This article was adapted from an announcement by the Florida Museum of Natural History.
Seagrass ecosystems along the northern half of Florida’s Gulf Coast have remained relatively healthy and undisturbed for the last several thousand years, according to a new study published in the journal Marine Ecology Progress Series and co-authored by researchers from the University of South Florida College of Marine Science.
Elsewhere, seagrasses have not fared as well. Nearly 30 percent of seagrass ecosystems have disappeared since 1879 and an estimated seven percent of seagrass beds were lost each year between 1990 and 2009. Those that remain are often struggling — the discovery of a healthy refugium is a rare event.
To assess the long-term health of seagrass communities along Florida’s Nature Coast, the study authors turned to fossils of mollusks, which can be used as a surrogate for species that aren’t normally preserved. If fossils indicate that mollusks have been doing well, it’s likely that other organisms in the ecosystem have been doing well, too.
The team sampled from 21 locations in six estuaries, from the mouth of the Steinhatchee River in the north to that of the Weeki Wachee River 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,” said Michal Kowalewski, senior author of the study and chair of invertebrate paleontology at the Florida Museum of Natural History. “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.”
IMAGE ABOVE: Efforts by the National Oceanic and Atmoshperic Administration to restore seagrass beds along Virginia's eastern shore ensure that bay scallops like these can continue to thrive. Credit Bob Orth / NOAA.
The odious task of counting and identifying the specimens took the team 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 environments across the globe.
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 percent 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 percent of seagrass coverage had been lost in a 56-mile stretch of the Indian River Lagoon.
Read more: Seagrass areal cover in Tampa Bay over the last 30 years (1990- 2021) observed by satellites
This loss of seagrass is 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.
IMAGE ABOVE: Thomas Frazer, professor and dean of the USF College of Marine Science, co-authored the recent study.
“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.
Many of the meadows have since rebounded with the aid of pollution mitigation and habitat restoration efforts. But algal blooms are now an annual occurrence in areas across Florida, 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.
Read more: Hungry herbivores threaten subtropical seagrass meadows
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. 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.
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 percent of the ocean floor, they’re responsible for 50 percent 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 percent of the fish caught by commercial fisherman and recreational anglers spend some part of their life history in those seagrass beds.”
The deep roots and slender green blades of seagrass 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 percent.
“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.
Funding for the study was provided in part by the University of Florida IFAS seed grant.