Species Description: A descriptive guide to the morphology, including keys and illustrations of plant and flower, occurrence, and global distribution of the 7 species of seagrass (Thalassia testudinum, Halodule wrightii (formerly H. wrightii), Syringodium filiforme, Ruppia maritima, Halophila engelmannii, Halophila decipiens and Halophila johnsonii) occurring in the Indian River Lagoon is presented by Eiseman (1980).
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 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 (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 wrightii, 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 1970s 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 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 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) are now available in GIS format (ARCINFO) ( see Fletcher & Fletcher 1995).
Depth: The lower limit of seagrass depth distribution for both Syringodium filiforme and Halodule wrightii 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 has been documented as occurring 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 from 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 Guadeloupe.
When occurring in a mixed seagrass flat, Halodule wrightii 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).
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 wrightii) or climax species (Thalassia testudinum) of seagrass (Gallegos et al 1994).
Abundance: In 1978, Thompson listed Syringodium filiforme and Halodule wrightii 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 wrightii.
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 wrightii, 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 wrightii 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 wrightii, 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.
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).
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 wrightii 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 wrightii 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 three 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). 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 four species, three 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 across 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 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 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).
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).