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II. HABITAT AND
DISTRIBUTION
Regional Occurrence:
Ruppia maritima is distributed
worldwide, occurring in both temperate and subtropical estuaries, bays and
lagoons (Eiseman 1980). Ruppia maritima occurs in brackish waters along the U.
S. Atlantic coast as well as in alkaline lakes, ponds and streams in the western
U.S. (Phillips 1960). R. maritima has also been reported from Newfoundland
(Fernald
and Wiegand 1914 as cited in Phillips 1960). Phillips (1960) reported the
occurrence of Ruppia at several brackish water sites along the west coast of
Florida. Thorne (1954) ( as cited in Phillips 1960) reported Ruppia maritima
occurring from Bay County Florida to Aransas County Texas. Phillips (1960)
concludes that it is probable that Ruppia occurs from Newfoundland to Texas.
IRL Distribution:
Ruppia maritima grows patchily throughout the Indian River Lagoon in very shallow
water, often mixed with Halodule wrightii, and becoming less common in the south
(Virnstein 1995). Eiseman (1980) described the occurrence of Ruppia maritima
throughout the Indian River Lagoon as being sparse, mixed with Halodule
beaudettei,
and occurring just below the intertidal zone. Early reports of Ruppia occurring
in the Indian River Lagoon, included several locations in Brevard County
including Sebastian Inlet, as well as from the St. Lucie River near St. Lucie
Inlet (Woodburn and Ingle 1959).
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 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. beaudettei. 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 1995).
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 wrightii, 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.
Depth:
When occurring in a mixed seagrass flat, Ruppia maritima occurs in slightly
deeper water, Halodule wrightii occurs closest to shore. 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 is
found in the deepest parts of the mixed flat (Phillips 1960).
Distributional Changes:
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 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 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).
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
Reproduction:
Seasonality of both growth and
biomass is exhibited by all species of seagrass in the Indian River Lagoon, being maximum during
April to May and June to July respectively.
Ruppia maritima is the only
seagrass in the southeastern U. S. that exhibits hydroanemophilous pollination,
i.e., hydrophobic pollen grains float on the water surface. This allows for
either self fertilization or outcross fertilization (Moffler & Durako 1987).
Both sexual and vegetative
reproduction can account for new plant production in Ruppia maritima. Dense mats
of flowering Ruppia plants, 2 & 1/2 to 3 feet long, with most plants bearing
flowers, have been observed (Phillips 1960). Muenscher (1944 as cited in
Phillips 1960) described the flowers of Ruppia maritima. Phillips (1960)
reported no correlation between salinity and flowering in Ruppia, although he
reported observing Ruppia before, during and after flowering at salinities above
31.4 ppt. An earlier report (Bourn 1935 as cited in Phillips) indicated that
Ruppia needed a salinity of 28.0 ppt or less to set seed. 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 (McMillan 1982; Moffler & Durako 1987). Temperatures averaging
22.0 - 24.0 °C induced both flowering and fruiting in Ruppia maritima. It was
concluded that a temperature of 15.0 - 20.0 °C was required for germination and
seed maturation, while a range of 20.0 - 25.0 °C was necessary for vegetative
growth and reproduction. Ruppia seeds might remain dormant throughout the autumn
and winter and germinate the following spring (Setchell 1924 as cited in
Phillips 1960). It appears that vegetative growth and sexual reproduction
(flowering and fruiting) commence with warm water temperatures in the spring and
end when high summer temperatures persist (Phillips 1960).
Because R. maritima flowers so
profusely, its geographical distribution is probably due to sexual reproductive
activity (Phillips 1960, McMillan & Moseley 1967) with fruits/seeds being
dispersed by currents. Seed dispersal could also be accomplished by migratory
waterfowl that use Ruppia as a food source. Vegetative growth as well as sexual
reproductive activity probably serve to maintain and enlarge established beds of
Ruppia.
Growth of Ruppia maritima,
Halodule wrightii, 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
much slower for all species (Koch et al 1974).
Flowering and reproduction of 5
seagrasses including Ruppia maritima was compared between clones placed in
laboratory culture under controlled conditions of light, salinity and
temperature, with those occurring in Redfish Bay, Texas. Ruppia could not be
induced to produce flowers in the laboratory although it flowered in Redfish Bay
sporadically throughout the summer. In this study 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 Ruppia in the same manner (McMillan 1976). Ruppia
maritima occurs occasionally in the intertidal zone but probably does not flower
there (Phillips 1960).
IV. PHYSICAL TOLERANCES
Temperature:
Ruppia, a eurythermal species,
has been reported from the tropics to temperate regions. Ruppia maritima var.
oqliqua was found near Nova Scotia and Prince Edward Island (Fernald and Wiegand
1914 as cited in Phillips 1960). Ruppia was observed to overwinter in Tampa Bay,
Fl where temperatures ranged from 13.0 - 35.0 °C and survived extremes of 7.0
°C in February and 39.4 °C in July. Ruppia beds increase in abundance with warm
water temperatures in the spring, and are most abundant during the flowering
period. Temperatures of 20 - 25 °C are probably most favorable for growth and
development of Ruppia (Phillips 1960).
Salinity:
Ruppia maritima has been found in
water ranging from fresh to 32 ppt salinity, but is generally found in waters of
25.0 ppt or less (Phillips 1960).
In vitro rhizome growth and
rooting of Ruppia maritima occurred at various salinities using artificial
seawater. Rhizome production was most rapid at 0 & 5 ppt as opposed to 10,
15 and 20 ppt salinity. In vitro root production, although requiring an external
carbon source, was greatest at 5 and 10 ppt salinity. This study also showed
that ex vitro plantings were very successful suggesting that Ruppia maritima
would be a good candidate for propagation through in vitro culture (Bird et al.
1993).
When Ruppia maritima from Redfish
Bay, Texas, was transferred to outdoor ponds and controlled growth rooms where
salinity was artificially increased, in the latter environment, growth continued
until 70 ppt salinity in controlled growth rooms. In this same environment, R.
maritima tolerated a salinity of 74 ppt, for a brief period.
V. COMMUNITY ECOLOGY
Trophic Mode:
The protein, carbohydrate and
trace element composition, energy content and nutritive value of Ruppia maritima
and Thalassia testudinum were investigated. It was found that relative to other
aquatic plants, Ruppia and Thalassia contain substantial amounts of protein,
carbohydrate, energy and minerals, but that nutritional value of these plants
can vary seasonally (Walsh & Grow 1973).
Habitat:
In the Indian River Lagoon,
Ruppia maritima has been reported from sand as well as muddy, course sand substrata. Along the Gulf coast, the predominant substratum where
Ruppia
maritima occurred, was a mixture of mud and silt with fine textured sand. All
seagrasses, including Ruppia, aid in binding sediment and thereby lessen the
effects of erosion (Phillips 1960).
Ruppia maritima appears to prefer
brackish water less than 25.0 ppt salinity. In Florida, Ruppia was found from
the intertidal to a depth of 7 feet, with densest growth at 2 to 4 feet (mean
high tide). Phillips (1960) speculated that the restriction of Ruppia to shallow
water is a result of its tolerance to low salinity waters, usually occurring in
turbid bays and estuaries. Light penetration is compromised in these turbid
waters and hence will not support plant growth below certain depths.
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 epiphytes, and up to 120 macroalgal species have been
identified from Florida's seagrass blades and communities respectively
(Dawes1987).
Habitat Diversity:
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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