II. HABITAT AND
Thalassia testudinum is
distributed from just north of Sebastian Inlet, Florida south to the Gulf of
Mexico, Bermuda, the West Indies, Central America and Venezuela (Eiseman 1980).
Several factors, such as temperature,
salinity, water depth, turbidity and wave action can potentially limit the
distribution of Thalassia testudinum. The absence of T.
testudinum beds along the Louisiana Coast is thought due to increased
turbidity and low salinity.
Along the northwestern Cuban shelf, Thalassia
testudinum was by far the most abundant seagrass accounting for 97.5% of
seagrasses present, and was found at depths to 14 meters but occurred more
abundantly in the first 5 meters of depth. When occurring alone, Thalassia
was more abundant in substrata composed of mud and sand, colonizing better on
coarser bottoms (Buesa 1975). This study also reported that red light (620 nm)
promoted optimum growth of Thalassia.
Thalassia testudinum is the dominant seagrass in southeast Florida as
well as the Florida gulf coast.
Seven species of seagrass occur in the IRL. Of these, 6 are known throughout the
tropical western hemisphere, while Halophila johnsonii is known only
from coastal lagoons of eastern Florida, . 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 (Dawes et al
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.
Thalassia testudinum occurs in the
southern half of the Indian River Lagoon at mid-depths. T. testudinum can
be locally abundant, often occurring in monotypic stands and appears to be
increasing in abundance in the Indian River Lagoon (Virnstein 1995). In 1980,
Eiseman reported that Thalassia testudinum was distributed sparsely in
the Indian River Lagoon: small patches were found near St. Lucie inlet and from
Fort Pierce Inlet to Vero Beach, Thalassia testudinum occurred relatively
abundantly, but only in scattered patches from Vero Beach north to Sebastian
Philips (1960) reported on Thalassia
testudinum in the Indian River Lagoon occurring near St. Lucie, Fort Pierce
and Sebastian Inlets and speculated that Sebastian Inlet was probably the
northern most limit of Thalassia on the east coast of Florida.
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).
Phillips (1960) reported depth distributions of Thalassia testudinum in
Florida by various investigators. Depths ranged from the intertidal
zone to 100 feet
on Molasses Reef off Key Largo. He concluded that assuming favorable
temperatures, water clarity is the major factor in determining depth
distribution of Thalassia.
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 the neap tide low water mark, whereas Syringodium
was limited by the spring tide low water mark and will be found in the deepest parts
of the mixed flat (Phillips 1960).
Turtle grass was reported at depths deeper
than 30 feet in clear waters of the Bahamas and only to 6 feet in murky
conditions (Tampa Bay) (Stephens 1966). Thalassia is not tolerant of
strong wave surge, growing only in protected areas (Moore 1963).
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 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 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).
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
Age, Size, Lifespan:
Beds of Thalassia testudinum, destroyed from thermal effluent in
Biscayne Bay, FL, were restored by planting "thousands" of seeds in
late summer. Approximately 3 & 1/2 years later, blade density in restored
areas averaged 2030 blades per square meter, almost equivalent to control areas. Also,
after this time interval, flowering occurred in the restored bed in the spring
with subsequent fruiting in late summer. This temporally defined sexual maturity
in T. testudinum: 3.5 years from seed to flower and 4 years from
seed to seed (Thorhaug 1979).
T. testudinum undergoes seasonal
fluctuations in productivity. Productivity, standing crop, blade length and
density reach a maximum during warm summer months. Blades of Thalassia
testudinum can grow rapidly, up to 1 inch per week under ideal conditions
(Stephens 1966). Average growth rates for Thalassia were also estimated
at 2 - 4 mm/ leaf per day, with maximum growth at 12.5 mm/leaf per day (Zieman
Shoot longevity and rhizome turnover, rather
than capacity to support dense meadows, are key elements in determining either
pioneer species (Halodule
beaudettei and Syringodium filiforme) vs. climax species (Thalassia
testudinum) of seagrass (Gallegos et al 1994). Because of stored
starch in the rhizomes, Thalassia can withstand environmental stress for
some time (Zieman 1975). However, it
was estimated that it takes approximately 2 - 5 years for a Thalassia testudinum
bed to recover from physical disturbance of the rhizome system, most often
caused by motor boat propellers. Disturbance of this nature results in a loss of
the fine sediment component and a lowering of pH and EH (Zieman 1976).
Growth and Light:
Growth of Thalassia testudinum, Halophila engelmannii, Ruppia maritima,
Halodule beaudettei and Syringodium filiforme was investigated in the
laboratory, 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).
Because of the seasonal and spatial (flowering
plants more abundant in the Miami area than in Tampa Bay) nature of flowering,
often occurring when summer solstice occurred, the relationship of temperature
and photoperiod relative to reproduction had been suggested (Phillips 1960).
However, water temperature, as opposed to 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 1982).
