AN ENDANGERED SPECIES ACT ISSUE FOR SOUTHEASTERN FLORIDA: JOHNSON'S SEAGRASS
CRS Report for Congress
An Endangered Species Act Issue for Southeastern
Florida: Johnson’s Seagrass
Eugene H. Buck
Senior Analyst in Natural Resources Policy
Environment and Natural Resources Policy Division
Summary
Johnson’s seagrass, a small marine plant growing in shallow estuaries and coastal
lagoons only along the southeastern Florida coast, has been listed by the National Marine
Fisheries Service (NMFS) as a threatened species under the Endangered Species Act
(ESA), the first marine plant so listed. The continued existence of this species appears
to be most affected by water clarity and sediment disturbance (e.g., stormwater runoff,
boating and personal watercraft activities, and siltation) as well as by hurricanes and
storm surges. In addition, limited seed production by this species makes its recovery
uncertain. State and local government officials, marine industries, and the U.S. Army
Corps of Engineers as well as private citizens are concerned that protection under the
ESA could preclude, or increase expenses for, routine dredging projects and the
expansion of three Florida ports. NMFS will seek to protect this species through ESA
measures and address local concerns that species protection may interfere with
maintenance dredging projects. This report will be updated as this issue evolves.
Background
Johnson’s seagrass, Halophila johnsonii, is a very small (two-inch high) flowering
marine plant with the most limited geographic distribution of about 60 species of
seagrasses worldwide. It grows on a variety of sediment types ranging from mud to
coarse sand discontinuously and patchily in estuaries and coastal lagoons along about 1501
miles of the southeastern Florida coast from Sebastian Inlet to Virginia Key in north
Biscayne Bay. Although it most frequently grows from the intertidal zone to about six feet
below mean tidal height, Johnson’s seagrass has been reported growing at depths greater
than other seagrass species at 12 feet or deeper in very clear water on tidal deltas adjacent
to inlets and in Hobe Sound. As an opportunistic species, Johnson’s seagrass expands the
total area covered by seagrasses through its ability to survive in environments where other
Most patches are less than one square meter in size.1
Congressional Research Service ˜ The Library of Congress
seagrass species cannot — both in the intertidal area above and the subtidal area below
other seagrass species. Large patches of this species are reported to occur inside Lake
Worth Lagoon, south of West Palm Beach; in Indian River Lagoon, the largest patches are
usually on tidal deltas adjacent to inlets.
Johnson’s seagrass appears to have a relatively high tolerance for low light levels and
fluctuating salinity and temperature — features that may allow this species to colonize
environments where other species of seagrass cannot survive. However, severe storms can
damage seagrasses though erosion and siltation. During hurricanes, sediments moved by
storm surges and other turbulent flow (e.g., waves and currents) can easily dislodge,
erode, or bury existing seagrass communities, particularly shallow-rooted species such as
Johnson’s seagrass. Milder storms can resuspend sediments and limit light penetration.
In general, seagrasses stabilize sediments as well as provide shelter (habitat) and
dissolved oxygen for many marine animals. Seagrasses provide micro-habitats for small
marine animals and are a source of easily decomposed and digested organic matter (food)
for various invertebrates, fish, and larger animals, including endangered West Indian
manatees and threatened green sea turtles. Various species of commercially and
recreationally valuable fish and shellfish live within seagrass beds. Juvenile fish of various
commercially and recreationally important species hide in seagrass beds to escape
predators. It is reasonable to infer that a similar ecological role can be attributed to
Johnson’s seagrass, although study of this species has been limited. In addition, marine
biodiversity is greater where Johnson’s seagrass occurs.
Only female flowers have been observed for Johnson’s seagrass, and this species’
mechanisms of seed dispersal and establishment of new beds have not been studied.
Although some suggest that Johnson’s seagrass only reproduces asexually, fruit has been
described by several investigators. In addition, this species has a small, shallow rhizome2
(i.e., root) structure relative to total plant biomass, a high biomass turnover rate, and a
rapid lateral spreading rate of as much as 1.25 centimeters per day. The small rhizome
structure suggests that Johnson’s seagrass is unable to store large energy reserves. While
some suggest that these two characteristics — uncertain reproductive capacity and small
energy reserves — make Johnson’s seagrass vulnerable to stress/disturbance and less likely
to repopulate any area from which it disappears, others assert that species with higher
biomass turnover and lateral spreading rates are, in general, better at colonizing available
habitat. Johnson’s seagrass has been reported in some human-influenced areas of the
Intracoastal Waterway, suggesting that this species' ability to repopulate is significant.
However, regrowth and reestablishment of surviving populations of Johnson’s seagrass
could be more difficult than for species with a life history that features a wide dispersal of
seeds. The ecology of Johnson’s seagrass has not been extensively studied, making
generalizations from other species difficult and suspect.
