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II. HABITAT AND
DISTRIBUTION
Regional Occurrence:
Halophila johnsonii occurs only in the coastal lagoons of east Florida,
from Sebastian Inlet to Biscayne Bay (Eiseman 1980).
IRL Distribution:
Halophila johnsonii occurs in
the southern half of the Indian River Lagoon, often in dense patches, in deep
water and on shoals (Virnstein 1995). Eiseman reports that H. johnsonii
occurs primarily in the intertidal
zone, between stands of Halodule beaudettei and
the high tide line, but may also occur in mixed patches with H. wrightii.
When H. wrightii is not present, H. johnsonii can occur below
the low tide level.
Halophila johnsonii is probably
restricted to coastal lagoons in eastern Florida from Sebastian Inlet south to
Virginia Key, Biscayne Bay. H. johnsonii was also documented in Sebastian
Inlet, Pepper Park (north of Fort Pierce Inlet) and Big Mud Creek (Eiseman &
McMillan 1980).
Distribution, biodiversity, productivity and
ecological significance of seagrasses in the Indian River Lagoon (IRL), FL, are
summarized by Dawes et al (1995). Seven species of seagrass, including all 6
species occurring throughout the tropical western hemisphere, as well as Halophila
johnsonii, occur in the IRL. Halodule beaudettei is the most common. Ruppia
maritima is the least common and is found in the most shallow areas of the
lagoon. Syringodium filiforme can be locally more abundant than H.
wrightii. Thalassia testudinum occurs in the southern portion of the
IRL (Sebastian Inlet and south). Halophila decipiens, Halophila engelmannii and
Halophila johnsonii can form mixed or monotypic beds with other species.
Because of their abundance in deeper water and high productivity, the
distribution and ecological significance of the 3 Halophila species may
have previously been underestimated
The northern area of the Indian River Lagoon
supports the most developed seagrass beds, presumably because of relatively low
levels of urbanization and fresh water inputs. Four species of seagrass -
Halodule beaudettei, Syringodium filiforme, Halophila engelmannii and Ruppia
maritima - can be found north of Sebastian Inlet, while all 7 species occur
to the south (Dawes et al 1995). Seagrasses were ranked in order of decreasing
percent cover by Virnstein and Cairns (1986) as follows: Syringodium
filiforme, Halodule beaudettei, Halophila johnsonii, Thalassia testudinum,
Halophila decipiens, Halophila engelmannii and Ruppia maritima.
Depth:
In the Indian River Lagoon, Phillips (1970) noted water depth for Halophila
ranged from 6 inches (slack low tide) to 6 feet (at flood tide).
Distributional Changes:
Changes in seagrass distribution and diversity pattern in the Indian River
Lagoon (1940 - 1992) are discussed by Fletcher and Fletcher (1995). It was
estimated that seagrass abundance is 11 % less in 1992 than in the 1970's and 16
% less than in 1986 for the entire Indian River Lagoon complex (Ponce to Jupiter
Inlet). Decreases in abundance occurred particularly north of Vero Beach. In
this area of the lagoon, it was also estimated that maximum depth of seagrass
distribution has decreased by as much as 50 % from 1943 to 1992. Alteration of
such factors as water clarity, salinity and temperature could affect the
diversity and balance of seagrasses in the Indian River Lagoon system and should
be considered when developing management strategies for this resource (Fletcher
& Fletcher 1995).
Mapping:
Sources of mapped distributions of Indian River Lagoon seagrasses include the
following: 1) Seagrass maps of the Indian & Banana Rivers (White 1986); 2)
Seagrass maps of the Indian River Lagoon (Virnstein and Cairns 1986); 3) Use of
aerial imagery in determining submerged features in three east-coast Florida
lagoons (Down 1983); and 4) Photomapping and species composition of the seagrass
beds in Florida's Indian River estuary (Thompson 1976). Data from the first two
sources (White 1986; Virnstein & Cairns 1986) is now available in GIS format
(ARCINFO) (see Fletcher & Fletcher 1995).
III. LIFE HISTORY AND POPULATION BIOLOGY
Abundance:
H. johnsonii can be locally abundant. It occurs in dense
monospecific stands, or in mixed patches only within the confines of the Indian
River Lagoon system.
Locomotion:
Sessile
Reproduction:
Pistillate flowers were observed in Halophila
johnsonii from April to July (Eiseman 1980). Staminate flowers were not
observed. This could imply that H. johnsonii is apomictic i.e.,
reproduces without fertilization. However, the possibility that male flowers in H.
johnsonii could be rare, was mentioned by Eiseman and McMillan (1980). An
Indian River Lagoon population of H. johnsonii near Jim Island in Fort
Pierce, FL was observed flowering in May and June (Dawes et al 1989). Moffler
and Durako (1987) report that H. johnsonii flowers April through July and
fruits in August.