Beds of Thalassia testudinum, destroyed from thermal effluent in Biscayne
Bay, FL, were restored by planting "thousands" of seeds in late
summer. Approximately 3 .5 years later, blade density in restored areas
averaged 2030 blades per square meter (m2), almost equivalent to control areas. Also, after
this time interval, flowering occurred in the restored bed in the spring with
subsequent fruiting in late summer. This temporally defined sexual maturity in T. testudinum:
3.5 years from seed to flower and 4 years from seed to seed.
In a transplant feasibility study, fragments
of Thalassia testudinum and Halodule (Diplanthera) wrightii were
transplanted to both aquaria and flow-through seawater systems. In the aquaria, Thalassia
survived for 7 months, whereas Halodule survived for only 3&1/2
months. In the flow-through seawater tanks, Thalassia survived 12 months
and produced new leaves, roots and rhizomes. Only a few Halodule plants
survived in the flow-through system. These results suggested that
transplantation of Thalassia fragments could provide a means of restoring
seagrass beds impacted adversely by coastal development (Fuss & Kelly 1969).
Thorhaug (1979) discussed restoration and
mitigation efforts of seagrasses in the Gulf of Mexico, Florida and the
Caribbean. Thalassia testudinum was the dominant species throughout much
of the Caribbean and Gulf of Mexico. It was concluded that: restoration efforts
including seeding, plugging and turion planting of various seagrasses can be
successful in one area, but not in another; both Halodule and Syringodium
can be successional stages to a Thalassia community; food webs can differ
between Thalassia and Halophila; and faunal diversity and
abundance as well as epibionts and associated macroalgae can also differ between
Thalassia and Halodule in many locations (Thorhaug 1979).
Plant increase and growth of Thalassia
testudinum can occur either by sexual or vegetative reproduction.
Seasonality of both growth and biomass is exhibited by all species of seagrass
in the IRL, including Thalassia testudinum, being maximum during April -
May and June - July respectively. However, since vegetative reproduction occurs
at least to some extent during 9 months of the year, it was felt that this type
of reproduction probably accounts for the maintenance and spread of (Thalassia)
seagrass beds (Phillips 1960). Zieman (1975) also concluded that sexual
reproduction in T. testudinum is not that extensive and that vegetative
reproduction probably accounts for significant spreading of turtle grass beds.
T. testudinum has both staminate and pistillate flowers. Reports of
flowering in Thalassia testudinum indicate reproductive seasonality. In
Biscayne Bay, FL, flowers were seen only during the third week in May, with
fruits appearing 2 - 4 weeks later. Fruits remained attached to the parent plant
until the third week in July at which time they detached and floated off. In
Tampa Bay, FL, although evidence of bud development in Thalassia testudinum
is apparent in May - June, when water temperatures increase, early bud
development was observed in January (Moffler 1981).
T. testudinum was seen flowering in the
Dry Tortugas in July (1916) and both male and female flowers were seen in early
June (1926) (as cited in Phillips 1960). Among several sites investigated by
Phillips (1960), 10% of plants collected in the Florida Keys in late May (1958)
were flowering and temperature ranged from 25.5 to 33.5 °C. In Tarpon Springs in
July (1958), 5 - 15 % of Thalassia plants collected had female flowers,
temperature range was 27.2 - 31.6 °C. Flowering plants (female inflorescence)
were found in Tampa Bay in June (1959). It was noted that when Thalassia
flowers were found, only one sex was observed (Phillips 1960).
Reproduction and flowering of Thalassia
testudinum was compared between clones placed in laboratory culture under
controlled conditions of light, salinity and temperature, and those in Redfish
Bay, Texas. Thalassia could not be induced to produce flowers in the
laboratory, nor was Thalassia observed flowering in Redfish Bay. In
contrast, Halophila engelmannii produced flowers continuously in the
laboratory (January - September), as well as in the field (April - mid-June)
implying that conditions inducing flowering in Halophila do not affect Syringodium
similarly (McMillan 1976).
IV. PHYSICAL TOLERANCES
Temperature probably limits the northern
distribution of Thalassia testudinum in Florida. In the Gulf of
Mexico, T. testudinum is apparently capable of enduring a warm temperate
climate; however, this is not the case along Florida's east coast where temperatures of 35.0 -
40.0 °C will kill the leaves of T. testudinum (Glynn 1968).
Phillips (1960) speculated that water
temperatures between 20 - 30 °C are most inducive to T. testudinum leaf
growth and that temperatures above or below this range may cause leaf mortality.
Zieman (1975) also reported a temperature optimum of 30 °C for turtle grass.
Thalassia testudinum does not
tolerate extreme fluctuations in salinity and apparently will not tolerate fresh
water. Moore (1963) speculated that salinities of 20 ppt or lower will have
deleterious effects on turtle grass beds.
Phillips (1960) reported salinity ranges for T.
testudinum from various sources: 35.0 - 38.5 ppt in the Dry Tortugas; 28.0 -
48.0 ppt in Everglades National Park; and 25.0 - 34.0 ppt in bays along
Florida's west coast. The maximum and minimum salinities reported for T.
testudinum were 48.0 ppt in Florida Bay, and 10.0 ppt in Crystal Bay (on the
west coast of Florida). Turtle grass is probably intolerant of salinities over
45 ppt for extended periods of time (Moore 1963). For example, in the Laguna
Madre, where salinity ranges from 27.3 - 79.2 ppt, Thalassia beds are not
found (Simmons 1957). Phillips (1960) concluded that the optimum salinity for T.
testudinum growth in Florida was 25.0 - 38.5 °C.