N.J. Eiseman and C. McMillan. “A New Species of Seagrass, Halophila johnsonii, from the2
Atlantic Coast of Florida.” Aquatic Botany, v. 9 (1980): 15-19; C.J. Dawes, et al. “A
Comparison of the Physiological Ecology of the Seagrasses Halophila decipiens Ostenfeld and
H. johnsonii Eiseman from Florida.” Aquatic Botany, v. 33 (1989):149-154.
The National Marine Fisheries Service (NMFS) proposed to list Johnson’s seagrass
as a threatened species on September 15, 1993, and a definition of the critical habitat for3
this species was proposed on August 4, 1994. On April 20, 1998, NMFS reopened the4
proposed listing of Johnson’s seagrass as a threatened species for additional public5
comment. On September 14, 1998, NMFS published the final rule listing Johnson’s
seagrass as a threatened species, effective October 14, 1998. This is the first marine plant6
listed under the Endangered Species Act (ESA) as a threatened or endangered species.
Under the ESA, NMFS is required to consider additional protective regulations for this
species.
Human Activities Stressing This Species
Many possible risks for this species are inferred from environmental risks to other
species of seagrass. Based on data from other seagrass species and from observations of
Johnson’s seagrass, continued existence of this species appear to be most affected by7
reduction of water clarity (e.g., water quality and sediment disturbance, especially from
poor stormwater control practices), with additional concerns for boating and personal
watercraft activities (propeller scars, anchor mooring, and out-current jet blowouts),8
dredging of navigation channels, and shading from over-water construction (e.g., docks).
The time required for recovery of damaged seagrass ecosystems is affected by a variety
of factors, such as the cause and extent of damage, the seagrass species affected, and
physical characteristics of the damage site.
Reduction of water clarity caused by human activity and increased coastal land use
can threaten seagrass by reducing photosynthesis. Erosion caused by boat wakes may also
increase turbidity and siltation. Siltation not only reduces incident light, but can also create
anoxic conditions in the sediment by limiting oxygen diffusion. Such anoxic conditions are
problematic to seagrasses as they may cause problems with root metabolism or result in
the production of compounds, such as sulfides, that can be toxic to the seagrass. With as
much as 70% of runoff pollution contributed by non-point sources, water quality
problems, especially poor stormwater control practices, can increase water turbidity as
well as nutrient loading. Being a deeper water species, Johnson’s seagrass is more
susceptible to subtle changes in water quality. Excessive nutrients (e.g., nitrogen and
phosphorus) from urban and agricultural land runoff can stimulate increased algal growth
W.J. Kenworthy. The Distribution, Abundance, and Ecology of Halophila johnsonii Eiseman7
in the Lower Indian River, Florida. Final Report to the Office of Protected Resources, NMFS.
Silver Spring, MD, 1993.
Propeller scars are damage caused to vegetation and bottom habitat by propellers on boat motors8
operated in shallow water. Anchor mooring refers to damage caused by anchors from boats which
can disturb and damage vegetation and bottom habitat, especially if these anchors are dragged
along the bottom as boats are pulled by currents. Out-current jets on personal watercraft direct
water straight down instead of outward, increasing the potential for sediment “blow-outs.”
that severely reduces water clarity and increases epiphyte loading, shading seagrasses. In
addition, seagrass habitat may be affected by the timing and duration of stormwater runoff.
Although Johnson’s seagrass is reported to tolerate a wide range of salinities, additional
research is required to better understand this species’ ecological response to freshwater
inflows.
Propeller scars and anchor mooring can break and excavate seagrass root systems,
severing rhizomes, completely removing them from the sediment, and reducing the
physical stability of the community. Severing seagrass rhizomes removes what little stored
materials and energy these plant accumulate, making them highly susceptible to any stress,
e.g., light limitation due to turbidity. In addition, propellers and anchors often alter bottom
topography (e.g., propellers make trenches in the sediment), which influences seagrass
recovery time. For example, boats are reported to have damaged or destroyed about
(e.g., jet skis) are also recognized as a potential hazard to seagrass beds in very shallow
water. Some seagrass beds can require as long as 5 years to recover from minor damage
and a decade or more to recover where the wash from a boat motor or personal watercraft
operated in shallow water digs a crater in a seagrass bed. However, since seagrass species
vary in their ability to recover from scarring, one must be cautious in inferring that such
characteristics are also relevant to Johnson’s seagrass, which is capable of spreading10
quickly.
Dredging of navigation channels can resuspend and redistribute sediments, severely
decreasing available light, burying plants, and altering bottom topography. However,
dredging in Florida is highly regulated to prevent such damage, and Johnson’s seagrass
continues to occur on tidal deltas adjacent to several inlets that have been regularly
dredged for many decades. Major ports, harbors, and navigation channels are maintained
at depths greater than 6 feet and Johnson’s seagrass has not been reported within such
dredged basins or navigation channels — only in adjacent shallow areas. Thus, it would
be new dredging in shallow areas, rather than maintenance dredging of previously dredged
basins and navigation channels, that would be more likely to affect Johnson’s seagrass
habitat. Although some have suggested that dredging may have enhanced this species,
scientific evidence does not support this argument.
