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II. HABITAT AND
DISTRIBUTION
Regional Occurrence:
Syringodium filiforme occurs
throughout the Gulf of Mexico and Caribbean Sea as well as Bermuda and the
Bahamas (Eiseman 1980).
IRL Distribution:
Seven species of seagrasses occur in the Indian River Lagoon. Of these,
6 species are known to occur throughout the tropical western hemisphere, while one, Halophila johnsonii, is known only
from coastal lagoons of eastern Florida. Among the seagrasses 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
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.
The northern area of the Indian River Lagoon
supports the most developed seagrass beds, presumably because of 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.
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 estimated that seagrass abundance was 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 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:
The distribution of 3 species of seagrass was mapped in a 15 ha area in
mid-Indian River Lagoon. Halodule beaudettei and Syringodium filiforme
were more abundant in shallow and deeper water respectively. Thalassia
testudinum occurred in patches. Areal coverage (%) of monospecific stands of
these three species was 35% for Syringodium, 14% for Halodule and
6% for Thalassia. Mixed beds, mostly Syringodium and Halodule
accounted for 25% coverage. Biomass (above-ground) was greatest during the
summer and minimum in late-winter. In this same study area, drift algae,
primarily Gracilaria spp. was initially mapped and then sampled in order
to estimate its abundance. It was concluded that at times drift algae can be
quantitatively more important than seagrass in terms of habitat, nutrient
dynamics and primary production (Virnstein & Carbonara 1985).
Sources of mapped distributions of Indian
River Lagoon seagrasses include: 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).
Depth:
The lower limit of seagrass depth distribution for both Syringodium filiforme
and Halodule beaudettei in the southern region of the Indian River Lagoon
is controlled by light availability. Both species occur approximately to the
same maximum depth, in Hobe (1.75 - 2.0 m depth) and Jupiter (2.5 - 2.75 m
depth) sounds, indicating similar minimum light requirements. In more
transparent waters, e.g., in the Caribbean, these species can occur at
considerably deeper depths (Kenworthy and Fonseca 1996).
Although Syringodium has never been
observed in intertidal areas, it did occur in shallow areas caused by spring
tides and strong winds (Phillips 1960). Near St. Lucie Inlet in the Indian River
Lagoon, dense growth of Syringodium was seen at 2 feet (mean low tide)
but was collected up to 10 feet. Phillips (1960) reported that densest growth of
Syringodium occurred in water 2.0 to 4.5 feet, at mean low tide, although
it occurred sparsely in much deeper water, probably a function of light
penetration. In Florida, Syringodium has never been reported as deep as Thalassia
or Halodule. However, Syringodium did occur at 25 meters in
Gaudeloupe.
When occurring in a mixed seagrass flat,
Halodule beaudettei occurred closest to shore. Ruppia occurred in slightly
deeper water. Thalassia testudinum, although probably preferring continuous
submersion, was limited by neap tide low water mark, whereas Syringodium
was limited by spring tide low water mark and was found in the deepest parts of
the mixed flat (Phillips 1960).
III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
Leaf length in Syringodium filiforme varies with water depth. Overall
leaf length was greater in deeper water, although maximum leaf length can occur
at any depth (Phillips 1960).
Shoot longevity and rhizome turnover, rather
than capacity to support dense meadows, are key elements in determining either
pioneer species (Syringodium filiforme and
Halodule beaudettei) or climax species (Thalassia
testudinum) of seagrass (Gallegos et al 1994).
Abundance:
In 1978, Thompson listed Syringodium
filiforme and Halodule beaudettei as the most abundant seagrasses in
the Indian River Lagoon, but between Fort Pierce Inlet and the southern tip of
Merritt, Syringodium declined sharply in abundance. North of the southern
tip of Merritt Island, Syringodium again was numerically dominant in
terms of erect shoots (Thompson 1978).
S. filiforme occurs abundantly at mid-depths throughout the IRL, rarely
occurs in shallow water and is often mixed with other species. Syringodium
is absent in areas of poor water quality (Virnstein 1995). Eiseman (1980)
reported S. filiforme occurring throughout the Indian River Lagoon
(where waves and current are not strong) often in mixed stands with Thalassia
testudinum and Halodule beaudettei.
Locomotion:
Sessile.
Reproduction:
Seasonality of both growth and biomass is exhibited by all species of
seagrass in the Indian River Lagoon, being maximum during April - May and June -
July respectively (Dawes et al 1995).
