Seven species of seagrass: Thalassia testudinum, Halodule wrightii,
Syringodium filiforme, Ruppia maritima, Halophila engelmannii, Halophila
decipiens and Halophila johnsonii occur in the Indian River
Lagoon, FL. An illustrated key and guide to their morphology and distribution is
presented by Eiseman (1980).
HABITAT AND DISTRIBUTION
Halodule beaudettei can be found south from North Carolina along the
Atlantic and Gulf coasts, in the Caribbean to warm, temperate South America,
northwestern Africa and possibly in the Indian Ocean and the Pacific coast of
Mexico (Eiseman 1980).
Biodiversity, distribution, 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. Of these,
Halodule wrightii 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, south of Sebastian Inlet. Halophila
decipiens, Halophila engelmannii and Halophila johnsonii can form
mixed or monotypic beds with other species. Because of their abundance in deeper
water and their high productivity, the distribution and ecological significance
of the 3 Halophila species may have previously been underestimated (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 wrightii, Halophila johnsonii, Thalassia testudinum,
Halophila decipiens, Halophila engelmannii and Ruppia maritima.
Distributions of the 7 species of seagrasses
in the Indian River Lagoon are summarized individually by Eiseman (1980) and
Virnstein (1995). Halodule wrightii is common throughout the Indian
River Lagoon, and is the dominating seagrass species in shallow subtidal areas
and is occasionally exposed at low tide (Virnstein 1995). H. wrightii was
reported from the intertidal
zone to 2 m depth in the Indian River Lagoon, usually in
pure stands but rarely with Halophila johnsonii. From the intertidal zone
to 1 m depth, H. wrightii is most often mixed with Ruppia maritima,
below this depth, it can be mixed with Thalassia testudinum or
Syringodium filiforme (Eiseman 1980).
The distribution of 3 species of seagrass was
mapped in a 15 ha area in mid-Indian River Lagoon. Halodule wrightii
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
Depth Distribution in the IRL
Halodule wrightii can be found from the intertidal zone to relatively deep water
and probably grows in pure stands closer to the beach than any other species of
seagrass. In the Indian River Lagoon, H. wrightii was collected to 6 feet
where the densest patches occurred at 2 - 3 feet, close to shore (Phillips
1960). When occurring in a mixed seagrass flat, Halodule wrightii
occurs closest to shore. Ruppia occurs in slightly deeper water. Thalassia
testudinum, although probably preferring continuous submersion, is limited
by neap tide low water mark, whereas Syringodium is limited by spring
tide low water mark and will be found in the deepest parts of the mixed flat
(Phillips 1960). The lower limit of seagrass depth distribution for both
Halodule wrightii and Syringodium filiforme in the southern region of the
Indian River Lagoon is controlled by light availability. Both species occur to
the same maximum depth, in Hobe Sound (1.75 - 2.0 m depth) and Jupiter Sound (2.5 - 2.75 m
depth), 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).
In 1960, Phillips discussed the distribution of Halodule in Florida and
the Gulf Coast. Distributional data for northeast Florida was "meager"
although Halodule was observed at the mouth of the Mantanzas River south
of St. Augustine and was also reported from the Mosquito Lagoon and the Indian
River Lagoon in Brevard County. South of Cape Canaveral, dense growth of Halodule
was seen near Sebastian, Fort Pierce and St. Lucie Inlets (Phillips 1960).
Substantial changes in seagrass distribution
and diversity pattern in the Indian River Lagoon, FL (1940 - 1992) have occurred
(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. The 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).
LIFE HISTORY AND POPULATION BIOLOGY
H. wrightii is the most common of the seagrasses that occur in the
IRL. It is most abundant in shallow water.
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). 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
Halodule wrightii is capable of both
sexual and vegetative reproduction. However, flowering in this species is
thought to be extremely rare, at least in Florida. H. wrightii can rapidly and
densely recolonize denuded areas in warm months. Most bed maintenance and new
shoot production probably occurs through rhizome elongation (Phillips 1960).
Halodule wrightii does not grow well in established beds of Thalassia, but
can quickly invade an area where Thalassia was removed. In Old Tampa Bay,
Halodule and Ruppia were secondary to large stands of Syringodium.
In areas where Ruppia was dense, Halodule and Syringodium
were sparse. However, dense beds of Halodule can be found in high
salinity areas where Thalassia and Syringodium are not found.
Shoot longevity and rhizome turnover, rather
than capacity to support dense meadows, are key elements in determining either
pioneer (Halodule wrightii and Syringodium filiforme) vs. climax
(Thalassia testudinum) species of seagrass (Gallegos et al 1994).
Growth of Halodule wrightii, Ruppia
maritima, Halophila engelmannii, Syringodium filiforme and Thalassia
testudinum was investigated at various light intensities in the laboratory.
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
significantly slowed for all species (Koch et al 1974).
