Emerald green and resplendent with glittering baby fish and tiny crabs, eelgrass meadows are among the most productive and ecologically important places in Puget Sound.
Already highly valued as nurseries for sea life, researchers have discovered a new eelgrass superpower, as living urban systems that reduce human pathogens in seafood by as much as 65%.
The researchers' findings, published in an Aug. 2 paper in Nature Sustainability, point to yet another reason to protect eelgrass here. Climate change has taken a devastating toll on eelgrass, a kind of seagrass, in some areas of the Salish Sea. For example, declines of 90% and more have been documented in the San Juan Islands because of a wasting disease stoked by warming water.
To find out how much eelgrass can clean up the water it grows in, researchers employed a species with exceptional filtering and concentrating abilities: mussels. In the study, they acted as a sort of test strip, to find out what is in the water.
Working with volunteers in the Washington Department of Fish and Wildlife's Puget Sound mussel monitoring program, the researchers obtained clean mussels from Penn Cove on Whidbey Island, then placed them around urban waters of Puget Sound, in cages so predators wouldn't get them. Put out in late fall, the mussels were collected three months later. In 20 locations around Puget Sound, mussels placed in areas with eelgrass were 65% cleaner (of bacteria that can sicken people) than mussels placed in areas without eelgrass.
In that way, eelgrass boosts the health of the environment, people and wildlife all in one, said Drew Harvell, an author on the paper and senior researcher at the University of Washington Friday Harbor Labs and affiliate faculty for the UW School of Aquatic and Fishery Sciences.
The superpowers of eelgrass
Eelgrass beds provide ideal nurseries for fish, crab and other animals. They anchor sediment, cycle excess nutrients and sequester carbon from the atmosphere. In Puget Sound, scientists found that mussels placed near eelgrass had 65% fewer pathogens than mussels placed in areas without eelgrass. The exact filtering mechanism is unknown and invites further research.
Dawkins et al. 2023, Lamb et al. 2017, NOAA (Fiona Martin / The Seattle Times)
"It's not exaggeration or hyperbole to talk about superpowers of seagrass," Harvell said. "When you think about all the services it provides, for baby salmon, for baby forage fish, for baby crabs, without seagrass we would have even fewer salmon, baby sea stars, baby crabs. Their role as nurseries is substantial; they are a crucial link in the chain of survival."
Joleah Lamb, an author on the paper, and now an associate professor at the University of California Irvine, noted some of the mussels sampled had a strong smell of sewage — but not the ones growing in eelgrass meadows.
There are about 55,000 acres of eelgrass meadow in Puget Sound, according to the Puget Sound Partnership, and while the overall acreage has remained relatively stable, some areas, including the San Juan Islands, have seen big declines. Eelgrass can only grow on soft sediments. It reproduces through seeds, and also can also spread through below-ground rhizomes, creating large, thick beds that can even shelter uplands from wave action. It thrives in clean, clear water. Pollution, sedimentation and damage by propeller chop, scour, structures that shade the eelgrass and shoreline development that eliminate habitat all damage, limit and destroy it. It can be very hard to reestablish if lost.
Eelgrass in the Salish Sea
Eelgrass is found near shorelines throughout the Salish Sea. The state Department of Natural Resources surveys seagrasses and tracks their abundance.
Eelgrass (Zostera marina L.) is an aquatic flowering plant. It photosynthesizes just like any plant and stands up when submerged at high tide, but lies down on the intertidal zone in beautiful long, green strands when the tide draws back. It is a crucial zone of life in Puget Sound for its many important functions. Herring will lay their eggs on it, waterfowl nibble it and all sorts of tiny lives shelter in it.
To watch an eelgrass bed on a sunny summer day is to enter a dreamy underwater wonderland captured in this video by the SeaDoc Society. The grasses fizz with air bubbles produced by photosynthesis — so many that underwater they can even be heard popping. The lush vegetation sways in the swish, swish of the tide and nurtures a menagerie of life: red rock crab aglow amid the green, tiny snails lugging their shells up and down the grassy green blades and quicksilver darting fish.
The idea for testing the ability of eelgrass to clean the water — as revealed by the lesser contaminant loading in the mussels growing in seagrass — came from earlier work by Lamb, Harvell and other researchers in tropical waters, where the cleansing ability of seagrass also was documented.
Just how the grasses cleanse water is not fully understood. Could it be the oxygen the grasses produce? The settling effect of the grasses on the water? Possible action of a microbiome on the eelgrass? The presence of resident filter feeders like mussels, clams, oysters, sponges and sea squirts? While it's always wise to qualify that a correlation could be due to other factors, what is known is that two studies have now documented a cleansing effect on water where seagrasses are present.
"We had done the project in Indonesia, and I was really stunned by the value of the result," said Harvell, who with other researchers in a 2017 paper reported seagrass meadows there reduced harmful bacteria by as much as 50%. "We thought, we need to bring this to Seattle and to our domestic shores and see what we find."
Mussels have long been used to assess the water quality of Puget Sound, in both federal and state monitoring programs. Because they stay put and filter the water, shellfish are perfect bio-monitors of what is in local waters.
The results can be surprising: in a 2020 paper, researchers found clean mussels transported to 18 sites in Puget Sound picked up all kinds of contaminants, including the opioid oxycodone.. Chemotherapy and diabetes drugs, pesticides, DEET, antibiotics, antidepressants and contaminants from stormwater all showed up in mussel tissues.
The ability of seagrass to help cleanse any of those pollutants is untested.
In addition to their cleansing powers, globally, seagrasses are spectacular at absorbing carbon dioxide from the atmosphere, according to a 2022 paper published in Nature, helping to blunt the effects of climate warming, including ocean acidification.
"Seagrass ecosystems are called the marine powerhouse because they support commercially important fish nurseries, protect coasts from erosion, mitigate ocean acidification, filter and cycle nutrients, and capture carbon dioxide up to 30 times faster than a rainforest," Lamb wrote in an email.
And yet seagrass meadows in the intertidal zone are at high risk from climate warming, globally and here at home, noted Harvell, emeritus faculty at Cornell University, and author of the book "Ocean Outbreak: Confronting the Rising Tide of Marine Disease."
Over 20 years of monitoring, DNR found that declines in San Juan Island eelgrass outnumbered increases by 3 to 1, according to their paper with Harvell just published in the journal Integrative and Comparative Biology. Losses were the most pronounced at sites with shallower eelgrass meadows, such as Westcott Bay and Blind Bay, where eelgrass completely disappeared from the head of the embayment. When Olivia Graham of Cornell University, DNR and Friends of San Juan Islands surveyed the extent of subtidal eelgrass meadows in the San Juan Islands in 2023, they found deeper areas had less disease, and eelgrass beds there were stable, and had even increased.
Conservation should be prioritized for areas where seagrass meadows have proved themselves most resilient, the researchers found, to "future proof" what seagrass remains.
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