Scientists use fossils to assess the health of Florida’s largest remaining seagrass bed. Surprisingly, it’s doing well!
Florida Museum of Natural History
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Seagrasses stabilize sediment, reducing erosion and enhancing the accumulation of nutrient-rich biomatter. They’re so good at doing this that even though seagrass meadows cover only 0.2% of the ocean floor, they’re responsible for 50% of marine carbon burial.
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The seagrass is greener along Florida’s Nature Coast … figuratively, that is. A new study published in the journal Marine Ecology Progress Series shows that seagrass ecosystems along the northern half of Florida’s Gulf Coast have remained relatively healthy and undisturbed for the last several thousand years.
This is not the case for most other seagrass ecosystems the world over, nearly 30% of which have disappeared since 1879. An estimated 7% of seagrass beds were lost each year between 1990 and 2009. Those that remain are generally not faring well, and the discovery of a healthy refugium is a rare event.
“Nothing is really pristine today, because humans have altered all of Earth’s environments, but this is about as good as it gets,” said Michal Kowalewski, senior author of the study and the Thompson chair of invertebrate paleontology at the Florida Museum of Natural History.
Figuring this out wasn’t easy, though. The extraordinary changes humans have made to the planet not only jeopardized the health of entire ecosystems, but they’ve also made it nearly impossible for us to know what a healthy ecosystem should look like in the first place.
“Most of the contemporary biological data we have postdates the Industrial Revolution,” Kowalewski said. “If you think about any type of real-time instrumentation that collects physical and chemical information about an environment, or if you think about the rigorous bio-inventory surveys, all of those things are from the last 50 to 100 years at most.”
In other words, humans have been altering their surroundings much longer than they’ve been systematically observing them. Fortunately, we aren’t the only thing that keeps a record of the past. The Earth does a pretty good job of it, too.
That’s the idea behind a relatively new branch of science called conservation paleobiology, which uses the most recent fossil record to reconstruct past ecosystems. For this method to work well, scientists need to analyze a large number of fossils, but there are only a few types of organisms that are preserved in sufficient quantities. Seagrasses, which are entirely composed of soft tissues that rapidly decompose after death, are not one of them.
This isn’t a hindrance to paleobiologists, though. Unlike modern grass lawns, which are ecologically barren and in which hardly anything lives but the grass itself, seagrass meadows are underwater oases for coastal marine organisms. This includes a variety of animals that produce hard shells, which are disproportionately represented in the fossil record. The shells of oysters, clams and other mollusks disintegrate so slowly that they stay around the ocean floor from hundreds to millions of years.
Kowalewski and his colleagues have previously conducted extensive research showing that the fossils of mollusks and other marine organisms with tough exteriors are so tightly connected and dependent on their environments that they can be used as a surrogate for species that don’t normally get preserved. If mollusks are doing well, it’s likely that everything else is too.
To find out if seagrass communities along Florida’s Nature Coast have recently degraded, the study authors sampled from 21 locations in six estuaries, from the mouth of the Steinhatchee River in the north to that of the Weeki Wache in the south. At each site, they used a long hose made from PVC pipe to suction up sections of the seafloor.
“We collect sediment samples while scuba diving, and then we sieve those samples and extract all that we find in it,” Kowalewski said. “The samples are dominated by dead material, because it has accumulated there over many centuries. Typically, for every live bivalve or snail, we find thousands of dead specimens.”
The odious task of counting and identifying the specimens took his team members several years to complete. Once they’d crunched the numbers, their results showed that mollusk diversity — and the health of seagrass meadows, by extension — hasn’t changed much over the last several millennia, including the most recent one in which humans have left their mark on even the most challenging and inhospitable environments.
“Only rarely do we find historical evidence that can make us optimistic about the current state of a local ecosystem,” Kowalewski said. “Most conservation paleobiology studies tell depressing stories about shrinking habitats, declining biodiversity and diminishing ecosystem services. For once, at least, this is not the case. What’s thrilling to me is we can show this system is still in very good condition, which makes it even more important to protect it.”
Establishing that the seagrass meadows found along the Nature Coast are relatively unchanged is also important because they can be now used with more confidence as a benchmark for assessing the state of heavily altered seagrass habitats and guiding their restoration.
