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Johnson's seagrass (Halophila johnsonii)

Status | Taxonomy | Species Description | Habitat | Distribution |
Population Trends | Threats | Conservation Efforts | Regulatory Overview |
Key Documents | More Info

 

underwater close up of Johnson's seagrass in the vicinity of Sebastian Inlet, FL
Johnson's seagrass
(Halophila johnsonii)
Photo: Lori Morris, St. Johns River Water Management District


 

 

Status
ESA Threatened - throughout its range

Taxonomy
Kingdom: Plantae
Class: Liliopsida
Order: Hydrocharitales
Family: Hydrocharitaceae
Genus: Halophila
Species: johnsonii

Species Description
Weight: n/a
Length: leaves are 0.2-1.0 inches (0.5-2.5 cm)
Appearance:  green with pairs of linearly shaped leaves
Lifespan: unknown
Diet: unknown
Behavior: reproduces only asexually, may be limited in distribution because of this characteristic

Johnson's seagrass can be identified by its smooth margins, spatulate leaves in pairs, a creeping rhizome (that is, a horizontal subterranean plant stem, like the runners on a strawberry plant) with petioles, sessile (that is, attached to their bases) female flowers, and long-necked fruits. The male flowers are unknown.

Outstanding differences between Johnson's seagrass and other similar species are its distinct asexual reproduction and leaf shape and form. The species is known to reproduce only asexually and may be limited in distribution because of this characteristic.

Johnson's seagrass plays a major role in the health of "benthic" resources as a shelter and nursery habitat. It has been documented as a food source for endangered West Indian manatees and threatened green sea turtles. We continue to conduct ecological research on Johnson's seagrass to better understand its life history and inform conservation decisions affecting the seagrass ecosystems.

 
johnson's seagrass critical habitat
Johnson's Seagrass Critical Habitat
(click for larger view PDF)

Johnson's seagrass range map
Johnson's Seagrass Range Map
(click for larger view PDF)


Habitat
Johnson's seagrass prefers to grow in coastal lagoons in the intertidal zone, or deeper than many other seagrasses. It does worse in the intermediate areas where other seagrasses thrive.

The species has been found in coarse sand and muddy substrates and in areas of turbid waters and high tidal currents. Johnson's seagrass is more tolerant of salinity, temperature, and desiccation variation than other seagrasses in the area.

Critical Habitat

NMFS designated critical habitat on April 5, 2000 in areas of Florida.

Distribution
Johnson's seagrass has a very limited distribution; it is the least abundant seagrass within its range. It has a disjunct and patchy distribution along the east coast of Florida from central Biscayne Bay to Sebastian Inlet. The largest patches have been documented inside Lake Worth Inlet. The southernmost distribution is reported to be in the vicinity of Virginia Key in Biscayne Bay, near Miami.

Population Trends
Johnson's seagrass is the rarest species of its genus. It has a limited distribution, limited ability to disperse and colonize habitats because of its asexual reproduction, and dependence on substrate stability.

Data on the species are rare, though one study found that abundance of all seagrass species is 16% less than in 1986 for the entire Indian River Lagoon complex (Ponce to Jupiter Inlet). Longer-term losses are thought to be approximately 50% for all seagrasses since the 1970s (Woodward-Clyde 1994).

 


underwater close up of Johnson's seagrass in the vicinity of Sebastian Inlet, FL
Johnson's seagrass
(Halophila johnsonii)
Photo: Lori Morris, St. Johns River Water Management District


Threats

  • Boating activities, such as
    • propeller scarring of the substrate
    • anchoring
    • mooring
  • Dredging
  • Storm action and sedimentation
  • Degraded water quality

Continued existence and recovery of Johnson's seagrass may be limited due to habitat alteration by a number of human and natural causes.

Alteration and subsequent destruction of the benthic community from boating activities, propeller scarring of the substrate, anchoring, and mooring has been observed in Johnson's seagrass sites. Such activities result in broken root systems, severed rhizomes, and significant reductions of the physical stability of the substrate.

Dredging waterways for boat access redistributes sediments, buries plants, and destroys bottom topography.

Erosional forces and sedimentation associated with severe storms are likely to affect some abundant populations located near inlets. During hurricanes, storm surge may scour and redistribute sediments, thereby eroding or burying existing populations.

Siltation due to human disturbance and land-use practices can also threaten viability of the species.

Nutrient enrichment, caused by inorganic and organic nitrogen and phosphorus loading from urban and agricultural land run-off, can degrade water quality by stimulating increased algal growth that smothers Johnson's seagrass by shading rooted vegetation and diminishing the oxygen content of the water.

 


Johnson's seagrass patch in the vicinity of Jupiter Inlet, FL
Johnson's Seagrass Bed
(Halophila johnsonii)
Photo: Lynn Lefevbre, USGS


underwater close up of Johnson's seagrass in the vicinity of Sebastian Inlet, FL
Johnson's seagrass
(Halophila johnsonii)
Photo: Lori Morris, St. Johns River Water Management District

Conservation Efforts
Research is ongoing on ways to successfully grow the species in captivity and transplant it to suitable locations. The Recovery Plan [pdf] calls for research on basic reproductive biology and life history of the species as well as general management and coordination among responsible local, state, and Federal agencies.

Regulatory Overview
On September 15, 1993, we published a proposed rule to list Johnson's seagrass as a Threatened species under the Endangered Species Act (ESA). Designation of critical habitat was subsequently proposed on August 4, 1994. A public hearing on both the proposed listing and critical habitat designation was held in Vero Beach, FL, on September 20, 1994. The public comment period was reopened until October 13, 1994, to include comments on both of the proposed actions.

Johnson's seagrass was listed as threatened [pdf] on September 14, 1998, and we designated critical habitat on April 5, 2000.

NMFS published a recovery plan [pdf] on October 4, 2002.

Key Documents
(All documents are in PDF format.)

Title Federal Register Date
5-Year Review 72 FR 68129 12/04/2007
Recovery Plan 67 FR 62230 10/04/2002
Critical Habitat Designation 65 FR 17786 04/05/2000
ESA Listing Rule 63 FR 49035 09/14/1998
Status Review n/a 10/15/1997

More Information

References

Updated: March 1, 2013

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