Eelgrass Anatomy

There are two major parts of eelgrass: the leafy shoots and the flowering shoots. The inconspicuous eelgrass flowers are the source of the eelgrass seeds, which resemble grains of wild rice. These seeds mature in July and are released and dropped to the bottom where they germinate. The long ribbon-like green leaves in a sheath attach directly to the roots. These are buried in the soft, mucky sediment of the bottom. Rhizomes are horizontal stems that connect plants together and from which new shoots grow. In our climate, eelgrass grows most quickly in the late spring and early summer when sunlight is at its peak.

Fourteen Fascinating Facts about Eelgrass

  1. The scientific name for eelgrass is Zostera marina.
  2. Eelgrass is an SAV (submerged aquatic vegetation).
  3. Because eelgrass is an SAV, people do not usually see live eelgrass unless they are boating, fishing, snorkeling, or scuba diving.
  4. The life cycle of eelgrass including flowering, pollination and seed germination all occur underwater.
  5. Beach goers encounter the long, thin, black decomposing blades of eelgrass washed up on the shore as part of the wrack.
  6. Eelgrass is not seaweed or algae. It is a sea grass.
  7. There are only two species of sea grass in Massachusetts coastal waters: eelgrass and widgeon grass.
  8. Of these, eelgrass is the most abundant, forming extensive submerged meadows, patchy underwater beds and individual stands.
  9. Eelgrass forms on the most valuable shallow marine habitats in Massachusetts’s coastal waters.
  10. Eelgrass is and indicator species. The presence and health of eelgrass is an important indicator of a marine embayment’s water quality.
  11. Eelgrass is a keystone species. Without eelgrass, water chemistry and the underwater landscape is greatly altered, affecting populations of plants and animals in the estuary.
  12. Eelgrass beds are shellfish and finfish nurseries. Bay scallop and mussel larvae (about the size of a pinhead) float through the water and settle on eelgrass blades.
  13. Much as deciduous trees lose their foliage, eelgrass loses its leaves in the late summer and autumn. These float away and become part of a nutrient rich “chowder” eaten by marine life.
  14. In the 1920s and 1930s eelgrass was used in some Vineyard homes for insulation. It was sold and stitched between two layers of building paper. According to one researcher, a six-inch layer of eelgrass spread to a density of 1.5 pounds per square inch is equivalent to six inches of fiberglass insulation.
The anatomy of an eelgrass plant. Drawing by Dana Gaines.

The anatomy of an eelgrass plant. Drawing by Dana Gaines.

A healthy stand of eelgrass can be seen from the surface.

A healthy stand of eelgrass can be seen from the surface.

Recent eelgrass meadows and former extension beds as determined by 1951 aerial photography.  Mapping of eelgrass over the decades documents losses

Recent eelgrass meadows and former extension beds as determined by 1951 aerial photography.  Mapping of eelgrass over the decades documents losses

The Value of Eelgrass to Estuaries

Eelgrass uses sunlight for photosynthesis, releasing oxygen into the water. Many inhabitants of the salt marsh are dependent on this dissolved oxygen to live. Dense sea grass leaves promote cycling of nutrients by trapping decaying matter and providing this material to decomposers.

Eelgrass is an important part of the marine food chain. Microscopic organisms grow on eelgrass blades and are a source of food for many crustaceans, shellfish, and finfish. Animals from tiny sea urchins to waterfowl feed on eelgrass leaves.

The habitat created by eelgrass is an underwater landscape teeming with life. Eelgrass meadows protect commercially important finfish and shellfish. Large predators cannot easily hunt for juvenile fish in this aquatic jungle. Eelgrass blades also safeguard juvenile shellfish from predators such as whelk that cannot climb the blades.

Eelgrass’s firmly anchored roots trap sand and help to prevent coastal erosion. Dense eelgrass meadows absorb wave energy and calm the water.

Eelgrass is a major component of the wrack line, the tangle of decaying plants and animals deposited by the tides on the beach. Wrack helps to trap sand and is a rich source of nutrients for wildlife.

The Eelgrass Ecosystem, drawing by Dana Gaines.

The Eelgrass Ecosystem, drawing by Dana Gaines.

The Problem

Anchored buoys scrape the pond floor and damage or destroy eelgrass plants

Anchored buoys scrape the pond floor and damage or destroy eelgrass plants

In the 1930s a widespread “die-off” of eelgrass occurred along the North Atlantic coast. This was attributed to the “wasting disease” whose cause is thought to be a marine slime mold (Labyrinthula) that attacks weakened plants. Eelgrass in Sengekontacket Pond became reestablished in the 1940s and 1950s only to be decimated in the early 1990s by what was thought to be a similar outbreak. The only remaining stands of eelgrass in the Sengekontacket Pond system are now in Majors Cove and Trapps Pond. Storms with heavy winds causing severe erosion, suspending silt and clouding the water are also implicated.

The health of eelgrass is affected both by human induced and natural causes. Because eelgrass grows in shallow waters, it is affected by near shore development. Nitrogen from septic systems, landscape fertilizers, road runoff and atmospheric deposition cause over-enrichment and promote the growth of alga. This forms floating mats which smother the eelgrass. Alga blooms are also stimulated and make the clear water turbid. Eelgrass is deprived of sunlight and photosynthesis is impaired. Dissolved oxygen decreases and pond water quality is degrade, affecting the entire ecosystem. The construction of piers over eelgrass beds also deprives plants of needed sunlight. Boat propellers can damage seagrass leaves and stir up silt, further reducing water clarity and dragging anchor chains destroy roots and rhizomes. Dredging projects are carefully scrutinized by environmental agencies and replication is required for any eelgrass bed damage.

The Sengekontacket Pond Project

Eelgrass seeds will be released in mesh bags as part of a restoration project

Eelgrass seeds will be released in mesh bags as part of a restoration project

In 1999, efforts to restore eelgrass beds in Sengekontacket Pond by transplanting healthy stalks from Trapps Pond faild due to predation by spider crabs. In 2003, seed stalks were harvested from Trapps Pond, and stored underwater in mesh bags until the seeds developed and were released from the stalks. However, due to the large size of the mesh openings some of the thousands of seeds drifted away before they could be sown. This seeming reversal led to the idea that seeds could be dispersed of using a “floating mesh bag technique” that would be an effective and far less time consuming seeding method than planting eelgrass seed in furrows by hand. In 2004, seed stalks were again collected and placed in mesh bags attached to a float in five seeding locations. This year, the project is expanded to eight locations in Sengekontacket Pond. The Friends of Sengekontacket, in cooperation with the Martha’s Vineyard Commission staff, will keep the community informed of the success of this project.

This project was carried out by the Friends of Sengekontacket, Inc. and the Martha’s Vineyard Commission with funding provided by Farm Neck Foundation and the Vineyard Open Land Foundation.