Eelgrass (Zostera marina)

by Prentice K. Stout

To the scuba diver, eelgrass is a jungle, to many marine invertebrates and fish it is a haven and nursery. It is food for ducks and geese, and a fascinating study area for the scientist.

Eelgrass communities are excellent study habitats for those willing to get wet. Situated as they are close to the shores of quiet salt ponds and lagoons, a person with flippers and a snorkel can spend many productive hours swimming over these aquatic fields. A study of these areas heightens an understanding of their value both in the total coastal process and as a habitat for many commercially valuable marine species.

Recognizing Zostera is not hard - they have thin leaves about 12 to 20 inches long (31 to 53 centimeter) with parallel edges and three veins running along their length. While they are alive they are green, but when cast up on the shore they turn black, and eventually grayish-white when bleached by the sun.

Eelgrass is not a seaweed or algae; it is a true flowering plant and a monocot (a plant having a single seed leaf). This places it in a category unique among marine species. During the spring, as the water warms, millions of pollen grains are released and come in contact with the style of the female flowers. Deep in the ovary, fertilization takes place, and in August the eelgrass plants produce hundreds of tiny seeds. These units of reproduction sink to the muddy bottom or are carried away by currents to other sites, where they create new colonies.

Studies reveal that eelgrass communities are valuable as sediment traps that help stabilize the coastal zone. Because their leaves are so closely packed together they also act as dampers reducing the motion of the water. Suspended materials carried by currents move into these areas, where the waters are calmer, and there they sink to the bottom. The closely packed leaves also provide a haven for young fish species such as flounder and mummichogs. Larger, predatory species find it difficult to hunt in this aquatic jungle.

Birds such as geese and ducks consume the leaves of Zostera as a principal food source. In September, the leaves break away from the roots. Some float away, carried by currents; others fall to the bottom where at least 85 percent of them decompose. Certain microscopic organisms called Detritivores begin to break it down into smaller particles, and these become surrounded by bacteria and fungi. They, in turn, are consumed by filter feeders such as clams and scallops. Deposit feeders (sea urchins) and the sediment feeders (worms and snails) also consume this marine "soup." The adult and larval forms of these invertebrates become food for larger life forms such as fish. It is a continuous cycle of life and death and life again. So important is Zostera's role in this food cycle that estimates reveal that more than 20 species of commercially valuable fish species feed in these eelgrass meadows at some point in their lives.

The surface of the leaves form a substrate for many invertebrate species. In 1937, R.C. Stouffer subdivided the eelgrass invertebrate community into four major categories: those on the plants, among the plants, on the mud surface, and in the mud. Perhaps the invertebrates most easily seen in New England waters are two related worms that secrete about themselves a hard tube of calcium carbonate. Spirobus borealis and Spirobus spirillum look alike, but a little study reveals that Spirobus spirillum coils to the right and Spirobus borealis to the left. If placed in a marine aquarium, these worms will emerge displaying delicate plume-like appendages. Another species, the Bryozoans, will appear as a flat crust growing on the blades, but a magnifying glass or microscope will show the individual animal, or Zooecia, that makes up this colony.

A nursery, or shelter, and a food source for animals, eelgrass has also provided many benefits to humans. In some Scandinavian countries eelgrass was used as roof thatch and upholstery. Burned, it gave not only heat but soda and salt. Early historical records indicate that in the United States eelgrass brought $20 to $30 a ton as insulation and sound-deadening material. In the 1920s and 30s, the Samuel Cabot Company (which still markets stains) sold a product called Cabot's Quilt, which consisted of two layers of building paper with a layer of eelgrass stitched between in quilt fashion for insulation. A researcher's studies indicate that a six-inch layer of eelgrass spread to a density of 1.5 pounds per square foot has the insulation efficiency of six inches of fiberglass insulation. Further studies reveal that Zostera will burn if subjected to a flame but will not support combustion by itself.

The vital importance of eelgrass was first noted by Danish biologists in 1890, but it was revealed dramatically in 1931 when a serious fungal disease and a change in ocean currents that brought warmer waters to the extensive Atlantic Zostera meadows teamed up to kill this species. With this catastrophic decline, which killed over 90 percent of the North Atlantic eelgrass population, many species of ducks and geese vanished. In addition, lobster, crabs, scallops, clams, and other invertebrates declined. A vital part of the food chain in coastal areas had been removed, and the decline in Zostera also caused significant problems with coastal erosion. It was not until 1945 that a recovery began. Zostera is now once again abundant.

It is easy to see that eelgrass and the organisms that live in its grassy confines do not exist in isolation; each species is involved in a number of relationships and interactions. These relationships and interactions have two consequences: a flow of energy from the autotrophs (green plants and algae that make their own food) to the heterotrophs (organisms that eat green plants or each other) and the continuous cycling of inorganic materials which move through living (biotic) organisms and back to the environment. Such a complex combination of living and nonliving elements in a natural setting is referred to as an ecosystem. Ecosystems are large and complex. We can narrow this complex down to "habitat": that portion of an ecosystem in which a particular organism lives. Eelgrass lives in the salt pond ecosystem but has its own habitat within that ecosystem. A further refinement can be made by stating that organisms sharing a common habitat and interacting with each other create a community.