|
II. HABITAT AND
DISTRIBUTION
Regional Occurrence:
Eiseman (1980) considers H.
decipiens essentially pantropical, occurring on the continental shelf (at
about 20 m) adjacent to the Indian River Lagoon, in the Gulf of Mexico, the West
Indies and Indo-Pacific.
Along the northwestern Cuban shelf, Halophila
decipiens occurred to depths of 24.3 meters. Although occurring the deepest,
H. decipiens was the least abundant (0.1 % composition) of 4 seagrasses
in the area (H. decipiens, Halophila engelmannii, Thalassia testudinum and
Syringodium filiforme) (Buesa 1975).
IRL Distribution:
Individual distributions of the 7 species of seagrasses in the Indian River
Lagoon (IRL) are summarized by Eiseman (1980) and Virnstein (1995). In the
Indian River Lagoon, Halophila decipiens occurs in the southern half and
can be locally abundant and dense in deep water (Virnstein 1995).
Distribution, biodiversity, productivity and
ecological significance of seagrasses in the Indian River Lagoon, 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, known only from coastal lagoons of eastern Florida, occur in the
IRL. 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 -
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, beaudettei, Halophila johnsonii, Thalassia testudinum,
Halophila decipiens, Halophila engelmannii and Ruppia maritima.
Distributional Changes:
Changes in seagrass distribution and diversity pattern in the Indian River
Lagoon (1940 - 1992) are discussed by Fletcher and Fletcher (1995). These
authors estimate 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 is 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:
Can be moderately abundant where it occurs.
Locomotion:
Sessile
Reproduction:
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 (Moffler & Durako 1987). Laboratory experiments
showing flowering induction under continuous light suggests that photoperiod
probably plays a limited role in sexual reproduction (McMillan 1982).
H. decipiens is monoecious, with male
and female flowers occurring on the same spathe. Female flowers produce
approximately 30 seeds. 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 (Dawes et al 1995).
IV. PHYSICAL TOLERANCES
Temperature:
stenothermal.
Salinity:
Halophila decipiens is considered a stenohaline species. When Halophila johnsonii, an
intertidal to shallow subtidal species,
was compared with deeper water populations of H.
decipiens, H. johnsonii showed greater tolerance to higher irradiances, and
to variations in temperature and salinity (Dawes et al 1989).
V. COMMUNITY ECOLOGY
Trophic Mode:
Autotrophic
Habitat:
Although Eiseman (1982) reported Halophila decipiens occurring at 20
m on the continental shelf adjacent to the Indian River Lagoon, Dawes (1987)
found it common in deeper water, 5 - 100 m.
Associated Species:
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). At least 113, and up to
120 macroalgal species have been identified from Florida's seagrass blades and
communities respectively (Dawes1987).
Halophila decipiens occurs in Salt
River Canyon, St. Croix, U.S. Virgin Islands. Although the net production of H.
decipiens is less than other Caribbean seagrasses, in Salt River Canyon, H.
decipiens represents a major source of primary production. It has been shown
that bacteria attached to H. decipiens detritus do not efficiently
recycle primary production of this seagrass in Salt River Canyon (Kenworthy et al 1989).
Virnstein (1995) suggests 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).
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
Submit additional information, photos or comments to:
irl_webmaster@si.edu
Page last updated: July 25, 2001
|
