On a sunny day, the incoming tide at Sebastian Inlet is a tropical turquoise hue, easily distinguishable from the darker waters of the Indian River Lagoon. The daily slugs of fresh Atlantic water are part of the reason why the seagrasses in the inlet’s flood shoals have fared better than most lagoon seagrasses in recent years.

Although seagrass in the inlet’s flood shoals decreased by slightly more than six acres in 2022, scientists say year-to-year fluctuations are not uncommon. What’s more, all seven species of seagrass endemic to the Indian River Lagoon can still be found in the inlet’s shoals.

The Indian River Lagoon is one of the most biologically diverse estuaries in North America. An estuary is any water body where freshwater from the mainland mixes with saltwater (in this case, from the Atlantic Ocean that flows through Sebastian Inlet) creating a brackish water body. The seagrasses in the lagoon are critical to the health of the 156-mile waterway.

Seagrass beds are vital to the health of the estuary, providing  a nursery for juvenile fish, supporting invertebrates like worms, clams and snails, and serving as a food source for creatures that include manatees and sea turtles.

In 2010, a series of algal blooms triggered a seagrass die-off in the Indian River Lagoon, wiping out more than 47,000 acres of seagrass. In many areas of the lagoon, seagrasses have still not recovered, but the inlet’s flood shoals offer a ray of hope, said Stephen Trbovich, a marine biologist from Atkins North America, which has conducted annual monitoring for the Sebastian Inlet District since 2007. 

“Sebastian Inlet might be one of the best areas in the lagoon to see how long seagrass recovery takes,” Trbovich said. “Compared to parts of the Indian River Lagoon where additional die-offs occurred in 2016 and 2020, Sebastian Inlet hasn’t had repeated setbacks since that 2011 algal bloom. Since we’re right next to the inlet, there’s frequent tidal flushing. We may be at a point now where the focus is not so much on year-to-year acreage, but on species composition and density.”

Using aerial surveying, analysis, and field verification — also called ground truthing — marine biologists from Atkins North America have found that seagrass coverage shoal-wide shrank slightly from 123 acres in 2021 to approximately 117 acres in 2022. The previous year, seagrass beds has increased by eight acres.

“These may be short-term losses; we’re talking a variation of five percent of what was mapped in 2021,” “Prior to the large-scale seagrass losses in 2011-2012, we saw similar year-to-year fluctuations in some of these same areas from changes in sand accumulation.”

Considering the widespread seagrass losses throughout the Indian River Lagoon that have not been regained, Trbovich said it’s encouraging to see that the total acreage of seagrass in the District’s mitigation area exceeds the acreage in 2008 (115 acres), 2009 (110 acres), and 2010 (112 acres) – the three years prior to the region-wide seagrass loss in 2011 and 2012.

In 2007, following a realignment of the inlet channel, the District was required to monitor seagrass in the inlet flood shoals for five years. The Commission chose to continue biological monitoring beyond the permit requirement as part of its commitment to  protecting this sensitive habitat.

“Because we were monitoring the flood shoals prior to the 2011 algal blooms that impacted much of the Indian River Lagoon, we can say with certainty that the seagrass beds near the inlet have held their own rather well,” said District Commissioner Beth Mitchell. “We’re also ensuring that our data is compatible with data collected by other agencies, such as St. Johns River Water Management District. Data sharing can provide scientists with a more robust understanding  of seagrass trends throughout the estuary. "

 Seagrass beds may be sparser than they were prior to 2011, but they may fill in over time, Trbovich said: “After 2011 we lost much of the dense manatee grass beds that covered the shoals; we’ve replaced the acreage but most of that is sparser beds of shoal grass and Johnson’s seagrass. It’s almost like the aftermath of a wildfire – we’ve seen the smaller, faster-growing plant species settle in and expand; in time that should facilitate the settlement of taller, slower-growing plant species.” 

Fast Facts:

·  The District’s 145-acre study area is divided into six zones and contains all the seven different species of seagrasses found in the Indian River Lagoon

·  Seagrass growth within the inlet shoals has constantly increased since the lowest point in 2012 except for a period of leveling between 2014 and 2016, when algal blooms once again impacted seagrass growth.

·  According to the St. John’s River Water Management District, 2.5 acres of seagrasses can support up to 100,000 fish, up to 100 million invertebrates and $5,000-$10,000 in economic activity.

 About the Sebastian Inlet District

The Sebastian Inlet District was created in 1919 as an independent special taxing district by act of the Florida State Legislature and chartered to maintain the navigational channel between the Atlantic Ocean and the Indian River.  The Sebastian Inlet District’s responsibilities include state mandated sand bypassing, erosion control, environmental protection and public safety.  The Sebastian Inlet supports a rich and diverse ecological environment that is unparalleled in North America.  The Inlet is vital not only to the ecological health of the Indian River Lagoon, but it is also an important economic engine for local communities in the region.  Known as the premier surfing, fishing, boating and recreational area on the east coast of Florida, the inlet is one of only five navigable channels that connect the Indian River lagoon to the Atlantic Ocean.

 Posted on June 1, 2023