Eelgrass
by Diane Gussett
Download coloring poster The Eelgrass Meadow: A World of Microhabitats
What is Eelgrass?
Underneath the PTMSC pier, long, slender, emerald-green leaves of eelgrass catch the sun and wave to and fro with the sea's constant motion. They soften the impact of waves and currents, creating a calm space in the midst of turbulence. Crabs shuffle through leaf litter while schools of juvenile salmon and cod find a safe haven from predators. Flounder and sole nose through the nutrient-rich sediment. Tiny snails crawl to the tips of the sunlit leaf blades to lay their eggs. Delicate stalked jellyfish and brooding anemones anchor to the swaying blades; their tentacles feeding from the rich plankton soup flowing by.
Eelgrass meadows are a vital part of nearshore food web relationships. Since the 1980's, habitat managers have been increasingly protective of eelgrass due to its importance as habitat for fish and wildlife and its vulnerability to shoreline development. Fish and shellfish which depend on eelgrass for all or part of their life cycle account for a multimillion dollar industry in Washington State alone, leading our resource managers to adopt a "no net loss" policy for eelgrass. But the importance of eelgrass goes well beyond the fisheries that it supports.
Eelgrass is not a seaweed, but a flowering plant! Zostera marina (from the Greek word zoster meaning 'belt', describing the ribbon-shaped leaves), or Common Eelgrass, is not technically a grass, but one of a group of flowering land plants that seem to have migrated into the sea in relatively recent geologic times. Unlike their land relatives, eelgrass flowers need no elaborate shapes or bright colors to attract insects, or animals to transfer pollen from flower to flower. Hidden behind a transparent leaf sheath, eelgrass flowers are seldom seen. They release their long filamentous pollen into the water, where it remains in suspension for days and is spread by waves and currents.
Plant Description: Grass-like flowering plant with dark green, long, narrow, ribbon shaped leaves 20 - 50 cm in length (exceptionally up to 2m long) with rounded tips. Leaves shoot from a creeping rhizome that binds the sediment. Leaves and rhizomes contain air spaces, lacunae, that aid bouyancy. Numerous flowers occur on a reproductive shoot similar to those of terrestrial grasses. Forms dense swards in the subtidal, supports a diverse fauna and flora and may act as a nursery for fish and shellfish. Source: Dr Harvey Tyler-Walters, Marine Life Information Network of Britain and Ireland.
Eelgrass is a perennial plant that grows both by vegetative growth, i.e. by spreading rhizomes, and by seed germination. Most growth of eelgrass in the Pacific Northwest is vegetative. The depth of eelgrass growth is limited by incident light and water clarity. In the Northwest, the maximum depth is about 22 feet, but in the extremely clear water off California, eelgrass has been found growing at depths of more than 100 feet.
In deep (subtidal) water eelgrass develops: longer and wider leaves, a high root/rhizome biomass, low shoot densities, few or no reproductive (flowering) shoots, low productivity and respiration, and no seasonal biomass cycle. By contrast, in the shallow (intertidal) waters, eelgrass has shorter, narrower leaves, low root/rhizome biomass, a high density of shoots, a large number of reproductive shoots, a high respiration rate, and exhibits a seasonal biomass cycle. Shallow water eelgrass meadows are also more accessible to feeding waterfowl.
What's so important about Eelgrass?
An Eelgrass Meadow:
- Creates a highly structured habitat from loose and shifting sands.
- Softens the impact of waves and currents, stabilizing the shoreline and providing a calm space where organic matter and sediments are deposited.
- Provides shelter and protection from predators for many juvenile fish and shellfish of ecological, commercial and recreational importance.
- Absorbs and concentrates nutrients from the sea and transfers them to the sediment or to animals.
- Decomposes into an important part of the food web for the coastal marine ecosystem.
- Provides diverse habitats.
- Provides an important pathway for food for both local and distant communities.
A Variety of Habitats: Compared to nearby barren, shifting sands, the eelgrass meadow is full of life. As in an old-growth forest, there are habitats in every niche. The leaves, stems, roots and rhizomes provide multitple habitats and support a great variety of animals living in, above, and under but not directly feeding on, the eelgrass.
The development of the eelgrass community starts with a coating of diatoms on the eelgrass blades to which bacteria and other algae are then able to attach. This seemingly insignificant "brown felt" is of immense importance to the entire eelgrass ecosystem. Without this initial layer which has the ability to colonize and complete a life cycle in a short time, and which has an extremely high growth rate, many of the nursery and nutritional functions of the eelgrass meadow would never develop.
A Marine Food Factory: Eelgrass is very efficient at converting solar energy into plant tissue and concentrating numerous elements that occur only sparingly in the sea. With its high productivity and rapid growth, eelgrass forms the food-base for fish, shellfish and waterfowl in shallow seas, as plankton does for marine life in deeper waters. From predator to increasingly larger predator, the food chain that began with eelgrass ends on our dinner tables.
Eelgrass is the main diet for such waterfowl as the black brant and canada goose as well as animals as snails and green urchins. Yet less than 5% is eaten by direct grazers. The main function of eelgrass is the production of detritus (decaying plant matter) from dead leaves and plants.
Much recent evidence shows that the real nourishment for animals eating the detritus is the microbial coating of bacteria and fungi. After stripping off the microbial layer, the animals expel the eelgrass particles which again become coated with a microbial layer. As this continues, the enzymatic activity of the microbes reduces the particle size, making them available to different classes of consumers. The same particle is repeatedly ingested until it is completely utilized. The breakdown of particles also increases the surface area available for microbes. The long decay period of eelgrass may be an important factor in prolonging the availability of food for its consumers.
These simple food chains are actually complex food webs. Birds export a great amount of nutrition and energy, as well as adding nutrients to the eelgrass meadow. In estuaries, where eelgrass beds adjoin marshes, eelgrass litter is trapped and nutrients are recycled within the wetlands, preventing loss to the deeper Puget Sound and ocean systems. Nutrients are then passed back and forth between the marsh and the eelgrass meadow.
Eelgrass detritus driven offshore sinks and can support large animal populations at depths up to 30,000 feet. Eelgrass blades were found in the stomach of an abyssal rattail fish caught far off the Oregon coast. In deep sea trenches, eelgrass detritus provides a substrate, shelter, and food and it also adds organic carbon and nitrogen to the sediment.