Massive seagrass beds in Western Australia’s Shark Bay — a UNESCO World Heritage Site — haven’t recovered much from the devastating heat wave of 2011, according to a new study demonstrating how certain vital ecosystems may change drastically in a warming climate.
RJ Nowicki, JA Thomson, DA Burkholder, JW Fourqurean, MR Heithaus. Predicting seagrass recovery times and their implications following an extreme climate event. Marine Ecology Progress Series, 2017; 567: 79 DOI: 10.3354/meps12029
Every continent save Antarctica is ringed by vast stretches of seagrass, underwater prairies that together cover an area roughly equal to California.
Seagrass meadows, among the most endangered ecosystems on Earth, play an outsize role in the health of the oceans. They shelter important fish species, filter pollutants from seawater, and lock up huge amounts of atmosphere-warming carbon.
The plants also fight disease, it turns out. A team of scientists reported on Thursday that seagrasses can purge pathogens from the ocean that threaten humans and coral reefs alike. (The first hint came when the scientists were struck with dysentery after diving to coral reefs without neighboring seagrass.)
But the meadows are vanishing at a rate of a football field every 30 minutes. Joleah B. Lamb, a postdoctoral researcher at Cornell University and the lead author of the new study, said she hoped it would help draw attention to their plight….
…The plants also draw fertilizer runoff and other pollutants out of the water, locking them safely away in meadow soil. Scientists have estimated that an acre of seagrass provides more than $11,000 worth of filtering every year….
In one survey, they collected seawater and put it in petri dishes to see if colonies of disease-causing bacteria known as Enterococcus grew from the samples. Levels of the bacteria in water from seagrass meadows, they found, were a third of the levels in water from other sites. In a second search, the scientists grabbed fragments of DNA floating in seawater. By examining the sequences, they identified 18 kinds of disease-causing bacteria. Water from the seagrass meadows had only half the level of this DNA, compared with water collected at other sites. The scientists next turned their attention to coral reefs around the islands. Reefs next to seagrass meadows, they found, were half as diseased as those without meadows.
…Seagrass meadows can store enormous amounts of carbon. Their soils don’t decompose because they have very little oxygen in them. As a result, seagrass meadow soil around the world has accumulated an estimated nine billion tons of carbon.
As seagrass meadows disappear, that carbon is being released back into the ocean. Some of it may make its way into the atmosphere as heat-trapping carbon dioxide. As dire as the situation has become, there is cause for hope. In recent years, Dr. Orth and his colleagues have successfully restored seagrass meadows off the coast of Virginia. “Now we have 6,200 acres of seagrass,” he said, “where in 1997 there wasn’t a single blade of grass.
Seagrass meadows — bountiful underwater gardens that nestle close to shore and are the most common coastal ecosystem on Earth — can reduce bacterial exposure for corals, other sea creatures and humans, according to new research.
…While research is beginning to reveal the mechanisms driving bacterial-load reductions in these ecosystems, it is evident that an intact seagrass ecosystem — home to filter-feeders like bivalves, sponges, tunicates (marine invertebrates) — removes more bacteria from water.
As seagrass meadows and coral reefs are usually linked habitats, Lamb’s team examined more than 8,000 reef-building corals for disease. The researchers found lower levels — by twofold — of disease on reefs with adjacent seagrass beds than on reefs without nearby grasses. “Millions of people rely on healthy coral reefs for food, income and cultural value,” said Lamb….
Joleah Lamb et al. Seagrass Ecosystems Reduce Exposure to Bacterial Pathogens of Humans, Fishes and Invertebrates. Science, 2017 DOI: 10.1126/science.aal1956
A new study links a long-term decline in Chesapeake Bay’s eelgrass beds to both deteriorating water quality and rising summertime temperatures. It also shows that loss of the habitat and other benefits that eelgrass provides comes at a staggering ecological and economic cost…….”Declining water clarity has gradually reduced eelgrass cover during the past two decades, primarily in deeper beds where lack of light already limits growth. In shallow beds, it’s more that heat waves are stressing the plants, leading to the sharp drops we’ve seen in recent summers.”
“Not only have we lost a huge ecological resource, there have been real economic and recreational consequences….Blue crab fisheries, for example, have probably lost a year or more of catch based on the amount of eelgrass we’ve already lost. For silver perch, it’s 10-20 years. In all, we estimate the potential economic cost to citizens at $1-2 billion.”…
…mean summertime water temperature in the lower Chesapeake Bay has already increased by more than 2 degrees Fahrenheit since 1984 — from 76.8° to 79.5°F — and that the frequency of extreme warm spells with water temperatures exceeding 82° has doubled in the last decade….As global warming continues to raise the area’s water temperatures — a conservative estimate is a further 3.5° F increase by 2040 — the team predicts a further 38% decline in eelgrass cover. And if water clarity follows its current downward trajectory during the next 30 years, eelgrass would decline an additional 84%. When both declining clarity and warming are considered, say the researchers, the predicted increases in temperature and turbidity would result in a loss of 95% of Bay eelgrass — a near total eradication…
…”Our analysis suggests that eelgrass could still persist in the face of moderate increases in temperature, if the water remains clear enough” adds Lefcheck. “But that will only happen if managers adopt an integrated perspective, and continue with their efforts to curb inputs into the Bay.“
Jonathan S. Lefcheck, David J. Wilcox, Rebecca R. Murphy, Scott R. Marion, Robert J. Orth. Multiple stressors threaten the imperiled coastal foundation species eelgrass (Zostera marina) in Chesapeake Bay, USA. Global Change Biology, 2017; DOI: 10.1111/gcb.13623
Seagrasses are disappearing at rates that rival those of coral reefs and tropical rainforests, losing as much as seven percent of their area each year. Replanting success rates have been unpredictable — but scientists are making new advances that could change that…
….In the Bay Area, the long strands of eelgrass provide shelter for fish nurseries, prime substrate for the sticky eggs of herring, and food for the small “grazers” that eat the algae coating on the grass. The plant beds also forestall erosion by trapping sediments and slowing down waves or currents. Although researchers estimate around 9,490 hectares (more than 23,000 acres) of the San Francisco coastline could support eelgrass beds, these plants grow on less than one percent of that shoreline. “There are so many different environs in San Francisco Bay, we’d like to come up with a template that helps us determine what methods would work best for each site,” says Boyer, who keeps her office at the Romberg Tiburon Center for Environmental Studies stocked with wetsuits….
…One failure, still unexplained, occurred in Corte Madera Bay, where healthy grass beds died suddenly after years of apparently healthy persistence. Although Boyer wants to keep looking for an answer, she says there isn’t time for that — not if scientists plan to meet the goal of getting 36 more acres of seagrass planted before the nine-year project ends.
If there have been surprising failures, there have also been unexpected successes. Eelgrass is thriving in Elkhorn Slough, an estuary about 100 miles south of San Francisco. Bordered by agricultural fields, the runoff is rich with fertilizer, making the water prone to algal blooms that usually kill seagrass. When algae coats the eelgrass, it diminishes the light available for photosynthesis and plant survival. Despite these conditions, the eelgrass in the Slough is abundant. That anomaly made Brent Hughes, a marine biologist from the University of California at Santa Cruz, wonder: Why?…