Eelgrass
(Zostera marina)
Flowering plants of the sea
Prentice K.
Stout
P934
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.
