Seagrass meadows that were devastated by cold weather and severe algae blooms between 2010 and 2012 are making a slow recovery in the vicinity of Sebastian Inlet.  But propeller scars caused by careless boaters could compromise their progress.

That's the preliminary finding of recent aerial photography and a fins-in-the-water survey commissioned by the Sebastian Inlet District.

A comprehensive report will be released later this fall on the status of more than 100 acres of shallow lagoon bottom surveyed by marine ecologist Don Deis and his team from Atkins North America, an engineering and project management company.

Meanwhile, “we’re at about 75 percent recovery since the die-off of 2012,” said Marty Smithson, Sebastian Inlet District Administrator.  "It's gonna take a few more years to get back to where it was in 2009."

Seagrass recovery is important because the underwater meadows are the foundation of the lagoon’s ecosystem, serving as a nursery for juvenile fish, habitat for shrimp and other marine creatures, and a food staple for manatees.

Boaters could aid progress by using marked channels to navigate the inlet instead of cutting across sea grass shoals and by not attempting to power off the shallows at low tide.

"Prop dredging is picking up frequency," Smithson said.

Still, the grass cover surrounding the inlet is in better shape than seagrass in many areas of the lagoon, especially the brown algae-laden Banana River to the north and the blue-green algae-plagued Martin County area to the south, where seagrass has pretty much disappeared, according to Vero Beach marine scientist Grant Gilmore.

Gilmore and Smithson said Sebastian's regular tidal flushing has helped to dissipate the destructive algae.

Much of the Sebastian Inlet area's recovery is due to Johnson's seagrass, a threatened species found only in the Indian River Lagoon and parts of Miami-Dade County's Biscayne Bay.

“Johnson’s seagrass was first recognized as a separate species in 1980 . . . [and was] named in honor of J. Seward Johnson Sr., the founder of Harbor Branch Oceanographic Institution in Ft. Pierce, Florida,” according to the Florida Museum of Natural History.

Deis, who's been studying the lagoon bottom for years by walking and snorkeling, said that manatee grass and shoal grass were the predominant species in near the inlet in 2009, with Johnson's as a minor component. But in 2013, following the freeze and subsequent brown algae spread, Johnson's began to take over.

"It grows from season to season. It doesn't reproduce sexually – it breaks off [and roots]," Deis said. "It's been holding the shoals together."

Added Smithson:  "If it weren't for Johnson's, we would have an unstable mess out there."

Today, Deis said, the grass cover is a mixture of shoal grass and Johnson's. The surest sign of full recovery, according to Gilmore, would be a predominance of manatee grass.

Deis was heartened at the increased grass cover and appearance of manatees, juvenile green sea turtles, cow nose rays and bonnethead sharks he found in his recent survey. But he was dismayed to find new prop scars that weren't there last year at this time.             

"A pontoon boat plowed its way through the shoal [while I was out there],” Deis said.

Smithson said the frequency of prop scarring eased up dramatically after the District dredged a new navigation channel from the Intracoastal Waterway to the inlet in 2007. Signs were posted to direct boaters away from the shallows, and illustrated brochures were distributed to local marinas.

But lately, Smithson said, as many as 300 boats are showing up in and around the inlet on weekends, with many skippers unfamiliar with proper navigation. Some who anchor on the shoals while the tide is high get stuck when the tide falls and use their propellers to dig their way off instead of using a push pole or paddle or wading and walking the boat. Seagrasses can take many years to recover from such damage.

The District's seagrass report will be shared with the South Florida Water Management District and the National Estuary Program.