In a salinity tolerance study of seagrasses
from Redfish Bay, Texas, Thalassia testudinum showed less tolerance than
Halodule (Diplanthera) wrightii. When salinity was increased in temperature
controlled tanks, Thalassia's growth was limited at 60 ppt. In outdoor
ponds, little growth was seen past salinities of 67 ppt. (McMillan & Moseley
Although considered a stenohaline species, T.
testudinum showed sparse occurrence at a salinity of 10 ppt (Phillips 1960)
and an abundant population was reported at a salinity of 11.5 ppt during an
unusually wet summer (Moore 1961).
V. COMMUNITY ECOLOGY
Photosynthetic rates were determined
for three species of seagrass in the Indian River Lagoon. Photosynthetic
rates (mg C/g dry wt-h) ranged between 0.009 - 0.395 for Halodule beaudettei,
0.005 - 0.79 for Thalassia testudinum, and 0.009 - 1.72 for Syringodium
filiforme (Heffernan & Gibson 1983).
The protein, carbohydrate and trace element
composition, energy content and nutritive value of Thalassia testudinum
and Ruppia maritima were investigated. It was found that relative to
other aquatic plants, Thalassia and Ruppia contain substantial
amounts of protein, carbohydrate, energy and minerals, but that nutritional
value of these plants can vary seasonally (Walsh & Grow 1973).
Various substrata have been reported
to support stands of T. testudinum: e.g., hard packed to course, muddy
sand; soft marl or mud; silt and clay-sized sediment; very fine, loose grayish
calcium carbonate. Common to all these substrata was the presence of calcium
carbonate with the substrata itself presenting anaerobic conditions (Phillips
The rhizome of Thalassia testudinum is
usually buried from 2 to 4 inches in the substratum (Phillips 1960) but was also
observed at 25 cm and more in Florida Bay (Ginsburg & Lowenstam 1958).
Although Thalassia testudinum can be locally dominant, it is often
associated with other species of seagrass. For example, although preferring
slightly shallower water, Thalassia is often associated with Syringodium
below the low tide line. Halophila engelmannii (Moore 1963) can co-occur
inconspicuously with both Thalassia and Syringodium, because of
its small leaf size. Halophila is apparently tolerant of shade conditions
and can occur at depths of 73.2 - 91.0 meters (Moore 1963).
Grazers and Epiphytes:
Turtle grass beds serve as both habitat and food source for marine animals.
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 algae present on seagrass
leaves (Zieman 1982). At least 113 epiphytes and up to 120 macroalgal species
have been identified from Florida's seagrass blades and communities respectively
(Dawes1987). Although few animals graze directly on seagrass, its epiphytic
community (bacterial films, diatoms and algae) provide food for small animals at
the base of the food chain to be consumed by young fish and caridean shrimp
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.
In Card Sound, FL, although molluscan biomass (2.31 g dry/m2) associated
with turtle grass beds exceeded the polychaete and pericaridean crustacea
biomass (1.74 g dry/m2), it was thought that the former taxa accounted for the
main interaction between primary consumers and higher-level predators. The main
fish predators in this system were the syngnathids and the gold-spotted
killifish (Brook 1977).
An Indian River Lagoon, Fl, study compared 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 (Virnstein et al 1983). Both
groups, especially the epifauna, were found to be both more abundant in seagrass
habitats and also more heavily preyed upon and hence more trophically important
than seagrass infauna. The primary transfer path to higher trophic levels occurs
through the epifaunal macrobenthos in seagrass habitats and through the infauna of sandy habitats (Virnstein et al 1983).
A comparison of faunal communities between
thermally impacted and stable Thalassia testudinum beds was undertaken in
Biscayne Bay, FL. Species abundance and diversity between restored areas and
those that had not recovered from thermal impact were statistically significant.
No differences were seen between restored areas and those that were not
impacted. Certain groups of animals, e.g., pink and caridean shrimp as well as
juvenile fish were numerically higher in restored areas than at control sites,
and a magnitude higher than at non-recovered areas (McLaughlin et al 1983).
A high standing crop of Thalassia
testudinum does not necessarily indicate macrofaunal abundance. For example,
when five turtle grass communities were sampled (4 in Biscayne Bay and 1 in the
Everglades), abundance of macrofauna ranged from 292 to 10,728 individuals per
m2. Other factors such as sediment type and total organic carbon (TOC) could
affect organisms living in the sediment water interface as well deposit feeders
Amphipods are capable of detecting differences in density of seagrasses and will
choose areas of high blade density, presumably as a prey refuge. 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. 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).
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).
VI. 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).
Report by: J. Dineen,
Smithsonian Marine Station
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