “grass like” flowering plant with dark green, long, narrow, ribbon shaped leaves (which people first compared to the shape of an eel) about 20-50 cm in length (exceptionally up to 2 m long) with rounded tips. Leaves shoot from a creeping rhizome that binds the sediment.
Leaves and rhizomes contain air spaces, lacunae, that aid buoyancy. Numerous flowers occur on a reproductive shoot similar to those of terrestrial grasses. An eelgrass plant consists of a horizontal stem or rhizome that grows on or just below the surface of the sediment. Leaves originate from a meristem which is protected by a sheath at the actively growing end of the rhizome. As the shoot grows, the rhizome elongates, moving across or within the sediment, forming roots as it progresses. Male and female flowers are on the same plant (Monoecious). Root initiation is directly related to formation of leaves and occurs during the growing season. A new leaf is created approximately every 10 to 14 days. When this occurs, a new set of roots emerges from a node on the rhizome at the base of what was an old leaf. Roots emerge from the node at a downward angle alternating from the left side to the right side of the rhizome, possibly adding to the stability.
Eelgrass beds are highly productive communities, and are ecologically important because they act as a nursery, habitat, and feeding ground for many fish, waterfowl, and invertebrates. Eelgrass beds, as well as other seagrasses, often have become the center of resource management initiatives to protect them. Eelgrass meadows build up in the spring and summer, and then decay in the fall and winter. Eelgrass blades can grow up to 3 or more feet long. They are found in relatively shallow subtidal habitats and rarely are found much deeper then twelve feet.
Buzzards Bay National Estuaries program eelgrass expert, Dr. Joe Costa has been studying and diving and restoring eelgrass in the Cape Cod area for many years. www.buzzardsbay.org/eelgrass.htm. In addition, the Commonwealth of Massachusetts and Dr. Charlie Costello have been mapping the changes in eelgrass around the state for many years (http://www.mass.gov/dep/water/resources/eelgrass.htm). Dr. Costello has been using the Field Station as his base of operations on and off.
The Encyclopedia of Life (great name! www.eol.org/pages/1089042) tells us the life history of Zostera marina L. “It plays important roles in sediment deposition, substrate stabilization, as substrate for epiphytic algae and microinvertebrates, and as nursery grounds for many species of economically important marine vertebrates and macroinvertebrates.” In fact, the actual growth and incorporation of eelgrass beds in a coastal area helps hold sediment; allowing it to establish itself in secure beds which then can trap more sediment. Eelgrass beds provide hiding spots and substrate for many creatures from crabs to pipefish to whelks and scallops and they also are able to cushion waves as they approach the shore. Historically, it was at one time the principle material for the Dutch dikes and has been used as stuffing for mattresses and cushions. Seeds were harvested and used like wheat by the Seri Indians in the Gulf of California.
From a great online data base of plants, the “Flora of North America” (http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=200024703) “At the southern limits of its range, active growth mostly is in the cooler months of autumn and spring, with flowering and fruiting mostly in the spring and the plants dying in the hotter summer months, the vegetation becoming dislodged from the substrate and floating to the water surface. The fruits apparently remain in the floating vegetation for a period of time, eventually falling from the shoots to the substrate. Movement in dislodged vegetative material is the only adaptation the fruits have for dispersal.”
Asexual reproduction occurs when rhizomes elongate and winter buds (called turions) form. Eelgrass can also reproduce sexually: Flowers form in May and June. Female flowers are fertilized by drifting pollen and develop into shoots that contain seeds. The shoots eventually break off, float to the surface and release their seeds. Eelgrass is only one of two underwater flowering plants. You would think that with two methods of spreading, that eelgrass would be bullet proof; but it has fallen prey to excess nutrients, wasting disease, shifts in salinity and temperature, and dredging and storms.
Some key stressors that can knock out an eelgrass bed include the blockage of light due to high sediment levels resulting from land erosion. High nutrient levels, caused by the excessive use of fertilizers, runoff and sewage outflow indirectly affect grasses by allowing excessive algae to grow both in the water and on the grass blades, further blocking the necessary light. Boat propellers and impellers have torn rooted vegetation out of bottom sediments and dredging has caused severe scarring of underwater grass beds. Back in the 1930s, eelgrass beds up and down the Atlantic coast were decimated by a disease called wasting disease.
Going back to the Encyclopedia of Life, we learn that the cause of the widespread wasting disease was a fungus-like protist, Labyrinthula zosterae, which dramatically reduces photosynthetic capability. In the 1930s, Zostera marina suffered dramatic die-offs on the Atlantic coasts of North America and Europe. The plants would develop large brown spots on the leaves and rhizomes and slowly die. This "wasting disease" eventually led to the disappearance of most of the eelgrass in the North Atlantic, along with much of the fauna that depended on it. Wasting disease continues to affect Eelgrass meadows in North America and Europe with variable degrees of loss, though none to date as catastrophic as the epidemic of the 1930s. Over the decades, Eelgrass gradually re-established itself in many (though not all) of the areas it had previously occupied.
Ah, controversy! From this Chesapeake Bay site: (1979 Field office report (http://www.fws.gov/chesapeakebay/savpage.htm) “The slime mold, Labyrinthula, was often cited as the reason for the eelgrass decline in the 1930s, but subsequent attempts to infect healthy plants with the isolated pathogen were generally unsuccessful, suggesting that this organism was only a secondary cause of infections. Researchers in Australia found that Labyrinthula was normally associated with healthy eelgrass as well as with 'diseased' plants. Therefore, disease seems an unlikely explanation in the Chesapeake Bay decline--especially since a very wide spectrum pathogen would be necessary to attack ten different species of submerged grasses.” Interesting article!
