Skip to main content

2023 report: Seagrass holding steady

A person's open hand holding small plants/seaweed, with a water background and another person's legs partially visible.
Johnson's seagrass, (Halophila johnsonii), is prevalent in the flood shoals of Sebastian Inlet. 

2023 was a reminder that the weather has a say in any plans over, on, or in the water.

Since 2007, the Sebastian Inlet District has contracted aerial surveyors and marine biologists to monitor seagrass growth in the inlet’s flood shoals. In 2022, the scientists determined that seagrass covered 117 acres of the inlet’s flood shoals. Last year? Well, there’s no definitive answer, due to persistent poor water clarity and unsuitable weather conditions that prevented the experts at GPI Geospatial, Inc. and AtkinsRéalis from capturing aerial images of the 145-acre shoal study area in 2023.

However, data gathered at the ground level indicates that seagrass receded in parts of the District’s six monitoring zones but expanded in others, says Stephen Trbovich, an AtkinsRéalis marine biologist who leads the annual monitoring project.

“While it seems probable that there was some net acreage loss of seagrass between 2022 and 2023, overall change appears to have been limited,” Trbovich says. “Some of those changes may have been because after trying to capture usable aerial images the fieldwork was pushed into early October, which is when we expect to see seasonal losses in species like Johnson’s seagrass. We’ve seen annual fluctuations in seagrass acreage at the inlet’s shoals, but since the late 2011 algal bloom that devastated seagrasses in much of the central and northern Indian River Lagoon, Sebastian Inlet has maintained an upward trend.”

The shoals are in the Indian River Lagoon, 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. Seagrass beds are vital to the health of the 156-mile-long 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 2007, the District was required by permit to monitor seagrass in the inlet’s flood shoals for five years due to the realignment of the inlet’s navigation channel. The Commission chose to continue biological monitoring beyond the permit requirement as part of its commitment to protecting sensitive inlet habitats; not only does seagrass provide food and shelter for the Inlet’s marine life but it also reduces sand movement and erosion on the shoals.

“Our commissioners value scientific monitoring to improve our understanding of long-term trends in seagrass growth,” said District Commission Chair Jenny Lawton Seal. “We share our data with other institutions and agencies, such as St. Johns River Water Management District. We’re all on the same team in terms of managing our natural resources. "

The inlet’s seagrasses have fared better than in other areas of the lagoon because the Sebastian inlet provides tidal flushing — and improved water quality for seagrass growth — from the Atlantic Ocean. The four most prolific species of seagrasses in the District’s monitoring zones are shoal grass (Halodule wrightii), Johnson’s seagrass (Halophila johnsonii), paddle grass (Halophila decipiens) and manatee grass (Syringodium filiforme).

Since the inlet’s seagrass acreage appears to be holding steady, Trbovich says future monitoring will focus on species composition and density in addition to acreage gained or lost. While the acreage has exceeded the pre-2011 extent, the seagrasses that have grown back are less dense and composed primarily of shoal grass and Johnson’s seagrass rather than the mix of manatee grass and shoal grass that covered the shoals previously.

“At Sebastian Inlet we can see how long it might take other areas of the Indian River Lagoon to recover in the absence of setbacks,” Trbovich says.

AtkinsRéalis uses aerial surveying, analysis, and field verification — also called ground truthing — to monitor the inlet’s seagrass coverage. Although water and weather conditions hampered this year’s calculations, the 2022 seagrass coverage estimate was 117 acres, down slightly from 2021 when seagrass covered 123 acres of the inlet’s shoals. Considering that much of the Indian River Lagoon’s seagrasses have still not recovered from the 2011 algal bloom, 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.

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 May 6, 2024