Protection Efforts
The state of Florida has taken steps to protect seagrass habitat. New and
redeveloped sources of stormwater discharge are required to meet state water
management district regulations. Several counties are reportedly installing point-source
stormwater management systems that may improve water quality from point sources. In
addition, improvements in wastewater treatment within the past two decades are reported
to have increased water clarity in Lake Worth and other lagoons, allowing seagrass cover
to expand. Although limited by federal navigation rules promulgated by the U.S. Coast
Guard, state of Florida efforts to mark navigation channels and establish speed zones to
protect seagrasses that are adjacent to Florida state parks and are important manatee
Associated Press. “Biscayne National Park Scarred by Boat Groundings.” August 10, 1998.9
If Johnson’s seagrass is a weedy species, it might recover more rapidly compared to turtle grass,10
which has been the subject of most studies on the effects of propeller scarring.
habitat may promote multiple public benefits. Non-motor and controlled access areas11
have resulted in improved seagrass habitat.
Since Johnson’s seagrass grows on its submerged lands, the state of Florida holds that
NMFS measures to protect this species must be consistent with state of Florida policies
and involve Florida as a necessary partner. NMFS has an active seagrass program and12
anticipates proposing separate regulations outlining specific protective measures for this
seagrass. Federal activities that may affect Johnson’s seagrass and require ESA §7
consultation are identified by NMFS as including Corps of Engineers project
authorizations, Environmental Protection Agency discharge permits, Coast Guard vessel
traffic regulation, management of refuges and species by the Fish and Wildlife Service,
authorization of state coastal zone management plans by the National Ocean Service, and
commercial fishery management by NMFS. NMFS anticipates reopening the public
comment period on the proposed designation of critical habitat before making a final
decision on whether and, if so, where to designate such critical habitat. The state of
Florida questions whether the best available scientific data as well as socio-economic13
evaluations support the proposed critical habitat. Protecting Johnson’s seagrass habitat
would also afford additional protection for the six other seagrass species found in Florida
waters as well as to the animal communities that depend on these plants.
Controversy
Some characterize the concern over Johnson’s seagrass as being a traditional conflict
between environmental preservation and economic development. Although NMFS
anticipates maintenance dredging can be authorized through ESA §7 consultations for
areas where Johnson’s seagrass occurs, some individuals are concerned that protection
under the ESA could preclude, or increase the expenses for, routine dredging projects and
the expansion of three southeast Florida ports. More specifically, Martin County public
services executives are concerned that listing of Johnson’s seagrass could jeopardize future
plans to reconfigure St. Lucie Inlet to retard beach erosion. Additional measures14
required to protect Johnson’s seagrass at dredging projects may include more detailed site
surveys and reduced mixing zones for dredge-generated turbidity — measures unlikely to
significantly increase overall project costs. In addition, NMFS may issue an ESA permit15
for “taking” or displacing one or several Johnson’s seagrass populations in St. Lucie Inlet
Frank J. Sargent, et al. Scarring of Florida’s Seagrasses: Assessment and Management11
Options. FMRI Technical Report TR-1. St. Petersburg, FL: Florida Department of
Environmental Protection, Florida Marine Research Institute, 1995. 43 p.
Mark S. Fonseca, et al. Guidelines for the Conservation and Restoration of Seagrasses in the12
United States and Adjacent Waters. NOAA Coastal Ocean Program Decision Analysis Series No.
For example, the state claims that most Johnson’s seagrass meadows have been found within13
Lake Worth Lagoon, rather than within the inlet areas previously proposed for designation.
Edward Filo. “Protected Sea Grass Threatens Inlet Plans.” Vero Beach Press-Journal, October14
Personal communication with Keith Mille, Environmental Specialist, Florida Department of15
Environmental Protection, Tallahassee, on October 30, 1998.
if this limited dredging project is determined unlikely to jeopardize the continued existence
of the species.
Although Johnson’s seagrass is uncommon in the Intracoastal Waterway, some are
concerned that its presence could impede routine maintenance dredging in the limited
segments of this transportation corridor where it does occur. However, NMFS has
concluded an ESA §7 consultation with the Corps of Engineers on maintenance dredging
of existing, authorized federal navigation projects in this area, and determined that
maintenance dredging of those channels constructed within the last 10 years is unlikely to
jeopardize the continued existence of Johnson’s seagrass.
The Florida Department of Environmental Protection has expressed concern to
NMFS that information on this species is inadequate for deciding on appropriate
protection and recovery efforts, and has suggested that NMFS consider a focused,
protection-oriented research effort on Johnson’s seagrass to assure that the most
appropriate management strategies for the species can be developed to reconcile
development with species protection. Until Johnson’s seagrass and its ecology are better
understood, scientists suggest protecting this species by taking a cautious and conservative
approach to dealing with problems affecting it and its habitat.16
Personal communication with Dr. Richard L. Turner, Associate Professor, Florida Institute of16
Technology, Melbourne, on October 23, 1998.