Species Description: 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).
Regional Occurrence: Halodule wrightii 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).
IRL Distribution: 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 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.
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 1985).
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).
Distributional Changes: 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).
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).
Abundance: H. wrightii is the most common of the seagrasses that occur in the IRL. It is most abundant in shallow water.
Reproduction: 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 (McMillan 1982).
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 3.5 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 (as opposed to Halodule fragments), could provide a means of restoring seagrass beds impacted adversely by coastal development (Fuss & Kelly 1969).
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).
Temperature: 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 (Phillips 1960).
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).
Salinity: 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 & Moseley 1967).
Trophic Mode: H. wrightii is autotrophic. 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 - 0.395 for Halodule wrightii; 0.005 - 0.79 for Thalassia testudinum; and 0.009 - 1.72 for Syringodium filiforme (Heffernan & Gibson 1983).
Habitat: 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 coarse muddy sand (Phillips 1960). See the above section on Physical Tolerances for salinity and temperature preferences.
A study in the northern section of the Indian River Lagoon 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 al 1983).
Associated Species: 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).
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 wrightii 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 al 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).
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 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).
Special Status: Habitat Structure
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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