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Manatee deaths in Florida point to a global decline in seagrass ecosystems

  • This year the U.S. state of Florida saw a record number of manatee deaths, and though investigations are still ongoing, experts attribute most of the deaths to the dieback of seagrass, a primary food source.
  • The manatee deaths are part of a larger trend: around the world, seagrasses are on the decline, mainly because of increasingly clouded waters due to coastal development.
  • Other drivers of this die-off include algal blooms, destructive fishing and boating practices, and the warmer, more acidic waters of climate change.
  • There are spots of hope, yet seagrass scientists warn that we are on the brink of losing many of these important wildlife habitats and global carbon sinks.

In July 2021, the U.S. Fish & Wildlife Service announced that 866 West Indian manatees (Trichechus manatus) had died in the state of Florida in the first seven months of the year alone. The news shocked and saddened many Floridians, for whom the gentle, slow-moving mammal is a statewide mascot, and served as a large blow to a statewide effort that had brought the Florida subspecies (T. m. latirostris) back from a slide toward extinction. 

It came as little surprise, however, to scientists studying the manatees’ primary food source: seagrasses. 

“Scientists have said time and time again that seagrass is declining, that there is a hell of a lot of nutrients in these waters, that there’s a big problem here that needs to be solved,” said Benjamin Jones, a seagrass ecologist and co-founder of the U.K.-based research organization Project Seagrass, in an interview with Mongabay. “But still nothing has really been done.”

The spate of deaths is still being investigated by the FWS as an unusual mortality event (UME), and so its cause is not yet official. Yet wildlife officers suspect the mass death is linked to the dieback of seagrass in the Indian River Lagoon, a major overwintering location for the Florida manatee, and where most of the deaths occurred. 

Healthy seagrass meadows thrive in clean water with ample sunlight. Image by Benjamin Jones/Project Seagrass.

Though West Indian manatees eat some other water plants, seagrass is their primary food source, particularly during cold months. Since 2009, seagrass in the lagoon has decreased by 58%, according to Charles Jacoby, supervising environmental scientist for the St. Johns River Water Management District.

Experts say the manatees’ deaths are a warning sign that the world needs to pay more attention to the health of these underappreciated habitats. 

Unnoticed importance

The plants included in the term “seagrass” aren’t taxonomic bedfellows, though all are submerged flowering plants. They include brackish-tolerant and marine species in many shapes and sizes. As such, seagrasses are an ecological grouping, lumped together for the lush canopy they create on the seafloor. Seagrass meadows grow in shallow coastal waters around the world, from cool temperate latitudes to the tropics — and in all of these places they are on the decline.

In 2009, the first global assessment of seagrass loss, published in the journal Proceedings of the National Academy of Sciences, estimated that at least 29% of known seagrass areas had disappeared in the last 200 years, and that this loss is accelerating. 

The decline has remained largely invisible to humans, at least until it manifests in something that catches our attention, such as the manatee deaths. Yet, seagrass meadows serve as vital habitat for thousands of marine species, from large animals that find food there — like sharks and turtles, as well as manatees — down to smaller fish, shellfish, and crustaceans. 

Seagrass meadows are also important nurseries for baby fish. By one estimate, one-quarter of the world’s 25 largest fisheries depend on seagrass meadows to nurture fish during their juvenile years. When these meadows disappear, it’s no surprise that fisheries can collapse and, as in Florida’s case, animals starve.

Unlivable conditions

From Australia to Southeast Asia to Europe and the Americas, experts say the reason for these declines is fairly universal: deteriorating water quality. Because seagrasses are plants just like those on land, they need sunlight to photosynthesize. When coastal development washes sediment into the water, or when nutrients from agricultural fertilizer and sewage cause algal blooms, seagrasses can’t get enough light. 

“That’s kind of the slow kill — it accumulates over time,” said Siti Maryam Yaakub, a Singapore-based marine ecologist with the environmental consulting group DHI, who works on ecological restoration and nature-based solutions. 

As seagrasses struggle to get by with less light, they become more susceptible to more intense stressors. After years of poor water quality, a storm or heat wave — made more intense globally by climate change — can suddenly wipe out an entire seagrass meadow.

In the Indian River Lagoon, declines were caused by nutrient runoff from local roads, septic tanks, and fertilizer use. Over the winter of 2020-2021, these inputs caused an intense algal bloom, which may have been the final blow to struggling seagrass. 

As nutrients run into the water, algae can begin to grow on and above seagrass, blocking sunlight and making it difficult for the grasses to grow. Image by Benjamin Jones/Project Seagrass.

Generally, the best way to prevent this is good catchment management, making sure that land-based pollution doesn’t run off into the sea. But Yaakub says there is often “a disconnect between what happens on land and on the coast.” 

“Anything marine falls under the ministry of fisheries, and then forestry takes care of plants on land,” she said. “And they just don’t meet in the middle.”

The solutions, however, are not as universal. 

“Part of the big problem with seagrass is that no one strategy works for everywhere, really,” said Jones, whose research has focused on seagrass meadows both in the U.K. and in the Indo-Pacific. “You need to be on the ground, looking at what the threats are, and asking local communities what they think are the biggest things.” 

Take, for example, restoration projects that aim to replant seagrass meadows. According to Yaakub, these efforts often fail if they fail to address the stressors that caused seagrass die-offs in the first place,  whether that might be fertilizer from farming further inland, changing water circulation in a bay due to a land reclamation project, or sediment washing into the water because people have cut down local forests.       