When the seasonal distribution of Syringodium
filiforme and associated macrophytes was studied in the northern Indian
River Lagoon, FL, minimum standing crop occurred during February through April;
maximum standing crop occurred in September. Halodule beaudettei, Halophila
engelmanii, and drift algae occurred in the study area but were not major
components of the system. Sandy patches within these seagrass beds were due to
the burrowing activity of the horseshoe crab, Limulus polyphemus. Because
the study area was at the northern distributional limit of Syringodium
filiforme, thermal stress may limit patch regrowth (Gilbert and Clark 1981).
Another study in the northern section of the
Indian River Lagoon, FL showed that the seagrass communities composed of Syringodium
filiforme, Halophila engelmannii and Halodule beaudettei responded to a
number of interrelated physical and biological variables some of which varied
seasonally (temperature, light, epiphytes). Other variables such as sediment
deposition and resuspension vary continuously. Vegetative growth of all three
species occurred in the spring and to a lesser extent during the fall (Rice et
al. 1983).
In a laboratory study, growth of Syringodium
filiforme, Ruppia maritima, Halodule beaudettei, Halophila engelmanii and
Thalassia testudinum were investigated at various light intensities. Optimum
growth for all five species was obtained at light intensities of 200 - 450
foot-candles. At light intensities above or below this range, growth was much
slower for all species (Koch et al 1974).
Flowering:
Water temperature, moreso than photoperiod, appeared 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 suggested that photoperiod probably plays a limited role
in sexual reproduction (Moffler & Durako 1982).
Phillips (1960) speculated that since
flowering was so rarely reported in Syringodium filiforme, most
dispersion of this seagrass probably occurred through vegetative growth, i.e.,
rhizome elongation and new branch production. New shoot production occurred in Syringodium
throughout the year, except in the coldest winter months.
Flowering and reproduction of seagrasses,
including Syringodium filiforme, was compared between clones placed in
laboratory culture vs. those in Redfish Bay, Texas. Flowering in Syringodium
could not be induced in the laboratory and this species flowered only scarcely
in Redfish Bay (McMillan 1976).
Embryology:
Seeds of Syringodium (like Halodule) can have prolonged dormant
periods up to 3 years. Fruits mature on reproductive shoots above sediment and
can be widely dispersed.
IV. PHYSICAL TOLERANCES
Temperature:
Syringodium filiforme is considered a tropical species because it
occurs throughout the Caribbean. However, because of its distribution in
northern areas of Florida, it can be considered eurythermal. Leaf kill in Syringodium
occurs when temperatures drop to approximately 20°C. The effect of cold water
on rhizome growth is not known (Phillips 1960).
Along Florida's east coast, Syringodium
does not occur north of Cape Canaveral. In the Indian River Lagoon, occasional
growth of Syringodium was seen in Brevard County and dense patches were
reported from near Sebastian, and between Sebastian, Fort Pierce and St. Lucie
Inlets (Phillips 1960). Cold winter water in the Tampa Bay area can cause leaf
damage in Syringodium filiforme but leaf kill occurs less frequently in
deeper Gulf waters (Phillips 1960).
Salinity:
Syringodium filiforme is euryhaline. In the Tampa Bay region where
salinity is usually under 25 ppt, Syringodium was found in dense stands
and Thalassia was sparse. Phillips (1960) speculated that dense stands of
Thalassia probably force Syringodium into lower salinity areas. In
the Indian River Lagoon, S. filiforme formed dense beds in salinities of
22.0 - 35.0 ppt where Thalassia occurred only rarely (Phillips 1960).
Syringodium filiforme does not occur in fresh
or low salinity water, although it can withstand periods of low salinity (10
ppt) (Phillips 1960). In Brevard county, Syringodium was found in a
salinity range of 20.1 - 20.6 ppt. From Sebastian to St. Lucie Inlet, Syringodium
was found in a salinity range of 22.0 - 35.0 ppt (Phillips 1960). Optimum
salinity for Syringodium is probably 20.0 - 25.0 ppt and over. Phillips
(1960) did not observe persistent growths of Syringodium in areas where
average salinity was under 20.0 ppt.
In a salinity tolerance study of 5 seagrasses
from Redfish Bay, Texas, including Syringodium filiforme, Thalassia
testudinum, Halophila engelmanni, Halodule (Diplanthera) wrightii and Ruppia
maritima, Syringodium showed the least tolerance when salinity was
increased. Under controlled conditions, growth of Syringodium ceased when
salinity reached 45 ppt (McMillan & Moseley 1967).
