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. An illustrated key with a guide to their morphology and distribution (IRL and global) is presented by Eiseman (1980).
Potentially Misidentified Species: Halophila baillonis
Regional Occurrence: Eiseman (1980) considers H. decipiens essentially pantropical, occurring on the continental shelf (at about 20 m) adjacent to the Indian River Lagoon, in the Gulf of Mexico, the West Indies and Indo-Pacific.
Along the northwestern Cuban shelf, Halophila decipiens occurred to depths of 24.3 meters. Although occurring the deepest, H. decipiens was the least abundant (0.1 % composition) of 4 seagrasses in the area (H. decipiens, Halophila engelmannii, Thalassia testudinum and Syringodium filiforme) (Buesa 1975).
IRL Distribution: Individual distributions of the 7 species of seagrasses in the Indian River Lagoon (IRL) are summarized by Eiseman (1980) and Virnstein (1995). In the Indian River Lagoon, Halophila decipiens occurs in the southern half and can be locally abundant and dense in deep water (Virnstein 1995).
Distribution, biodiversity, 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. H. 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.
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 - H. 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, H. 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 estimate 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 is 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).
Abundance: Can be moderately abundant where it occurs.
Reproduction: 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 (Moffler & Durako 1987). Laboratory experiments showing flowering induction under continuous light suggests that photoperiod probably plays a limited role in sexual reproduction (McMillan 1982).
H. decipiens is monoecious, with male and female flowers occurring on the same spathe. Female flowers produce approximately 30 seeds. 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).
Salinity: Halophila decipiens is considered a stenohaline species. When Halophila johnsonii, an intertidal to shallow subtidal species, was compared with deeper water populations of H. decipiens, H. johnsonii showed greater tolerance to higher irradiances, and to variations in temperature and salinity (Dawes et al 1989).
Trophic Mode: Autotrophic.
Habitat: Although Eiseman (1982) reported Halophila decipiens occurring at 20 m on the continental shelf adjacent to the Indian River Lagoon, Dawes (1987) found it common in deeper water, 5 - 100 m.
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, and up to 120 macroalgal species have been identified from Florida's seagrass blades and communities respectively (Dawes1987).
Halophila decipiens occurs in Salt River Canyon, St. Croix, U.S. Virgin Islands. Although the net production of H. decipiens is less than other Caribbean seagrasses, in Salt River Canyon, H. decipiens represents a major source of primary production. It has been shown that bacteria attached to H. decipiens detritus do not efficiently recycle primary production of this seagrass in Salt River Canyon (Kenworthy et al 1989).
Virnstein (1995) suggests 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
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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Down C. 1983. Use of aerial imagery in determining submerged features in three east-coast Florida lagoons. Fla Sci 46: 335 - 362.
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Virnstein RW. 1995. Seagrass landscape diversity in the Indian River Lagoon, Florida: The importance of geographic scale and pattern. Bull Mar Sci 57: 67-74.
Virnstein RW, Cairns KD. 1986. Seagrass maps of the Indian River lagoon. Unpublished report.
White CB. 1986. Seagrass maps of the Indian and Banana Rivers. Final Report to the Coastal Zone Management Program, Florida Department of Environmental Protection.
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