Just 50 miles south of the study’s sampling area, seagrass communities haven’t been as lucky. Between 1950 and 1980, the city of Tampa’s population increased from about 125,000 people to 270,000. During that same period, 46% of seagrass meadows in Tampa Bay disappeared. Aggressive nutrient reduction efforts in the region led to water quality improvements and the recovery of seagrass in Tampa Bay between 1999 and 2018 — however, recent assessments have again shown significant reductions in seagrass followed by modest recoveries. On the opposite coast, a survey from 1999 indicated as much as 60% of seagrass coverage had been lost in a 56-mile stretch of the Indian River Lagoon.
These die-offs are primarily caused by nutrient pollution from inland farms and coastal cities. Plumes of single-celled microalgae and photosynthetic bacteria feast on excess nutrients and multiply in the water column, creating what are, in effect, marine clouds. This significantly reduces the amount of light that reaches the seafloor, which seagrasses don’t tolerate well.
The Nature Coast, which was designated an aquatic preserve in 2020, has largely avoided these challenges.
“There’s not a lot of development in those watersheds, particularly from the area around Weeki Wachee all the way up into the Panhandle, so the effects of excess nutrient delivery are not as pronounced as is in other places that have suffered as a consequence,” said study co-author Thomas Frazer, dean and professor of biological oceanography at the University of South Florida College of Marine Science.
The good news is, many of the meadows have since rebounded with the aid of pollution mitigation and habitat restoration efforts. But in many places across Florida, algal blooms are now an annual occurrence, and these will continue to have negative effects on seagrasses.
Climate change creates additional challenges, particularly for species along the Nature Coast. Though they’re doing well now, temperate and subtropical species are being pushed toward the planet’s poles by increasing global temperatures. But Florida species in Gulf waters can only migrate so far before they hit land.
“There’s nowhere for them to go,” Frazer said.
Backed up against the Panhandle, Florida’s Gulf Coast seagrasses will encounter marine climate refugees from further south. This has the potential to disrupt the tenuous balance within seagrass ecosystems. A single seagrass leaf can host a bazaar of tentacled hydroids, encrusting bryozoans, olive-shaped sea squirts, bacterial colonies and algal fuzz. In fact, as little as half of what you see when looking at seagrass is actual plant.
In a healthy seagrass meadow, these seagrass dwellers are kept in check by grazing fish and invertebrates, but as these ecosystems change along with the Earth’s climate, the continued diversity and existence of grazers is far from guaranteed.
“We’re already seeing range extensions of mobile fauna,” Frazer said. “A number of fishes, for example, are moving up north along the Gulf Coast, and they may either eat seagrass or consume grazers that help keep seagrasses clean of organisms that live on them.”
Seagrass ecosystems were around before the dinosaurs went extinct, and the consequences of losing something this old and diverse are not trivial.
In her 1955 book “The Edge of the Sea,” Rachel Carson wrote that “thrusting their roots into the sand and shifting coral debris, the seagrasses achieve a firmer attachment than the rootless algae do; where they grow thickly, they help to secure the offshore sands against the currents, as on the land the dune grasses hold the dry sands against the winds.”
Seagrasses stabilize sediment, reducing erosion and enhancing the accumulation of nutrient-rich biomatter. They’re so good at doing this that even though seagrass meadows cover only 0.2% of the ocean floor, they’re responsible for 50% of marine carbon burial.
All of these tightly packed resources attract animals. Sea turtles, manatees and fish eat seagrass, while countless other species make the meadows their home.
“They provide very important nursery habitat,” Frazer said. “In Florida alone, more than 80% of the fish caught by commercial fisherman and recreational anglers spend some part of their life history in those seagrass beds.”
Their deep roots and slender green tongues also protect coastal environments on land. Most seagrasses average only a foot or two in length, but a bunch of them together creates a substantial amount of drag. In near-shore environments, where the water is shallow, seagrasses can reduce wave energy by up to 40%.
“They’re a front line for storm protection,” Frazer said.
For now, that protection remains in place. Florida’s Nature Coast Aquatic Preserve has the largest seagrass bed in the Gulf. With good management, it may stay that way.
Louis Grimmelbein and Sahale Casebolt of the Florida Museum of Natural History and Savanna Barry, Katherine Cummings and Alexander Hyman of the University of Florida are also coauthors of the study.
Journal
Marine Ecology Progress Series
Article Title
Mollusk shell assemblages as a historical tool for identifying unaltered seagrass beds
Article Publication Date
15-May-2025
