Seagrass in the Lagoon at Sebastian — Sebastian, Florida

Seagrass near Sebastian has fared better than in most of the Indian River Lagoon, but it has not returned to what it was. Across the whole lagoon, a 2022 peer-reviewed study by Morris and colleagues in Frontiers in Marine Science found that between 2011 and 2019, about 19,000 hectares, or about 58 percent, of the lagoon's seagrass was lost, as phytoplankton blooms shaded the bottom and the deep edges of beds moved toward shore. That loss came after a period of growth: the same study reports an increase of about 7,000 hectares from 1994 to 2009, setting up the steep drop that followed the 2011 algal bloom felt from Sebastian to the northern lagoon.

On the flood shoals just west of Sebastian Inlet, the story since 2011 is recovery. The Sebastian Inlet District reported that seagrass on its six monitored shoals totaled 115.38 acres in 2024, close to the 110 to 115 acres measured before 2011. The District attributes the difference to tidal flushing: Sebastian Inlet brings Atlantic water into the lagoon, improving water quality for seagrass growth on the shoals near the inlet compared with areas farther from an ocean connection.

Geography explains much of the pattern. Morris and colleagues note that south of the intermittent lock at Port Canaveral, the lagoon's next connection to the ocean is 140 km away at Sebastian Inlet, and that the three inlets south of it increase oceanic exchange in the southern part of the system. The lack of inlets in the northern lagoon leads to long water residence times, which favors the blooms that cut light to seagrass.

The inlet's influence on seagrass is older than the 2011 event. The Smithsonian Marine Station Indian River Lagoon Species Inventory states that in the central Indian River Lagoon, near Sebastian Inlet, seagrass coverage increased markedly from historic levels, with much of that increase due to the opening of the inlet at its present location. Closer to the mainland, conditions are harder. The City of Sebastian reports that grass beds immediately south of the mouth of the St. Sebastian River have become more fragmented over time, and links lagoon-wide turbidity from non-point source pollution to a reduced area suitable for seagrass growth.

The Sebastian Inlet District has tracked the same patch of lagoon bottom since 2007. The District reports that its channel realignment that year required five years of seagrass monitoring on the inlet's flood shoals, and that its Commission chose to keep monitoring beyond the permit requirement. The 145-acre study area on the western flood tidal shoal is divided into six zones, mapped from aerial photographs and checked in the field by marine biologists, a process called ground truthing.

The resulting record is one of the most detailed for any part of the lagoon near Sebastian. In 2019, the District's technical report recorded 108.71 acres of seagrass and a cumulative increase of 7.86 acres from 2018 to 2019, in contrast to the lagoon-wide trend. Coverage reached 123 acres in 2021 and 117 acres in 2022. In 2023, persistent poor water clarity and unsuitable weather prevented aerial images suitable for acreage estimates, so the 2024 report compared 115.38 acres in 2024 with 117.02 acres in 2022, an approximate 1.4 percent decrease. The same report states that seagrass growth on the Sebastian Inlet shoals has increased steadily since its lowest point in 2012, except for a leveling between 2014 and 2016, when algal blooms again affected the lagoon. The District shares its data with the St. Johns River Water Management District, and researchers from Harbor Branch Oceanographic Institute and Brevard Zoo have used its maps to sample the Sebastian Inlet shoals.

Acreage is only part of the picture near Sebastian. The District's 2024 report states that the 145-acre study area contains all seven species of seagrass found in the Indian River Lagoon, and names shoal grass (Halodule wrightii), Johnson's seagrass (Halophila johnsonii), paddle grass (Halophila decipiens) and manatee grass (Syringodium filiforme) as the four most prolific, with shoal grass and Johnson's seagrass dominant.

That mix has changed since 2011. The 2019 report found that from 2008 to 2011 the dominant species on the Sebastian Inlet flood shoal were manatee grass and shoal grass, with Johnson's seagrass in small amounts in two of six zones, and that since 2012 Johnson's seagrass and shoal grass have dominated. Johnson's seagrass, a federally listed threatened species, was present at 65 percent of sites with seagrass in 2019. The District's 2023 report adds that although acreage has exceeded the pre-2011 extent, the regrown beds are less dense than the earlier manatee grass and shoal grass meadows.

Boat propellers are one threat that local action can reduce. The Sebastian Inlet District reports that in 2007, as part of the project extending the channel from Sebastian Inlet to the Intracoastal Waterway, it installed signs reading Caution, Shallow Water, Seagrass Area across 145 acres of flood shoal habitat, and that monitoring then showed a steady decline in prop scars over the following four years. The District also distributed a Sebastian Inlet Navigation Guide marking the seagrass caution areas.

Elsewhere in Indian River County, restoration has moved to planting. The Indian River County Lagoon Division reports that, under a NOAA Transformational Habitat Grant shared with the Indian River Lagoon National Estuary Program (IRLNEP) and other partners, the county and Sea and Shoreline completed planting 12.8 acres of seagrass at Big Slough on June 13, 2025 and 20 acres at Preacher's Hole on August 5, 2025, using herbivory exclusion devices to protect the plantings. The Sebastian Inlet District's 2024 report cites St. Johns River Water Management District figures that 2.5 acres of seagrass can support up to 100,000 fish and up to 100 million invertebrates, a measure of what each acre on the Sebastian Inlet shoals represents.