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Sebastian Inlet sees eight-acre increase in seagrass shoal-wide in 2021

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Biologist Don Deis displays a few strands of Johnson's seagrass.

Seagrass in Sebastian Inlet’s flood shoals increased by slightly more than eight acres in 2021, continuing a steady recovery trend that offers a spark of hope for the Indian River Lagoon in the vicinity of the inlet, where diminished water quality and algal blooms wiped out most of the estuary’s seagrass beds in recent years.

Using aerial surveying, analysis, and field verification — also called groundtruthing — marine biologists from Atkins North America have found that seagrass coverage shoal-wide increased from 114.9 acres in 2020 to slightly more than 123 acres in 2021.

“There has been a steady increase in seagrass since 2018,” said Don Deis, marine biologist from Atkins North America and project lead for 15 years. “Of course, there is a limit to this expansion because the seagrass protection areas at the inlet shoals are limited in size.”

Atkins has been conducting the annual monitoring for the Sebastian Inlet District since 2007, when a District channel extension project connecting the Intracoastal Waterway (ICW) to Sebastian Inlet with a 3,120-foot marked channel for boaters required a comprehensive mitigation strategy to protect sensitive seagrass habitat. 

During a recent visit to the flood shoals, Deis waded through the shin-deep water to point out wisps of Johnson’s seagrass (Halophila johnsonii) and its cousin, shoal grass (Halodule wrightii), which more resembles land grass.

“Historically, the dominant species were manatee grass and shoal grass,” Deis says. “It was really, really hard to find Johnson’s seagrass because it isn’t a species that competes well with them. After the die-off (2011 superbloom in the Indian River Lagoon), what came back were the pioneering species: Johnson’s seagrass and shoal grass, with some manatee grass coming in on the periphery of the shoals.”

The steady increase in seagrass growth quantifies the inlet’s positive impact on water quality at the inlet flood shoals, especially when compared to other parts of the Indian River Lagoon where seagrasses have not returned, said James Gray, Sebastian Inlet District Executive Director.

“Our Commission chose to continue biological monitoring beyond the original permit requirement as part of its commitment to protecting this sensitive habitat,”  Gray said. “While our primary charge is to maintain the navigational channel between the Atlantic Ocean and the Indian River Lagoon, our Commissioners understand the value of documenting the environmental benefits of Sebastian Inlet’s tidal exchange.”

Six Zones Contain Seven Species

At first blush, an eight-acre increase in seagrass may not sound noteworthy, but it is one of few bright spots within the upper half of the 156-mile-long Indian River Lagoon, where major harmful algal blooms in 2011 and 2012 killed off 60 percent of the estuary’s seagrass.

The District’s 145-acre study area, divided into six zones representing the lobes of the flood tidal shoals and monitored annually, contains all the seven different species of seagrasses found in the Indian River Lagoon, including shoal grass, manatee grass (Syringodium filiforme), paddle grass (Halophila decipiens), Johnson’s seagrass, star grass (Halophila engelmannii), widgeon grass (Ruppia maritima) and turtle grass (Thalassia testudinum). The 123.5 acres of seagrass coverage represents 84.8 percent of the mitigation zone.

Deis said the 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. All zones exhibited net increases in seagrass coverage, ranging from 0.08 acres (Zone B) to 2.29 acres (Zone C) seagrass coverage in the study area in 2021.

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Aerial imagery of Sebastian Inlet flood tidal shoal (Florida). The image was taken on August 5, 2021 by GPI Geospatial, Inc. for the Sebastian Inlet District.

Considering the widespread seagrass losses throughout the Indian River Lagoon that have not been regained, what is most encouraging is 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.

However, while total acreage of seagrass is now above pre-loss coverage, the species composition and density has undergone a drastic shift, the report shows.

“Manatee grass and shoal grass and a mixture between the two used to dominate the protection areas before 2011,” Deis said. “Now, Johnson’s seagrass and shoal grass, along with mixtures of those two dominate those areas. We also find three other species in low presence mostly around the edges of the shoals (and protection areas) – paddle grass, star grass and turtle grass.”

 Read the 2021 Sebastian Inlet Annual Seagrass Monitoring Report here

2021 Seagrass Monitoring Report

 Download a copy of the Sebastian Inlet Navigation Guide:

Sebastian Inlet Navigation Guide

About Seagrasses

Seagrasses are flowering, saltwater plants that live in the shallow areas of an estuarine system. They not only act as a nursery and food source for juvenile fish, shellfish and manatees, but also improve water quality by trapping and removing sediment and nutrients suspended in the water column.  Seagrass roots and rhizomes stabilize sediment in the lagoon bed, and established beds help protect shorelines and limit erosion.  The drift algae collected within the seagrass is an important component in organism habitat and the collection of nutrients within the lagoon.

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.

Seven species of seagrasses are found in the Indian River Lagoon;

Shoal grass (Halodule wrightii) is the most common of seagrasses.  Most abundant in shallow water (less than 6.5 feet) and it tolerates a range of salinities.  It grows to between 4-10 inches.  Shoal grass is considered a pioneer species because it can grow and spread quickly to stabilize the sediment.

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Shoal grass

Widgeon grass (Ruppia maritimagrows in patches in very shallow water (less than 1 foot) and can be found in areas with low salinity.

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Widgeon grass

Johnson’s seagrass (Halophila johnsoniiis only found in the southern half of the lagoon.  Johnson’s seagrass is short, one to two inches long, with paired leaves originating from a single rhizome.  It can form dense patches and is found in both shallow and deeper water.

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Johnson's seagrass

Manatee grass (Syringodium filiforme) is found at mid-depths (5-feet) throughout the lagoon and can grow to lengths of 14 inches. It is rarely found in shallow water and is often in mixed beds with other species.

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Manatee grass

Paddle grass (Halophila decipiens) is the only species in the lagoon that is annual where blades are lost in cold weather and reemerge in warm weather.  It is generally found in deeper water and is most abundant at the southern end of the lagoon. 

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Paddle grass

Star grass (Halophila engelmannii) is generally more common in the northern lagoon and grows at a range of depths, sometimes in mixed beds with other species.

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Star grass

 Turtle grass (Thalassia testudinum) can be found in the southern half of the lagoon at mid-depths with flat, ribbon-like blades that grow to 14 inches long and ½ inch wide.  Thalassia has the highest requirement for light of all the seagrasses in the lagoon and is an excellent indicator of healthy, stable water quality.

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Turtle grass

 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.