|Listed||September 14, 1998|
|Description||Seagrass with smooth, paired, foliageleaves, creeping rhizome stem, sessileflowers, longnecked fruits.|
|Habitat||Flood tidal of the inlets, the intertidal zone, areas with poor water quality, sandy shoals, turbid muddy basins, and near the mouths of canals with fluctuating salinity and water color.|
|Threats||Dredging, prop scoring, storm surge, altered water quality, siltation.|
Johnson's seagrass is one of 12 species of the genus Halophila. Halophila species are distinguished morphologically from other seagrasses in their possession of either a pair of stalked leaves without scales or a pseudo whorl of leaves. Identifying characteristics of Johnson's seagrass include smooth foliage leaves in pairs 0.4-0.8 in (1-2 cm) long, a creeping rhizome stem, sessile (attached to their bases) flowers, and longnecked fruits. Most Halophila species are reduced in size, more shallow rooted, and have two to three orders of magnitude less bio-mass per unit area compared to all other seagrasses.
The most outstanding difference between John-son's seagrass and other species is its distinct differences in sexual reproductive characteristics. While H. decipiens is monoecious (has both female and male flowers on the same plant) and successfully reproduces and propagates by seed, Johnson's seagrass is dioecious (has flowers of a single sex on the same plant). However, the male flower has never been described either in the field or in laboratory culture. The absence of male flowers supports the hypothesis that sexual reproduction is absent in this species, and propagation must be exclusively vegetative. However, the female flowers observed in Juniper Sound and Sebastian Inlet had three well-developed stigmas per flower. Flowers with unfertilized ovules were common, indicating a substantial allocation of reproductive efforts to the formation of female sex organs, leading to the conclusion that male flowers are probably present. Numerous efforts to find seeds in the sediments beneath patches of Johnson's seagrass have failed to locate any that germinated and developed into plants.
That male flowers have never been observed does not mean they do not exist. However, vegetative growth and rhisome branching may be the only means of propagation for Johnson's seagrass.
Its enormous potential for vegetative expansion, a perennial and intertidal growth habit, and a relatively high tolerance for fluctuating salinity and temperature enable Johnson's seagrass to colonize and thrive in environments where other seagrasses cannot survive. The range of these environments include the flood tidal of the inlets, the intertidal zone, and in areas with poor water quality. It has been observed on sandy shoals, turbid muddy basins, and near the mouths of canals with fluctuating salinity and water color. It also occurs on channel margins near inlets with very high current velocities, in areas of the lagoon with the poorest water transparency and in highly eutrophied conditions.
The maximum lower depth of Johnson's seagrass is deeper than other seagrasses. However, even though it grows deeper than larger bodied species and may photosynthesize more efficiently at lower irradiances, it is nonetheless sensitive to water transparency.
Johnson's seagrass is known to occur only between Sebastian Inlet and northern Biscayne Bay on the east coast of Florida. It has one of the most limited geographical distribution of any seagrass known.
Habitat within the limited range in which John-son's seagrass exists is at risk of destruction by a number of human and natural perturbations including (1) dredging; (2) propeller scarring; (3) storm surge; (4) altered water quality; and (5) siltation. Due to the fragile nature of H. johnsonii 's shallow root system, the plants are vulnerable to human-induced disturbances in addition to the major natural disturbances to the sediment, and their potential for recovery may be limited. Destruction of benthic communities due to boating activities (propeller scarring and anchor mooring) was observed at all Johnson's seagrass sites during the National Marine Fisheries Service (NMFS) study. Further, this condition is expected to worsen with the predicted increase in boating activity. This severely disrupts the benthic habitat by breaching root systems and severing rhizomes, and significantly reducing the viability of the community.
Turbidity is a critical factor in the distribution and survival of seagrasses, especially in deeper regions of the lagoon, where photosynthesis can be limited. Shallow regions are less affected by turbidity unless light is rapidly attenuated. In interior lagoonal areas where salinity is low, highly colored water typically is discharged via drainage systems. Stained waters attenuate shorter wavelengths rapidly and potentially stress plants due to the lower levels of salinity. This is a critical factor, especially in the vicinity of Sebastian, St. Lucie, Jupiter, and Ft. Pierce Inlets, and Lake Worth and North Biscayne Bay, where freshwater reaches the flood tide delta and nearby seagrass meadows via rivers and canal systems that discharge into the lagoon.
Trampling due to human disturbance and increased land-use induced siltation can threaten viability of the species. Degradation of water quality due to human impact is also a threat to the welfare of seagrass communities. Nutrient over-enrichment caused by inorganic and organic nitrogen and phosphorous loading ia urban and agricultural land runoff can stimulate increased algal growth that may smother the understory of Johnson's seagrass, shade rooted vegetation, and diminish the oxygen content of the water. Such low oxygen conditions have a demonstrated severe negative impact on seagrasses and associated communities. Continued and increased degradation of environmental quality also will have a detrimental effect upon Johnson's sea-grass communities.
There are two known herbivores that occur in the range of H. johnsonii —the green sea turtle (Chelonia mydas ), and the West Indian manatee (Trichechus manatus )—both of which feed upon the seagrass. Herbivorous fish also feed upon the seagrass community. Predation pressures alone are not likely to be a threat to the species' existence.
The existence of the species in a very limited range increases the potential for extinction from stochastic events. Natural disasters such as hurricanes could easily diminish entire populations and a significant percentage of the species. Seagrass beds that are in proximity to inlets are especially vulnerable to storm surge from hurricanes and severe storm events.
Conservation and Recovery
Despite existing Federal and Florida state laws to conserve and protect seagrass habitat, there is a continued and well-documented loss of seagrass habitat in the United States and elsewhere. For example, seagrasses have declined in many areas of the Indian River Lagoon. The Florida Department of Natural Resources and the Florida Department of Environmental Regulation have recently merged, greatly increasing the assignment of enforcement responsibilities without an associated increase in staff for the Marine Patrol. Although stormwater management systems are installed or being installed, the Florida Indian River Lagoon Act of 1990 does not cover other large inputs that will affect water quality, which in turn could affect seagrasses (e.g. industrial discharges, brine disposal, canals, processing plants).
Previous transplantation efforts to mitigate for the loss of seagrass beds have failed. Until recently, Halophila species have not been transplanted successfully in the field and studies underway are incomplete. Many seagrass ecosystems are known to recover very slowly even under the most natural, pristine conditions. Current efforts are insufficient to protect critical seagrasses. This was also the conclusion and recommendation of scientists attending the International Seagrass Workshop in Kominato, Japan in August 1993. A recovery plan for Johnson's Sea Grass was released by the NMFS in the summer of 2000.
U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
National Marine Fisheries Service
Protected Resources Division
9721 Executive Center Drive
St. Petersburg, Florida 33702-2432
National Marine Fisheries Service
Office of Protected Resources
1315 East-West Highway
Silver Spring, Maryland 20910
National Oceanic and Atmospheric Administration.14 September 1998. "Threatened Status for Johnson's Seagrass." Federal Register 63 (177): 49035-49041.