Seagrass in the Indian River Lagoon at Sebastian

Lagoon-wide, more than half the Indian River Lagoon's seagrass died after 2011, but the beds on the Sebastian Inlet flood shoals have grown back to about their pre-2011 acreage.

A peer-reviewed study by Morris and colleagues in Frontiers in Marine Science (published January 17, 2022) found that between 2011 and 2019 about 19,000 hectares, roughly 58 percent of the lagoon's seagrass, was lost, as the deep edges of beds moved shoreward and shallower. The same study reports a gain of about 7,000 hectares from 1994 to 2009 and an earlier loss of about 3,000 hectares between 1943 and 1994, so the 2011 collapse followed a period of gains in lagoon-wide maps that include the waters off Sebastian.

On the shoals just west of Sebastian Inlet the trend since 2011 has been recovery. The Sebastian Inlet District reports that seagrass on its six monitored shoals totaled 115.38 acres in 2024, against a pre-2011 range of approximately 110 to 115 acres. Seagrass growth on those shoals has increased steadily since its lowest point in 2012, except for a leveling between 2014 and 2016 when algal blooms again hit the lagoon.

The Sebastian Inlet District's most recent published count is 115.38 acres in 2024, a 1.64-acre drop from the 117.02 acres mapped in 2022.

The 2024 report found net increases in Zones A, B and D, from 0.06 acres in Zone B to 1.93 acres in Zone D, and net decreases in Zones C, E and F, from 0.53 acres in Zone F to 2.62 acres in Zone E (Sebastian Inlet District, 2024 report). The earlier record is uneven: the 2019 report counted 108.71 acres and a cumulative increase of 7.86 acres from 2018 to 2019, in stark contrast to the lagoon-wide trend (Sebastian Inlet District, 2019 report), and the 2023 report gave 123 acres for 2021 and 117 acres for 2022 (Sebastian Inlet District, 2023 report).

One gap matters for anyone tracking the numbers from Sebastian. Persistent poor water clarity and unsuitable weather prevented aerial images of the 145-acre study area in 2023, so no acreage exists for that year. As of October 14, 2026, the District's seagrass reports page still opens with the 2023 report, posted May 6, 2024, and the 2024 report is the newest acreage published; no 2025 figure had been posted.

Seagrass near Sebastian Inlet has fared better because the inlet brings Atlantic water into the lagoon, flushing it and improving the water clarity seagrass needs.

The District says the inlet's seagrasses have fared better than some other areas of the lagoon because Sebastian Inlet provides tidal flushing and improved water quality from the Atlantic Ocean (Sebastian Inlet District). Morris and colleagues explain the geography: south of the intermittent lock at Port Canaveral, the next connection to the ocean is 140 km to the south at Sebastian Inlet, and the lack of inlets in the northern lagoon leads to residence times that can exceed 1 year (Frontiers in Marine Science).

The inlet's effect predates the 2011 crash. The Smithsonian Marine Station Indian River Lagoon Species Inventory states that in the central lagoon, near Sebastian Inlet, seagrass coverage has increased markedly from historic levels, though much of this increase is due to the opening of the inlet at its present location. Closer to the mainland the picture is weaker: the City of Sebastian reports that grass beds immediately south of the mouth of the St. Sebastian River have consistently become more fragmented over time, and notes the suggestion that turbidity from non-point source pollution has greatly reduced the area suitable for seagrass throughout the lagoon.

All seven Indian River Lagoon seagrass species grow in the Sebastian Inlet study area, with shoal grass and Johnson's seagrass now dominant.

The 2024 report names shoal grass (Halodule wrightii), Johnson's seagrass (Halophila johnsonii), paddle grass (Halophila decipiens) and manatee grass (Syringodium filiforme) as the four most prolific species (Sebastian Inlet District). In 2019 the same shoal held six of the seven species, so the seventh has since been found there (2019 report).

The mix has shifted since 2011. From 2008 to 2011 the dominant species were manatee grass and shoal grass, with Johnson's seagrass identified in small amounts in two of the six zones; since 2012 Johnson's seagrass and shoal grass have dominated. Johnson's seagrass, a threatened species, was present at 65 percent of all sites that contained seagrasses in 2019, up from 43 percent in 2018 (Sebastian Inlet District). The 2023 report adds that while acreage has exceeded the pre-2011 extent, the beds that grew back are less dense and composed primarily of shoal grass and Johnson's seagrass (Sebastian Inlet District). Acreage alone overstates the recovery off Sebastian.

Near Sebastian Inlet, the Sebastian Inlet District urges boaters to stop their engines, tilt the motor and pole, drift or walk the boat to deeper water, and its navigation guide maps the marked channel and the seagrass caution areas.

In 2007, as part of the project extending the channel from the inlet to the Intracoastal Waterway, the Sebastian Inlet District installed signs reading Caution, Shallow Water, Seagrass Area over 145 acres of flood tidal shoal habitat, and monitoring showed a steady decline in prop scars in the following four years. The District also published a Sebastian Inlet Navigation Guide with a map of the seagrass caution areas, and it answers questions at (321) 724-5175. The 2024 report describes that 2007 work as a 3,100-foot marked channel linking the Intracoastal Waterway to Sebastian Inlet.

The shoals sit inside the state aquatic preserve off Sebastian, where FDEP states that seagrass is protected, asks boaters to avoid shallow water and raise the motor over seagrass, and warns that damage to seagrass can result in a fine up to $1,000 (FDEP).

Yes: in 2025 Indian River County and its partner Sea and Shoreline planted 32.8 acres of seagrass at Big Slough and Preacher's Hole in the county's stretch of the lagoon.

The Indian River County Lagoon Division reports completing 12.8 acres at Big Slough on June 13, 2025 (320 herbivory exclusion devices, 1,600 peat pots and 64,000 planting units) and 20 acres at Preacher's Hole on August 5, 2025 (250 exclusion devices, 1,250 peat pots and 50,000 planting units). The work is funded by a NOAA Transformational Habitat Grant won by the Indian River Lagoon National Estuary Program with Indian River County and 9 other partners. The county asks boaters to drive and anchor with care in aquatic preserves and restoration areas.

Each acre counts for the fishery off Sebastian. The District cites St. Johns River Water Management District figures that 2.5 acres of seagrass can support up to 100,000 fish, up to 100 million invertebrates and $5,000 to $10,000 in economic activity (Sebastian Inlet District). Morris and colleagues caution that even optimistic estimates put lagoon-wide recovery at 12 to 17 years, and that mitigating such a long-term loss may entail planting seagrass to accelerate recovery (Frontiers in Marine Science).