There are some bright spots in the world: in a research paper published in June, researchers found that seagrasses in 65% of Pacific Island countries and territories were either increasing or showing no signs of decline. The co-authors suggest this may be because these places, spanning Polynesia, Melanesia and Micronesia, exert fewer human impacts and have strong cultural connections to ocean ecosystems.

However, even these relatively resilient seagrass meadows face an uncertain future on a warming planet. 

An angler in the Indo-Pacific fishes on a seagrass bed. Healthy seagrasses are an important home for marine life, including an estimated one-quarter of the world’s 25 largest fisheries. Image by Benjamin Jones/Project Seagrass.

The climate conundrum

In Chesapeake Bay, the largest estuary in the U.S., seagrasses have mounted a shaky recovery thanks to the cooperation of seven jurisdictions — Delaware, Maryland, New York, Pennsylvania, Virginia, West Virginia, and the District of Columbia — to reduce nutrient runoff into the Bay’s watershed. Seagrass meadows in the Chesapeake rebounded from just 38,000 acres (15,400 hectares) in 1984 to nearly 109,000 acres (44,100 hectares) in 2018. (For comparison, prior to European colonization, scientists estimate that the Chesapeake’s seagrasses covered between 200,000 and 600,000 acres, or 81,000-243,000 hectares.) 

Yet over 2019 and 2020, intense storms and increased rainfall muddied water clarity, and inundated some salt-favoring seagrasses with too much freshwater. This caused the Chesapeake’s seagrass coverage to drop back down to just over 60,000 acres (24,300 hectares).

In this, the Chesapeake may be a good case study for seagrasses under climate change, as rising temperatures cause stronger storms and more frequent heat waves, in the water as well as on land. Seagrasses in the bay are at the very limit of their heat tolerance, a situation that many other temperate and tropical seagrasses may find themselves in before the end of the century. According to Jessie Jarvis, a professor at the University of North Carolina Wilmington who has worked on seagrass research and restoration in the Chesapeake and beyond, that leaves seagrasses even less prepared for our rapidly warming world. 

“When you add chronic stress, like the poor water quality in the Bay for decades, any time you have an acute event like these storms … because they’re already stressed they have less resilience,” Jarvis said. 

If there is good news, it is that seagrasses that grow in healthy temperature limits and good water quality can adapt well to climate change, using extra CO2 to boost photosynthesis. If they do, they can serve as a highly efficient tool to mitigate the planet’s warming: seagrasses store more than double the carbon of the average terrestrial forest. Though the total area of seagrass meadows covers less than 0.2% of the world’s oceans, these grasses currently store about 20 billion metric tons, or 10%, of its carbon. 

Seagrass meadows on the Susquehanna Flats, at the head of Chesapeake Bay. Image by Peter McGowan/USFWS (Public domain).

Rethinking protection

But to maintain seagrasses, experts say governments and the public alike need to start recognizing their importance. Jones says he was inspired to found Project Seagrass during his graduate work on U.K. seagrass meadows, when the sampling equipment he carried onto the train often led to conversations with curious strangers. Many of these people, he found, had no idea what seagrasses were. 

This ignorance can be compounded by a “surprisingly negative mindset about the grasses” when they are noticed, says Brooke Landry, chair of the Chesapeake Bay Program Submerged Aquatic Vegetation Workgroup. 

Though any Chesapeake-adjacent public school attendee gets a thorough education on the bay’s seagrasses, Landry told Mongabay that many transplants to the area don’t have the same knowledge. They don’t want seagrasses on their property, touching them while they swim, or interfering with boating. 

Yet Landry says the same people must recognize that if they enjoy eating fish and shellfish, or love charismatic creatures like turtles and, yes, manatees, then they also need to love seagrass.

A woman swims over dense seagrass meadows in Greece. Image by Benjamin Jones/Project Seagrass.

The mass die-off in Florida, therefore, could serve as a signal to rethink conservation as an effort that goes beyond just the health of creatures to which humans have ascribed value.

This is particularly the case when it comes to manatees and other long-lived mammals, for which conservation efforts have long focused primarily on preventing immediate death: things like vessel strikes or fishing net entanglement. However, “what we haven’t been thinking about is the carrying capacity of habitats,” Helene Marsh, professorial research fellow at James Cook University in Australia, told Mongabay. Marsh’s career has focused on the research and conservation of dugongs (Dugong dugon), a manatee relative that occurs in the Indian and Pacific oceans. 

Though preventing immediate death is still very important, “maybe what [the mass mortality event] is telling us is that we need to broaden our conservation outlook and to think more about habitats,” Marsh said. “We should be thinking more about the conservation of the plant communities on which these animals depend.” 

Citations:

McKenzie, L. J., Yoshida, R. L., Aini, J. W., Andréfouet, S., Colin, P. L., Cullen-Unsworth, L. C., … Unsworth, R. K. (2021). Seagrass ecosystems of the Pacific island countries and territories: A global bright spot. Marine Pollution Bulletin167, 112308. doi:10.1016/j.marpolbul.2021.112308

Unsworth, R. K., Nordlund, L. M., & Cullen-Unsworth, L. C. (2018). Seagrass meadows support global fisheries production. Conservation Letters12(1), e12566. doi:10.1111/conl.12566

Waycott, M., Duarte, C. M., Carruthers, T. J., Orth, R. J., Dennison, W. C., Olyarnik, S., … Williams, S. L. (2009). Accelerating loss of seagrasses across the globe threatens coastal ecosystems. Proceedings of the National Academy of Sciences106(30), 12377-12381. doi:10.1073/pnas.0905620106

Banner image: A Florida manatee in the Crystal River National Wildlife Refuge. Image by Keith Ramos/USFWS (Public domain).