Hospitality Returning to the Bottom of the Bay

We must consider many factors before settling our lives into a new place. Whether it is the school system, safety or convenience of transportation, people have a list of requirements for their suitable habitat. It is the services and infrastructure after all that distinguish our comfortable communities from hard life on the frontier. Whether dispersing from their population or remaining exactly where they were born, organisms seek to fill similar needs when settling into their habitat. Some plants and animals, known as foundation species, are specially equipped to satisfy these requirements for many different species. By virtue of their physical characteristics, foundation species provide three-dimensional habitat for entire ecological communities, and in doing so dramatically enhance biodiversity in a relatively small area. Reef-building organisms like corals and oysters, trees that form a dense canopy layer, and the impenetrable underwater forests of kelp are all examples of foundation species that host robust ecosystems.

As we have learned from the worldwide destruction of coral reefs, declines in foundation species pose an enormous threat to their associated, denizen species. For the same reason, the protection and restoration of foundation species could be the most important step to maximize the conservation of global biodiversity. This conservation strategy is currently underway in San Francisco Bay, as collaborative effort between Audubon California, Save the Bay, NOAA, and San Francisco State University strives to restore and enhance one of the most important foundation species in San Francisco Bay – Zostera marina, or eelgrass.

Eelgrass is a type of seagrass – a marine flowering plant (not a seaweed!) that grows submerged in shallow water areas worldwide. Seagrasses form vast meadows in coastal environments that resemble terrestrial grasslands, their dense root system and tall leaf canopy creating complex habitat in areas that are otherwise unvegetated. Seagrass beds provide food, cover, and spawning ground for a wide array of invertebrates, fishes, birds, and mammals. Found in tropical as well as temperate regions, seagrasses play host to hundreds of associated species including green sea turtles, manatees, seahorses, and countless species of fish. As a biodiversity hotspot, seagrass beds also attract hordes of predators that come to feed on their residents. Unfortunately, seagrasses are declining worldwide at an astounding rate. Like all foundation species, the destruction of seagrasses can have severe impacts on the many associated species that rely on seagrass bed habitat. A recent study by A. Randall Hughes of UC Davis found that nearly 15% of all seagrass species worldwide are currently listed as threatened in some portion of their range. For every species of seagrass, there is at least one associated species of concern, and in total there are 74 species of concern that are associated with seagrasses worldwide. These results come from a preliminary study, however, and the true conservation costs of seagrass declines have likely been underestimated. Moreover, there is a general lack of awareness of the importance of seagrass bed ecosystems on biodiversity.

Over the past few decades within the San Francisco Bay, the size and number of eelgrass beds has been steadily declining, mostly due to reduced light availability. Like all plants, eelgrass requires sunlight for photosynthesis; however, conditions within the Bay have severely limited the depth to which light can penetrate the Bay water. Dredging operations in the Bay destroy eelgrass beds either by physical disturbance or by stirring up sediments to increase turbidity. Construction activities in the Bay watershed release sediments to streams that eventually reach the Bay, smothering eelgrass beds. Even if there is enough light for the plants to growth, excess nutrients can accelerate algae growth beyond the feeding rates of grazers, allowing the algae to over take eelgrass leaves and block out light.

Biologists of the eelgrass restoration team are putting in a great effort to enhance existing eelgrass beds and restore this habitat to the fullest extent of its San Francisco Bay range. Richardson Bay in North San Francisco Bay is the ideal location for eelgrass restoration. It harbors the second largest eelgrass bed in the estuary, with plants that have the greatest genetic diversity of all beds sampled. Given its sheltered location and distance from dredging operations, Richardson Bay has the model environmental conditions for large eelgrass beds, but the genetic diversity of its plants also gives hope for successful transplanting to other sites in the Bay. Using a variety of techniques, biologists are hoping to discover the restoration method with the highest success rate. Both mature plants and seedlings are transplanted to sites at four different depths where they are either hand-planted by divers or tied to special grid-like frames which sink to the muddy bottom. At the same time the team is modeling Bay circulation patterns to better understand the potential for seed dispersal from existing beds. Divers are collecting mature eelgrass flowers from donor beds and using them for targeted seed dispersal at other sites in the Bay. Seeds can be deployed either by hand or from mesh bags attached to buoys which hold the flowers and distribute the seeds in a circular pattern, moving with the current.

The restoration team and Bay Area naturalists alike hope that these efforts will reverse the Bay’s long-term decline in native biodiversity, giving us an example of the multi-layered ecosystem that this foundation species once supported. At the base of its complex food web, dense mats of eelgrass roots hold sediments in place and keep them well-oxygenated to support the growth of important bacteria. Eelgrass leaves provide substrate for algae and epiphytic plants which are grazed upon by a number of invertebrates. Even dead eelgrass plants that settle within the bed, develop a film of bacteria, fungus and detritus, which also feeds small invertebrates. During low tides, eelgrass beds hold moisture, so that these small organisms are protected. As a result, waterfowl arrive in droves to feast on a surfeit of invertebrate prey. The Pacific Herring, the largest commercial fishery in the Bay which has seen recent population declines, depends on eelgrass beds for spawning and cover. The herring lay their sticky eggs on eelgrass leaves so that the young will be protected until they reach maturity. The success of these restoration efforts could enhance the Pacific herring population, allowing for increased takes by local fishermen. Birders would also be happy to see the return of the diversity of native and migratory waterfowl that hunt the in eelgrass beds. To learn more about the progress of the San Francisco Bay eelgrass restoration, visit www.tiburonaudubon.org

Cited: Hughes, A. Randall, Susan L. Williams, Carlos M. Duarte, Kenneth L. Heck Jr., and Michelle Waycott. 2009. Associations of concern: declining seagrasses and threatened dependent species. Frontiers in Ecology and the Environment. 7:242-246

Photo credit: www.ceoe.udel.edu