Neither eel nor grass, eelgrass (Zostera marina) can be a little confusing to get to know. Adding to the mix-up, eelgrass is a marine angiosperm: it is a flowering plant that lives in marine or brackish waters (where fresh water mixes with salt). Eelgrass is actually the most common marine angiosperm worldwide, occurring along both coasts of the United States and Canada and through Baja. Suitable eelgrass habitat has a sand or mud bottom, low wave or current speed, and relatively clear or shallow water so it can “see” the sun to photosynthesize. Eelgrass grows a network of rhizomes under the mud that helps stabilize the substrate, and once established, an eelgrass bed will actually slow currents and trap sediment down by its roots, leading to clearer water and even better eelgrass habitat.
Eelgrass beds are a great place to be if you’re a fish, especially a small one. The beds provide shelter and good hiding places which make them excellent fish nurseries; some, such as the pipefish, make it their permanent home. Pacific herring use eelgrass as their spawning grounds, laying their sticky eggs on the ribbon-like leaves. Ducks and geese come to eat the eelgrass, and predatory birds and larger fish come to eat the smaller fish. And let’s not forget about the invertebrates! Small crustaceans such as amphipods, crabs and shrimp graze on the eelgrass, while fish and birds graze on the crustaceans. Perhaps the most symbiotic relationship is that between eelgrass and oysters. Oysters feed by filtering particles from the water, making it clearer so that the sun can penetrate further.
The San Francisco Bay supports eelgrass beds mainly in the shallow, subtidal parts of San Pablo Bay and Richardson Bay (“subtidal” means that the area is not often exposed, except at very low tides). The largest bed lies between Point San Pablo and Point Pinole, accounting for fully half of all the eelgrass in the bay. Smaller beds are spread out between Hayward and the Carquinez Strait (for a map, see this report).
There are several threats to eelgrass beds in San Francisco Bay, and they’re the same threats that face eelgrass in many populated areas. As we’ve built our communities right up to the shoreline, we’ve eliminated much of the marshy habitat that would have been home to eelgrass, and what’s left is often too deep for the plant to grow. In addition, hard shorelines reflect waves, which bounce back to further disrupt to the eelgrass beds. Dredging, boat propellers, and anchors can mutilate beds, and these activities, as well as boat wakes, increases the amount of sediment in the water, which in turn blocks sun from the eelgrass.