Seasonality of both growth and biomass is
exhibited by all species of seagrass in the IRL, being maximum during April - May
and June - July respectively. The significance of seagrass beds as habitat,
nursery and food source for ecologically and economically important fauna and
flora, as well as various management strategies for seagrass beds of the IRL are
discussed in Dawes et al. ( 1995).
Water temperature, moreso than photoperiod,
appears to be more influential in controlling floral development as well as
subsequent flower density and seed production in seagrasses. Laboratory
experiments showing flowering induction under continuous light suggests that photoperiod
probably plays a limited role in sexual reproduction (Moffler & Durako 1987).
IV. PHYSICAL TOLERANCES
Temperature:
Water temperatures in the IRL ranged
from a low of 21.0 to a high of 36.0 °C, and cover the period from March
through September (Phillips 1970). When Halophila johnsonii, an
intertidal to shallow subtidal species, was compared with deeper water
populations of H. decipiens , H. johnsonii showed greater tolerance to
variations in salinity and temperature as well as higher irradiances (Dawes et
al 1989).
Salinity:
Salinity for Halophila johnsonii
occurring in the IRL ranges from 24.3 to 38.0 ppt. The occurrence of
Halophila johnsonii throughout the year at salinities as high as 43 ppt
was documented (Eiseman & McMillan 1970, Eiseman 1980). H.
johnsonii showed greater tolerance to variation in salinity than did a
related species, H. decipiens.
V. COMMUNITY ECOLOGY
Trophic Mode:
Autotrophic
Habitat:
In the Indian River Lagoon, FL, Halophila
johnsonii occurs both in firm substrata composed of fine sediment (Eiseman
& McMillan 1980), as well as in sandy, mud substratum (salinity - 38 ppt,
temperature - 33 - 36 °C) (Dawes et al 1989). Eiseman (1980) reported that H.
johnsonii prefers firm substratum.
Virnstein (1995) suggested the "overlap
vs. gap hypothesis" to explain the unexpectedly high (e.g., fish) or low
(e.g., amphipods) diversity of certain taxa associated with seagrass beds. In a
highly variable environment such as the Indian River Lagoon, diversity of a
particular taxa is related to its dispersal capabilities. For example,
amphipods, lacking a planktonic phase, have limited recruitment and dispersal
capabilities, whereas highly mobile taxa such as fish (which also have a
planktonic phase) would tend to have overlapping species ranges and hence higher
diversity (Virnstein 1995). For an extensive treatment of seagrass community
components and structure including associated flora and fauna, see Zieman
(1982).
Associated Species:
When not occurring in monospecific beds, Halophila johnsonii co-occurs with
Halodule beaudettei.
For an extensive treatment of seagrass
community components and structure, including associated flora and fauna, see
Zieman (1982). The significance of seagrass beds as habitat, nursery and food
source for ecologically and economically important fauna and flora as well as
various management strategies for IRL seagrass beds are discussed by Dawes et al
(1995).
Direct grazing on Florida seagrasses is
limited to a number of species, e.g., sea turtles, parrotfish, surgeonfish, sea
urchins and perhaps pinfish. Other grazers e.g., the queen conch scrape the
epiphytic algae on the seagrass leaves (Zieman 1982). One hundred and thirteen
epiphytes and up to 120 macroalgal species have been identified from Florida's
seagrass blades and communities respectively (Dawes1987).
VI. SPECIAL STATUS
Special Status:
Habitat structure
Notes on Special Status:
Virnstein (1995) stressed the importance of
considering both geographic scale and pattern (landscape) in devising
appropriate management strategies to maintain seagrass habitat diversity in the
Indian River Lagoon. It was suggested that goals be established to maintain
seagrass diversity and that these goals should consider not only the
preservation of seagrass acreage but more importantly, the number of species of
seagrass within an appropriate area. By maintaining seagrass habitat diversity,
the maintenance of the diverse assemblage of amphipods, mollusks, isopods and
fish, associated with seagrass beds, will be accomplished (Virnstein 1995).
Benefit in the IRL:
Because of the vital role of seagrasses as habitat, the health of the Indian
River Lagoon ecosystem is reflected in the health of its seagrass
communities. Thus, the implementation of sound management strategies
designed to protect and promote seagrass habitat helps insure protection for
many of the commercially and recreationally important species resident in the
Indian River Lagoon.
Economic Importance:
None.
Report by: J. Dineen,
Smithsonian Marine Station
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Page last updated: July 25, 2001
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