productivity and ecological significance of seagrasses in the Indian River
Lagoon 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. Halodule beaudettei is the most common.
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.
Thalassia testudinum occurs in the southern portion of the IRL, from
Sebastian Inlet 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.
Changes in seagrass distribution and diversity patterns in the Indian River
Lagoon from 1940 to 1992 are discussed by Fletcher and Fletcher (1995). These
authors estimate 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 system. Decreases in abundance occurred particularly north of Vero
Beach into the Melbourne area. In this region 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);
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:
Growth of Halophila engelmannii,
Ruppia maritima, Halodule beaudettei, Syringodium filiforme and Thalassia
testudinum 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).
A study in the northern section
of the Indian River Lagoon, FL showed that the seagrass communities composed of
Halophila engelmannii, Halodule beaudettei 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
H. engelmannii is among the least common of the seagrasses in the
IRL. Its highest abundance occurs in the northern area of the lagoon
Seasonality of both growth and
biomass is exhibited by all species of seagrass in the IRL, being maximum during
April to May, and June to 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 (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). McMillan (1982) concluded that temperature was more important in regulating flowering in Halophila
engelmannii. Temperatures of 22.0 - 27.5 °C and a photoperiod of more than 12
hours was needed for sexual reproduction to occur in Halophila engelmannii.
Flowering and reproduction of
Halophila engelmannii was compared between clones placed in laboratory culture
and those occurring in Redfish Bay, Texas. In the laboratory, flowers were
produced continuously from January to September whereas in Redfish Bay, H. engelmannii
flower production was restricted from April to mid-June. Temperature
(22.0 - 24.0 °C) was indicated as the most important factor relative to flower
production of Halophila under controlled conditions. It was felt that
temperatures above 23.5 °C and salinites above 40.0 ppt inhibit flower
production in Halophila (McMillan 1976).
IV. PHYSICAL TOLERANCES
Under laboratory conditions, temperatures between 22.0 - 27.5 °C and a photoperiod of
more than 12 hours was required for sexual reproduction to occur in Halophila
In a salinity tolerance study of
seagrasses occurring in Redfish Bay, Texas, "some" Halophila
were able to withstand a salinity of 74 ppt, without chlorophyll levels
being unaffected (McMillan & Moseley 1967). In this study, the reoccurrence
of Halophila from fragments that were thought to be dead, suggested that
Halophila had a higher salinity tolerance than Syringodium.
V. COMMUNITY ECOLOGY
Halophilia engelmannii is generally found to be more common in northern areas of
the IRL system, and is found in somewhat deeper water than other seagrasses.
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). 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 as habitat. 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 have a planktonic phase) would tend to have overlapping species
ranges and hence higher diversity (Virnstein 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 epiphytes and up to 120 macroalgal species have been
identified from Florida's seagrass blades and communities respectively
VI. SPECIAL STATUS
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.
Report by: J. Dineen,
Smithsonian Marine Station
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Page last updated: July 25, 2001