When fragments of Halodule (Diplanthera)
wrightii and Thalassia testudinum were transplanted to both aquaria
and flow-through seawater systems, in 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 (as opposed to Halodule fragments), could provide a means of
restoring seagrass beds impacted adversely by coastal development (Fuss &
Flowering and reproduction of 5 seagrasses
including Halodule wrightii, was compared between clones placed in
laboratory culture under controlled conditions of light, salinity and
temperature, with those occurring in Redfish Bay, Texas. Halodule could
not be induced to produce flowers in the laboratory although in Redfish Bay, Halodule
flowers continuously throughout the summer. In this same study, Halophila engelmannii
was induced to produce flowers continuously in the laboratory
(January - September), but also flowered in the field (April - mid-June). This
implied that the right combination of temperature, salinity and light was not
met for Halodule under laboratory conditions (McMillan 1976).
Given its distribution throughout the
tropical and subtropical Atlantic Ocean as far north as North Carolina, Halodule wrightii can be considered eurythermal.
Optimum temperatures for H.
wrightii are likely similar to those of Thalassia, and range between 20 - 30 °C
Several species of seagrasses, including
Halodule wrightii, from the western Atlantic and the Indo-Pacific were induced
to flower under continuous light, suggesting that day length plays an
insignificant role in reproductive periodicity. Rather, the author suggests that
temperature sequences are critical in controlling reproductive periodicity in
seagrasses (McMillan 1982).
Halodule wrightii is probably more euryhaline than Thalassia,
and was observed to withstand fresh water in the St. Lucie River for an unknown
time, although it did not survive prolonged fresh water coverage. In the Indian
River Lagoon, near St. Lucie Inlet, dense stands of Halodule were found
in salinities of 35 ppt. In Florida, Halodule has been reported in abundance in salinities ranging from 12.0 - 38.5 ppt (Phillips 1960). In the
upper Lagune Madre, Texas, H. wrightii was reported to be the most
abundant seagrass in salinities ranging from 1.0 - 60.0 ppt and the only
attached vegetation in salinities above 45.0 ppt (as cited in Phillips 1960).
When Thalassia testudinum, Halophila
engelmanni, Halodule (Diplanthera) wrightii, Ruppia maritima and Syringodium
filiforme from Redfish Bay, Texas, were transferred to outdoor ponds and
controlled growth rooms, while salinity was artificially increased, Halodule
(Diplanthera) was the most tolerant of higher salinities (McMillan &
H. wrightii is autotrophic. Photosynthetic rates were determined for three species of seagrass in the
Indian River Lagoon, Fl in March and July. Photosynthetic rates (mg C/g dry
wt-h) ranged between 0.009 - 0.395 for Halodule wrightii; 0.005 - 0.79
for Thalassia testudinum; and 0.009 - 1.72 for Syringodium filiforme
(Heffernan & Gibson 1983).
Halodule wrightii can be found on a wide variety of substrata, from
silty mud to course sand with varying amounts of mud. These types of substrata
are more likely to be found north of Miami, for example, in the Indian River
Lagoon, where the substratum is composed of extremely course muddy sand
(Phillips 1960). See the above section on Physical Tolerances for salinity and
A study in the northern section of the Indian
River Lagoon, FL showed that the seagrass communities composed of Halophila
engelmanni, Halodule wrightii and Syringodium filiforme 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
Because seagrasses function as habitat, nursery and food source for
ecologically and economically important fauna and flora, they are a highly
significant component of estuarine ecosystems (Zieman 1982, Dawes et al 1995).
Macrobenthos: When macrobenthos was sampled in Halodule wrightii grass beds at three
different sites in the Indian River Lagoon, over a 6 year period, temporal
patterns of community structure were asynchronous. Variation in abundance was
primarily due to variation in the arthropod and bivalve faunas. Trends in
abundance were both site and taxon dependent and were not linked to
differences in physical parameters (salinity, temperature) or depth, suggesting
that local rather than regional factors could account for the dynamic nature of
seagrass macrofaunal assemblages (Nelson et al 1996).
A study comparing the abundance of
macrobenthic and epifaunal organisms epifauna in seagrass (Thalassia testudinum,
Halodule wrightii 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 faunal groups, 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. Consequently, 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 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 wrightii; 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).
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 wrightii were offered to amphipods at equal
blade density, amphipods chose H. wrightii because of its higher surface
to biomass ratio (Stoner 1980).
Although only 15 species were collected over
the 4 year study period, the amphipod communities associated with Halodule
beaudettei seagrass beds in the Indian River Lagoon, FL showed variable
seasonal patterns of abundance and diversity. Abundance was usually higher
during November - May than during June - October. Seasonal variations in
amphipod abundance were due to seasonal variation in predators (fish and decapod
crustaceans) rather than seasonal variability of seagrass abundance (Nelson et
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).
At least 113 epiphytes, and up to 120 macroalgal species have been
identified from Florida's seagrass blades and communities respectively (Dawes
1987). A species list of seagrass epiphytes of the Indian River Lagoon,
FL, is provided by Hall and Eiseman (1981). Forty one species of algae occurred
on the seagrasses Syringodium filiforme, Halodule wrightii and Thalassia
testudinum. Epiphytic algal diversity and abundance were generally
higher in winter and spring and lowest during late summer and early fall.
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).
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.
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