After a storm has pounded the coast, you sometimes see long thin blades of eelgrass cast ashore by the waves, rolled into bright green clumps along the wrack line. Over time the drying sun reduces to them to black, rice paper-thin strips.
This is actually an underwater flowering plant that grows where sun can penetrate the shallow coastal waters of the northern hemisphere. Small beds or larger meadows of eelgrass create nutrient-rich habitat for diverse animal and algae life.
The realm beyond the low-tide mark can be a barren, rough-and-tumble place for marine animals, particularly where currents and waves dislodge and carry sediments along the bottoms of rivers and bays. The structure of eelgrass tempers these forces, providing a protective haven.
As eelgrass grows it sprouts rhizomes, basically stems running horizontally through the mud or sand. New shoots sprout from nodes along the top of the rhizome, and little roots grow down to secure the plant and bind the sediments around it. Through this vegetative reproduction, and also through seed dispersal in spring and summer, eelgrass plants eventually consolidate into beds.
The blades (also called leaves) of eelgrass are a half-inch wide and up to three feet long. Waving to and fro in the water column, they create a canopy that deflects the energy of currents up and over the beds, further protecting the bottom from erosion. Sediments actually have a chance to drop out and accumulate where water flow slows within the bed among the shoots and blades.
In this way fragments of decomposing algae, marsh plants, animal skeletons, feces and other drifting organic material can also become trapped in the meadow. These materials are shredded by small animals like crustaceans and snails, and decomposed by bacteria and fungi. The resulting, particle mixture, called detritus, feeds many animals in their niches on and within the sediments of the meadow.
Worms, clams, amphipods and other deposit feeders move among the mats of roots and rhizomes consuming organic material in the sediments. Some clams and worms burrow into the sediments where they draw water over their gills to strain out food particles. Mussels anchor themselves to the eelgrass shoots or the firm substrate and, like the clams, filter plankton out of the water.
Rising
up through the water column, the grass blades add an upper dimension to
the habitat. Their surfaces are colonized by microalgae, which are grazed
by the zooplankton. Tiny animals, like grass shrimp and small crabs, feed
on these microorganisms.
The meadow, with its provision of firm substrate, abundant food supply and refuge, is prime nursery habitat for crustaceans, mollusks and small fish. Some clams attach to eelgrass during the first stage of their lives, as they change from larvae to juveniles. They then burrow into the sediments. Female Atlantic silversides lay their eggs at the base of eelgrass blades. Male nine-spine sticklebacks construct their nests and rear young among eelgrass blades. Other juvenile fish, including herring, mummichogs and rainbow smelt, also seek refuge there. But the eelgrass meadow is not a completely safe haven. Large game fish like striped bass and blue fish swim through to feed on these small fish.
Canada and brant geese, mute swans and a variety of ducks swim along dipping or diving to feed on eelgrass blades and seeds.
Most animals, though, don’t have the capacity to digest the fibrous material of live eelgrass directly. This vegetation is most valuable as a food source only after it dies. The decomposing blades, roots and rhizomes—and even the bacteria and fungi decomposers themselves—are food for many animals. Some particles fall within the bed, enriching the bottom litter and sediments. Currents carry the remainder away to feed animals outside of the meadow.
Population Fluctuations
There are now about 2000 acres of eelgrass in the estuary, according to a 2002 assessment by Fred Short, a research professor of natural resources at the University of New Hampshire. In the past century, though, there have been drastic declines in the quantity of eelgrass.
A slime mold killed nearly 90 percent of the northern Atlantic eelgrass population in the 1930's. Again, after nearly recovering in the 1960's, the plants were beset by another slime mold outbreak in the late 1980's. The estuary's popluation dwindled to 300 acres. It rebounded in the 1990's, except for some of the beds Little Bay and the Piscataqua River.
Slime mold isn't the only threat to eelgrass. The growing population development of southern New Hampshire could lead to harmful amounts of nutrients in local water bodies.
One concern is the increase of nutrients flowing into estuarine. Excessive nutrient levels have been found to prompt algae blooms, which rob eelgrass of crucial light. In Waquoit Bay, Cape Cod, for example, nutrient loading has wiped most of the eelgrass population. As a result, its scallop population has also dropped.
Major contributors to nutrient loading include groundwater enriched by nutrients from septic wastes, and excess fertilizers in storm runoff from the landscape draining into the estuary. In the case of the Great Bay Estuary, this would include land from as far as the town of Candia to the west, and beyond the city of Rochester to the north.
On a local level, some docks and piers extending into water bodies sometimes affect eelgrass populations.