Seagrasses in Tomales Bay: The Unsung Heroes of Habitat

By Suzanne Olyarnik
Published: February 2007

Eelgrass. Credit: Randall Hughes, Bodega Marine Lab

It’s a calm, quiet morning on Tomales Bay; the water is motionless and glassy as the sun peeks over the hills.  A sudden, silver flash at the surface breaks the stillness for just a moment, sending a ripple across the water and reminding us that beneath the quiet surface there is an unseen world teeming with life. At its heart is eelgrass (Zostera marina), a unique plant that has adapted to living submerged in salty coastal waters.

In places like Tomales Bay, where conditions are favorable, eelgrass forms thick beds with thousands of simple, long, green, ribbon-like leaves all but filling the water column. It looks much as you might expect – like tall grass underwater – but its unassuming appearance belies the important role eelgrass plays in the bay ecosystem.

Eelgrass meadows can stretch for miles underwater, supporting a rich diversity of fish and wildlife, including many commercially and recreationally important fish species, shorebirds, waterfowl, crabs, shrimp, and many other invertebrates.  10 to 100 times more animals can be found in eelgrass beds compared to adjacent sandy and muddy habitats. The three-dimensional structure that eelgrass provides is a haven for hundreds of species. Some spend their entire lives in the eelgrass beds, while others use them as a nursery ground or as a stop on a long migratory journey.

Consider Coho salmon, for example. Estuaries such as Tomales Bay play a vital role in the salmon life cycle. Coho salmon and steelhead trout use streams that flow into Tomales Bay for spawning. In the estuary, returning adults and migrating smolts (juvenile salmon) can adjust to changes in the salinity of the water as they move from saltwater to freshwater and back again. When they are big enough, juvenile salmon move down from the streams and into the eelgrass beds of the bay where they feed on tiny invertebrates. To avoid becoming a meal themselves, young salmon hide among the eelgrass leaves.

Eelgrass beds also supply a nursery area for Pacific herring, a favorite salmon snack.  Herring spawn in the estuary in late winter leaving tens of thousands of sticky eggs that attach to eelgrass blades and other surfaces. Many of these eggs get eaten by diving waterfowl including surf scoters and greater scaup.

Critical for Birds

Black brant, also known as the Pacific sea goose, is another key member of the eelgrass community. Brant use California eelgrass beds — including those in Tomales Bay — during their extensive migration from the high Arctic to Mexico. These “refueling” stops help the birds survive the rigors of migration to produce next year’s offspring. Brant are one of the few species that consume eelgrass directly, and they depend on it as a major food source in their diet. Because eelgrass is available only at key sites, the entire population of brant is vulnerable to eelgrass losses at a single location. Eelgrass declines in other areas have resulted in reduced numbers of this beautiful bird.

Many other species of birds, including great blue herons, marbled godwits, willets, dunlins, brown pelicans, and black scoters, feed on fish and invertebrates here, too. With their rich food resources, eelgrass beds support a huge population of birds year round.

A Key Species Between Land and Sea

Drawing on the nutrients available in both the coastal sediments and the rich coastal waters, eelgrass is a highly productive plant, generating more plant material per acre than a fertilized cornfield. In addition, each blade of eelgrass provides a substrate for diatoms, bacteria, and detritus (decaying plant and animal matter). These things provide food for many marine invertebrates, small bug-like creatures such as isopods, and amphipods, as well as polychaete worms, clams, snails, anemones, and many others.

The large numbers of these invertebrates makes eelgrass beds rich feeding grounds for fish and birds. As eelgrass dies, bacteria and fungi feed on the dead leaves, breaking some of them down into tiny bits. These particles of plant material provide vital nutrients for the coastal food web. A significant amount of eelgrass often gets transported to deeper, offshore areas, providing important resources to those habitats as well.

Eelgrass provides many services for people, too. Like other plants, eelgrass photosynthesizes, using energy from the sun to convert water, carbon dioxide, and minerals into food. During this process, oxygen is released; it literally bubbles out of the plants, providing oxygen for fish and other animals living nearby. By sequestering carbon dioxide, eelgrass is contributing to the reduction of greenhouse gases; it’s estimated that one acre of eelgrass sequesters over 7,000 pounds of carbon per year, which equals the carbon dioxide emissions from an automobile traveling 3,860 miles.

On top of this, Eelgrass meadows guard against shoreline erosion by dampening wave energy and storms. By trapping sediments and nutrients, eelgrass also improves water quality.

Threats to Eelgrass

Occurring in coastal areas, eelgrass feels effects from both land and sea. A decline in eelgrass can alert us to greater problems in the coastal habitat. Because eelgrass needs sunlight to survive, water clarity is important. Nutrient pollution is one of the main threats to eelgrass; sewage and fertilizers from land runoff produce excess nutrients that stimulate growth of microscopic plants. These phytoplankton cloud the water, diminishing sunlight that the eelgrass requires in order to grow.  Other threats include physical disturbances (dredging, damage from boating activities), invasion of nonnative species, disease, and harmful algal blooms.

The next time you explore Tomales Bay, whether from the shoreline or out on the water, consider the hidden wealth below its glimmering surface. Evidence that eelgrass meadows are serving the ecosystem is everywhere: a flash of silver on the surface, a harbor seal diving for a fish, a pelican dipping its bill under the water, a flicker of a bat ray, the dorsal fin of a leopard shark, a hovering osprey…there are so many different animals you can encounter on a given day. They are the reminders that eelgrass is underneath it all, supplying food, providing habitat, cleaning the water, and enhancing the entire bay ecosystem.