But not true according to P.J. Ralph and F.T. Short (Marine Ecology Progress Series (2002) Volume: 226, Issue: 1936, Pages: 265-271 http://www.mendeley.com/research/impact-wasting-disease-pathogen-labyrinthula-zosterae-photobiology-eelgrass-zostera-marina-1/ “Impact of the wasting disease pathogen, Labyrinthula zosterae, on the photobiology of eelgrass Zostera marina” who indeed found that this pathogen did reduce the ability of eelgrass to photosynthesize and eventually caused death of the tissue. Every plant wants to photosynthesize and several other papers I found corroborate Ralph’s and Short’s work.
More recently, scientists and conservationists up and down the eastern Seaboard and abroad have become concerned about a cyanobacteria called Lygnbya spp. which can also be found here. Lyngbya occurs in large clumps attached to seagrass or other substrates in shallow marine waters. It differs from other ‘seaweeds’ with its dark brown-black coloration and dense filamentous growth form. Lyngbya is also known as “mermaids hair” as it has the consistency of wet matted hair when removed from the water. Lyngbya has gotten worse over the years in the harbor, and it is adept at adapting to high nutrient loads in the harbor.
Because many state fisheries are becoming extremely concerned about eelgrass restoration, huge efforts are made in various areas to re-seed, replant, or somehow restore missing eelgrass beds. Each summer, people come from as far as Richmond and Charlottesville to take part in the world's largest seagrass restoration project, mobilized by The Nature Conservancy in partnership with the Virginia Institute of Marine Science and the Virginia Coastal Management Program. Their seeding efforts have created 4500 acres from 400 acres. Healthy strong plants are brought to controlled habitat tanks and the plants are allowed to grow until they release their seeds which are then spread into areas that can support the growth of new plants http://weareseaborn.blogspot.com/2011/06/seagrass-restoration-is-worlds-largest.html. Some groups suspend healthy plants in the water column above potential beds until the plants flower and release their seeds below.
Fortunately on Nantucket, it is relatively easy to remove things that can reduce or damage eelgrass beds, like some of the old-style moorings and the Nantucket Land Council and eelgrass expert, Dr. Dave Burdick of the University of New Hampshire (Associate Research Professor Jackson Estuarine Laboratory, Department of Natural Resources) along with the Great Harbor Yacht Club are evaluating how eelgrass beds regrow after exchanging movable moorings for the fixed helical moorings and reinstalling shoots of eelgrass using the Transplanting Eelgrass Remotely with Frame System (TERFS) to regrow bald patches. If only it were that easy for guys losing their hair!
Scientists up and down the East Coast are working on a variety of methods to restore eelgrass beds from seeding to installing shoots by hand. Eelgrass has many adaptive traits that allow it to live, thrive and colonize underwater; the blade attachment at the sediment water interface (harbor “seafloor”) is very bendable, which allows eelgrass to survive being bent by dredges that drag over it. Instead of breaking like many grasses on dry land might do, the eelgrass sheath is very flexible at that point. Eelgrass also has natural bubbles that provide buoyancy in the center of the shaft of grass. What eelgrass is not good about adapting to is a lack of sunlight due to high concentrations of phytoplankton or other materials that reduces transmission of sunlight in the water column. When excess nutrients from fertilizers and septic systems and storm-water runoff contribute food to the water, phytoplankton blooms occur and these block out the light from the sun and slow or stop eelgrass growth. Docks or other over the water structures can do the same thing, which is why wetland regulatory agencies work hard to limit their use near productive beds. The collapse of the bay scallop fishery in many Cape area towns was attributed to the loss of eelgrass beds from both wasting disease and from eutrophication and shade out due to human influences. Over the past 70 years, according to the University of Delaware (http://www.ceoe.udel.edu/kiosk/eelgrass.html), approximately 90% of all eelgrass throughout the U.S. has been lost. Communities are now aggressively working to repair this damage.
Mapping eelgrass beds is done by aerial reconnaissance which is verified by boat surveys and sometimes underwater dive surveys. At the UMass Boston Nantucket Field station, junior science interns and older college age coastal ecology students map the underwater mass by throwing out a quadrat into the water and snorkeling in a straight line to count the density of plants in each subsection of the quadrat. This year one student looked at the range of density from nonexistent to sparse to moderate to dense coverage and then compared the types and number of benthic creatures that could be found in each of those different eelgrass patches. Some creatures are highly mobile like crabs and are relatively at home in most environments, while others like pipefish and scallops, prefer to take the shelter and buffet offered by denser stands of eelgrass.
Eelgrass habitats are an essential component of a functioning and healthy coastal area. When eelgrass and mudflats and salt marshes all exist in an undisturbed corridor of habitats, greater protection from storms and sea level rise and a “greenbelt” of habitats provide ideal ecological value. During a talk a week or two ago at the Nantucket Field Station, UMass Boston Assistant Professor Dr. Anamarija Frankic described how essential having all three of these components is in the protection and integration of coastal systems. During low tides, eelgrass shelters small animals and plants from extreme temperatures. On tideflats, eelgrass beds hold moisture like a sponge, offering additional protection for small creatures. Eelgrass meadows cushion the impact of waves and currents, preventing erosion. Eelgrass roots weave sediments in place. This protection helps preserve the highly productive bacteria in the sediments which nourish large amounts of invertebrates (http://www.ecy.wa.gov/programs/sea/pugetsound/species/eelgrass.html.) It all works together to form an environment that can support fisheries, so that we don’t forget, “No eelgrass, no scallops.”
Learn more about Dr. Frankic’s work around the world at http://blog-aauw.org/2011/08/03/meet-anamarija-frankic/. Teachers, it is almost time to go back to class! Here is a great eelgrass curriculum developed by MIT students: http://seagrant.mit.edu/eelgrass/project/eelgrass_manual.pdf
