Creature Feature: Eelgrass

February, 2015





Zostera marina

Eelgrass is a type of seagrass, a group of flowering plant species that thrive in saltwater. Often confused with seaweed, eelgrass has tiny flowers and is thought to be more closely related to lilies than to grasses. Eelgrass flowers bloom underwater and have filamentous pollen strands that drift onto stigmas of nearby flowers by flowing through the water column. The evolution of filamentous pollen in seagrasses is an adaptation to submarine pollination, which is quite uncommon. Even less common among plants is the adaptation to saltwater.

Eelgrass grows in coastal estuaries and bays in the north Pacific and Atlantic oceans. In Humboldt Bay, its horizontal stems are rooted in bay mud in the intertidal zone. It can thrive at varying depths in different regions, depending on water clarity. In Humboldt Bay, sediment from the tributaries limits eelgrass by blocking light penetration through turbid waters, particularly after rainstorms that carry sediment from dirt roads, clearcuts, landslides, and eroded streambanks.

Eelgrass provides important habitat for juvenile salmon, herring, and other fishes, as well as Dungeness crabs and the wide variety of invertebrates they prey upon. Eelgrass also provides food for resident and migratory waterfowl. Perhaps most notably, black brant geese make exclusive use of estuarine seagrasses as they migrate from overwintering areas in Mexico to spring staging areas in coastal California and the Pacific Northwest before moving to breeding areas in southwest Alaska. Brants generally graze Humboldt Bay’s eelgrass by the tens of thousands from mid-December to early May.

Although eelgrass and other seagrasses occupy less than two tenths of a percent of the world’s oceans, they capture and store a tremendous amount of carbon from the Earth’s atmosphere (12 to 20 percent). Combined with coastal marshes and tropical mangroves, seagrass ecosystems comprise a mere two percent of ocean area, but account for 50 percent of ocean carbon storage. One scientist has estimated that the carbon storage capability of one acre of seagrass is equal to 40 acres of terrestrial forest.

Unfortunately, 29 percent of the world’s known seagrass beds have been destroyed since 1879, and they continue to disappear at an annual rate of seven percent, re-releasing sequestered carbon back into the atmosphere.

Due to its critical ecological role in supporting so many species of wildlife, eelgrass and its habitat are protected by state and federal law. Projects that impact it must offset any loss of eelgrass by establishing new sites, which requires transplanting eelgrass shoots in suitable areas where it is not already present. Transplanting eelgrass is difficult work that often has fairly low success rates.

The primary threats to eelgrass are direct impacts from dredging, shellfish production, and dock construction. Another threat is poor water quality resulting from sediment-polluted runoff, which reduces eelgrass growth by decreasing the amount of sunlight that can be used for photosynthesis. Existing eelgrass habitat will undoubtedly be altered as sea level rises, although eelgrass may become more abundant as the footprint of the Bay expands with higher relative sea level.

Fortunately, Humboldt Bay’s eelgrass beds appear to be thriving, and many local scientists, land managers, and regulatory agency staff are actively engaged in protecting, restoring, studying, and managing this critical resource and the numerous species that rely on it for food and habitat.