Species Description: Seven species of seagrasses (Thalassia testudinum, Halodule wrightii (formerly H. wrightii), Syringodium filiforme, Ruppia maritima, Halophila engelmannii, Halophila decipiens and Halophila johnsonii) occur in the Indian River Lagoon, Florida. An illustrated key and guide to their morphology and distribution is presented by Eiseman (1980).
Orpurt and Boral (1964) redescribed the flowers, fruits and seeds of Thalassia testudinum and detailed fruit development and seed germination. It was estimated that it takes about 8 weeks for fruit to mature after pollination. Tomlinson and Vargo described the vegetative morphology of Thalassia testudinum (1966) and further described root functional morphology (1969 a), floral morphology and anatomy (1969 b), and leaf anatomy and development (1972).
Regional Occurrence: 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.
IRL Distribution: Thalassia testudinum is the dominant seagrass in southeast Florida as well as Florida's Gulf Coast.
Seven species of seagrass occur in the IRL. Of these, six are known throughout the tropical western hemisphere, while Halophila johnsonii is known only from coastal lagoons of eastern Florida. 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 three 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 seven 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.
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; it 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 Inlet.
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 northernmost limit of Thalassia on the east coast of Florida.
The distribution of three species of seagrass was mapped in a 15-hectare area in the 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 at a 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).
Depth: 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 wrightii 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).
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 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).
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) are now available in GIS format (ARCINFO) (see Fletcher & Fletcher 1995).
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 3.5 years later, blade density in restored areas averaged 2,030 blades/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 (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 1975).
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 wrightii 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).
Restoration: 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 3.5 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).
Reproduction: 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, nine months out 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.
Flowering: 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).
Temperature: 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.
Salinity: 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 1967).
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).
Trophic Mode: 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 wrightii, 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).
Habitat: Various substrata have been reported to support stands of T. testudinum: e.g., hard packed to coarse, 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 1960).
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).
Other Seagrasses: 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 (Dawes 1987). 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 (Moore 1963). 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.
Macrobenthos: 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 wrightii 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/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 (Brook 1978).
Amphipods: Amphipods are capable of detecting differences in density of seagrasses and will choose areas of high blade density, presumably as a prey refuge. 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).
Decapods: 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. In all, 38 species in 28 genera and 17 families were sampled. The crustacean community was regulated by aboveground 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).
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). For an extensive treatment of seagrass community components and structure including associated flora and fauna, see Zieman (1982).
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).