Eelgrass is growing again in areas where oyster racks once lined Drakes Estero. In presentations this month, Dr. Ben Becker, a science advisor and research coordinator for the National Park Service, described the first five years of what he called a passive eelgrass restoration as both variable and largely successful.
“Some of the restored areas have as much eelgrass as the control areas,” Dr. Becker told the Light. But in highly disturbed areas where there was major debris, eelgrass is “still growing back.”
Vital to coastal ecology, eelgrass is one of the only flowering plants, or angiosperms, that can grow in a marine environment. It provides important nursery habitat for fish and other marine organisms, cleans the water, protects the coastline and is thought to reduce ocean acidification. Though it is among the most productive marine habitats, it is also one of the rarest in California, where it is a designated species of concern.
Eelgrass is impacted by pollution and coastal development, as well as activities like aquaculture. Approaches to restoring it are varied on the West Coast, and scientists have learned much through trial and error. According to an assessment of restoration techniques by the Pacific Marine and Estuarine Fish Habitat Partnership, restoration methods play an important role in success, but a site’s environmental conditions have a greater impact.
Dr. Jennifer Ruesink, a biology professor at the University of Washington who studies wild oysters and eelgrass, said oysters grown on structures like racks can have physical effects on eelgrass, including shading and sediment erosion.
“The rack itself has a footprint where it’s anchored into the sediment,” she said. “The local effects of the racks are negative, and the effects of the racks beyond the eelgrass can be negative or positive.”
In Drakes Estero, the biggest impacts on eelgrass productivity have come from aquaculture debris and localized smothering from rack equipment, Dr. Becker said. Oyster operations were present in the estero from the 1930s to 2014, when the park service did not renew the last remaining 1,000-acre lease, held by seashore rancher Kevin Lunny. Between August 2016 and May 2017, a park contractor, T.L. Peterson, removed five miles of wooden oyster racks and other debris from the estero.
In a presentation for the Beyond Golden Gate Research Symposium on Jan. 20, Dr. Becker said that in areas where there was no eelgrass, crews used a specially designed excavator to scrape the seafloor to remove debris. Divers came out for several months to handpick additional debris in high-sediment, low-visibility areas where eelgrass was growing.
All told, roughly a million pounds of pressure-treated wood and nearly 3 million pounds of other debris—including P.V.C. tubes used for spacing oyster clusters, netting, rope, oyster shells, wires and plastic—were pulled from the waters. That weight did not include mud that was pulled up with the debris.
The presence of oyster shells and other debris in the estero in part reflected the fact that, over the years, farmers at Drakes Estero sold shucked oysters in jars rather than for the half-shell market. Mr. Lunny’s business was the last site farming and selling this type of product to California markets.
“Historically oysters were grown for that purpose, but now as preferences have changed, the industry has changed as well,” said Kirsten Ramey, a senior environmental scientist supervisor at the California Department of Fish and Wildlife. “When shucking happens on site, that’s where you have the big shell piles.”
A second component of the restoration project consisted of monitoring eelgrass beds through drone imagery and snorkel-based surveys before and after the rack removal.
Eelgrass naturally grows and shrinks year to year, becoming more or less dense in a given area. This is especially true around the perimeter of a large eelgrass bed—such as those growing in nearby Tomales Bay, which is among the most significant sites for eelgrass on the West Coast. Passive restoration success is typically measured in terms of increased shoot density.
The Drakes Estero restoration came with some mitigation strategies. For example, if contractors had to place pilings in the water to steady their barges and damaged 100 square feet of eelgrass in the process, the park was required to show that 120 square feet grew back over the next few seasons.
Dr. Becker said this requirement was met after two years. Five years after the project began, eelgrass showed a 249 percent increase, or a gain in 1,156 square yards, by 2019.
The removal of debris had an additional benefit: a decrease in the presence of an invasive tunicate called didemnum vexillium. The invertebrate attaches to hard surfaces, and eelgrass can become encased in it, impacting photosynthesis and possibly seed dispersal.
Research on both the West and East Coasts found the tunicate was starting to grow on eelgrass about 12 years ago, Dr. Becker told the Parks and Open Space Commission this month. But in Drakes Estero, most of the tunicate was growing on racks and debris. The park is now monitoring to see how much is growing on eelgrass, and is considering working with researchers to understand the tunicate population.
Ecologists are worried about the loss of seagrass generally, and in recent years, protective measures have seen some benefits. In California, all activities that might impact seagrass beds are regulated, and eelgrass is protected by no-net-loss provisions.
Though aquaculture equipment cannot be introduced in existing seagrass beds under current law, eelgrass can appear where there are pre-existing aquaculture structures. This may be because the presence of bivalves can have positive impacts on eelgrass, too.
Shellfish can reduce phytoplankton in the water, which improves seagrasses’ access to light. Ecologists like Dr. Reusink are studying the influence of aquaculture on eelgrass to help oyster growers develop best management practices. “Shellfish can transform phytoplankton into dissolved nutrients used by plants such as eelgrass,” she said.
Of course, altering an environment can have many effects, as different species use different habitats. An oyster bed is a different type of habitat from an eelgrass bed, so an oyster bed without eelgrass will benefit some species while pushing out others. These types of trade-offs can present complicated questions for management policies.
Terry Sawyer, a co-owner of Hog Island Oyster company, said growers like him are interested in understanding these relationships, along with farming techniques that minimize impacts on ecosystems. “We’re learning all along, and different practices have different levels of impact,” he said.
Mr. Sawyer said he finds eelgrass moving in and out of areas with aquaculture equipment. “You have to have five meters between [the grass] and where you install equipment, but what we find is that in a place with a lot of filter feeders, the plants can do better,” he said.
Indeed, ecologists increasingly recognize that “the system as a whole includes people,” Dr. Reusink said. Human activity combined with Mother Nature’s variability means that “what happens in one place may not be the same as what happens in another.”