Seagrass
There are seven species of seagrass in the Indian River Lagoon.
Indian River Lagoon Seagrass Species
Johnson's Seagrass
Johnson's Seagrass (Halophila johnsoni) is found only in southeast Florida, including the southern half of the lagoon. Johnson’s seagrass is short, one to two inches long, with paired leaves that have central veins originating from a single node on a rhizome. It can form dense patches, with patches often split between deep water and shallow shoals, possibly due to competition for light with larger species. Johnson’s seagrass was named in honor of J. Seward Johnson Sr., founder of Harbor Branch Oceanographic Institution in Fort Pierce. The National Marine Fisheries Service considers Johnson’s seagrass to be a threatened species under the Endangered Species Act, due to its limited ability to reproduce and its limited distribution.
Manatee Grass
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 found in mixed beds with other species. The blades of this seagrass are cylindrical, with two to four blades arising from each node in the rhizome. As denoted by its common name, manatees prefer to feed on Syringodium.
Paddle Grass
Paddle grass (Halophila decipiens) is the only species in the lagoon that is an annual, which means its blades are lost in cold weather and reemerge in warm weather. It generally is found in deeper water, and it is most abundant at the southern end of the lagoon, south of Sebastian Inlet. The blades, between one-half inch to an inch in length, resemble those of Johnson’s seagrass, but they have small notches on their margins, a broader paddle-shape and more rounded tips.
Shoal Grass
Shoal grass (Halodule wrightii) is the most common of the seagrasses, and it can be found throughout the lagoon. It is most abundant in shallow water (less than 6.5 feet), and it tolerates a range of salinities. Clusters of multiple leaves, with notches at their tips, originate from a single node along a creeping, branched rhizome (a stem that grows horizontally and gives rise to both leaves and roots) and can grow to lengths of four to 10 inches. Shoal grass is considered a pioneer species because it can grow and spread quickly to stabilize the sediment.
Star Grass
Star grass (Halophila engelmannii) generally is more common in the northern lagoon, but it can be found throughout the system. Star grass grows at a range of depths and sometimes with other species in mixed beds. Whorls of four to eight blades, one-half inch to an inch long, grow from a single stem forming a star-like pattern.
Turtle Grass
Turtle grass (Thalassia testudinum) can be found in the southern half of the lagoon (south of Sebastian Inlet) at mid-depths. Blades of turtle grass are flat and ribbon-like, growing to 14 inches long and a half inch wide. Blades can lengthen up to one inch per week under ideal conditions. Thalassia has the highest requirement for light of all the sea grasses in the lagoon, and it is an excellent indicator of healthy, stable water quality.
Widgeon Grass
Widgeon grass (Ruppia maritima) grows in patches throughout the lagoon in very shallow water (often less than 1 foot), and it can be found in areas with low salinity. The blades arise alternately from a sheath and they are wider at their bases and taper to long pointed tips. This grass closely resembles shoal grass, but its blades lack notches at their tips.