V. COMMUNITY ECOLOGY
Trophic Mode:
Photosynthetic rates were determined for three species of seagrass in the
Indian River Lagoon, Florida in March and July. Photosynthetic rates (mg C/g dry
wt-h) ranged between 0.009 - 1.72 for Syringodium filiforme, 0.009 -
0.395 for Halodule beaudettei and 0.005 - 0.79 for Thalassia testudinum
(Heffernan & Gibson 1983).
Habitat:
Favorable substratum for Syringodium
is very soft bottom, i.e., loose muddy sand; although Syringodium
has been reported from a wide variety of substrata including the soft black mud
near St. Lucie Inlet in the Indian River Lagoon, as well as in firm muddy sand
composed mostly of sand (Phillips 1960).
Currents:
In south Florida, it appeared that strong current
promoted the growth of both Thalassia testudinum and Syringodium
filiforme as evidenced by their luxuriant growth in tidal channels
separating mangrove islands, as opposed to growth observed in quiescent lagoons.
It is thought that rapid current will tend to break down diffusion gradients,
making more CO2 and inorganic nutrients available to the plant (Zieman 1982).
Associated Species:
Seagrasses:
Syringodium filiforme is usually sparse in areas of dense Ruppia
growth, where brackish water Ruppia apparently outcompetes Syringodium
(Phillips 1960). Along the northwestern Cuban shelf, Syringodium filiforme
was approximately 10 times more abundant than Halophila engelmanii and
H. decipiens combined, accounting for 2.2 % composition of seagrasses in the
area. Thalassia testudinum accounted for 97.5 % total biomass.
Epiphytes:
A species list of seagrass epiphytes of the Indian River Lagoon, FL, was
provided by Hall and Eiseman (1981). Forty one species of algae occurred on the
seagrasses Syringodium filiforme, Halodule beaudettei and Thalassia
testudinum. Epiphytic algal diversity and abundance was generally higher in
winter and spring and lowest during late summer and early fall.
At least 113 epiphytes and up to 120
macroalgal species were later identified from Florida's seagrass blades and
communities respectively (Dawes1987).
Direct grazing on Florida seagrasses is
limited to a number of species, e.g., seaturtles, parrotfish, surgeonfish, sea
urchins and perhaps pinfish. Other grazers e.g., the queen conch scrape the
epiphytic algae on the seagrass leaves (Zieman 1982).
Macrofauna:
Amphipods are capable of detecting differences in density of seagrasses and will
choose areas of high blade density, presumably as a prey refuge. In addition,
when 3 different species of seagrass, Thalassia testudinum, Syringodium
filiforme and Halodule beaudettei were offered to amphipods at equal
blade density, amphipods chose H. wrightii because of its higher surface
to biomass ratio (Stoner 1980).
A study of decapod crustacea associated with a
seagrass/drift algae community in the Indian River Lagoon, FL showed remarkable
diversity. The seagrass community sampled was composed of 4 species, 3 of which
were abundant: Syringodium filiforme; Halodule beaudettei; and Thalassia
testudinum. Brachyuran crabs and caridean shrimp comprised the majority of
decapods sampled. In all 38 species in 28 genera and 17 families were sampled.
The crustacean community was regulated by above ground plant abundance i.e., a
function of habitat complexity. It was concluded that competitive exclusion rather than predation was more important in regulating habitat diversity of the
macrocrustacean community in these seagrasses (Gore et al 1981).
A study comparing the abundance of
macrobenthic invertebrates and epifauna in seagrass (Thalassia testudinum,
Halodule beaudettei and to a lesser extent, Syringodium filiforme) vs.
adjacent sandy bottom habitats was conducted in the Indian River Lagoon, FL by
Virnstein et al (1983). Both groups, but especially the epifauna, were found to
be both more abundant in seagrass habitats and also more heavily preyed upon and
thus more trophically important than seagrass infauna. The primary transfer path
to higher trophic levels occurred through the epifaunal macrobenthos in seagrass
habitats and through the infauna in sandy habitats (Virnstein et al 1983).
A new anaspidean Phyllaplysia smaragda,
associated with manatee grass Syringodium filiforme, was described from
material collected between Titusville and Merritt Island, FL . P. smaragda
was observed feeding on scrapings of S. filiforme as well as on an
encrusting epiphyte Erythrocladia subintegra (Clark 1970).
For an extensive treatment of seagrass
community components and structure including associated flora and fauna, see
Zieman (1982).
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).
VI. SPECIAL STATUS
Special Status:
Habitat structure
Broad-scale Cost/Benefit:
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).
Economic Importance:
None.
Report by: J.
Dineen,
Smithsonian Marine Station
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irl_webmaster@si.edu
Page last updated: July 25, 2001
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