Geoscience Reference
In-Depth Information
Eelgrass is a green flowering plant that has “returned to the sea.” Not a true grass, it nevertheless has rib-
bonlike leaves that enclose a stem at their base. The blades contain a series of air canals that transport oxygen
throughout the plant, including the roots, which grow in soft sediment that is often starved for oxygen. The air
canals also confer buoyancy to the plant, thereby orienting the leaves toward the light. Eelgrass grows from an
extensive system of rhizomes, or creeping runners, which also put down fibrous roots that anchor the plants. It
reproduces by unique flowers that have adapted to function underwater. In late spring or early summer, eel-
grass puts out small flowers in spikelike clusters that house both pollen-bearing male and seed-bearing female
flowers. However, self-pollination is prevented because the male flowers only release their sticky threads of
pollen after the female parts have already matured. The clusters detach and float to the surface where they are
transported by the currents. Ripe fruits are shed during this time, and the seeds sink to the bottom. Some of the
seeds released from the ripe fruit germinate in early autumn, but most will do so the following spring, avoiding
freezing or, in the gulf, the grinding action of ice.
Eelgrass is a perennial plant, and in some cases it is more productive than salt marsh cordgrass. Few herbi-
vores consume the living blades directly, the exception being Canada geese and brant, which gather in large
flocks to feed on the eelgrass in the fall, fueling their southward migrations. Most of the productivity from eel-
grass enters the detritus food cycle. Eelgrass is constantly shedding old blades, which are broken down by bac-
teria. The decayed organic matter, or detritus, is then ingested by a variety of detritus-feeding snails, bivalves,
and amphipods. They pass the refractory detritus through their digestive tracks virtually unchanged, but in the
process strip from it the nourishing bacteria, protozoa, and microalgae. Their fecal pellets are then colonized
by bacteria and are themselves recycled.
Eelgrass beds perform other vital ecological functions, acting as a nursery area. The three-dimensional com-
plexity of eelgrass beds, as compared with unvegetated habitats, increases the habitable space, but more im-
portantly, provides protection from predators. Juveniles of many fish species have been found in the gulf 's
shallow lagoons: among them, Atlantic tomcod, winter flounder, striped bass, stickleback, cunner, and white
hake. Sand shrimp and rock crab juveniles also find shelter in the eelgrass beds. While reducing the foraging
efficiency of predators, the high productivity of eelgrass lagoons also attracts a variety of birds that prey on
small fishes, and consequently, double-crested cormorants, blue herons, gulls, and terns can often be seen
hunting in the shallows.
Among the organisms that find food and shelter in the gulf's eelgrass beds are sticklebacks (left) and green crabs
(right), the latter an invasive species that has caused widespread damage to eelgrass.
