Thalassia is a genus of the tropics and subtropics: T. testudinum occurs between 9°S and 32°N, and the distribution range T. hemprichii falls between 28°S and 32°N (Spalding et al., 2003). Thus, its global distribution is largely determined by incoming irradiation and related temperature. Regional distributions are considerably more complex, with temperature, salinity, light attenuation, sediment composition, and sediment depth playing major roles.
Seagrasses from tropical environments generally have reduced temperature tolerances compared with temperate-zone counterparts. Their range oftemper-ature tolerance is only half that of organisms from temperate waters (Moore, 1963), which is due to loss of resistance to cold environments, whereas their upper thermal limit is generally not much greater than that of organisms from warm-temperate regions (Zieman, 1975a). Thalassia can neither tolerate prolonged exposure to high temperatures nor long-term desiccation on intertidal flats (Brouns, 1985; Stapel et al., 1997). Phillips (1960) found that T. testudinum preferred temperatures of 20-30°C in South Florida, and the optimum temperature range for growth for this species is between 23 and 31°C (Barber and Behrens, 1985). This species tolerates short-term exposure to high temperatures (33-35 °C) but growth rapidly falls off when these temperatures are sustained (Zieman, 1975a). In Puerto Rico, Glynn (1968) found that temperatures of 35-40° C killed the leaves of T. tes-tudinum on a tidal flat, but that the rhizomes of the plants were apparently unaffected. On shallow vegetated banks, temperatures rise rapidly during spring low tides, and the high temperatures, coupled with desiccation, kill vast quantities of leaves that are later sloughed off. This defoliation process occurs sporadically throughout the year and seems to pose no long-term problem for the plants. Brouns (1985) reported similar negative effects of desiccation at extreme low tides for monospecific T. hemprichii meadows in Papua New Guinea. In the subtropical waters of Texas and Florida, the most severe mortality of T testudinum is usually caused by severe cold rather than heat, even though these seagrasses from the extreme northern distribution range showed a greater tolerance to chilling temperatures (at 2° C, during 412 h), than their southern counterparts (McMillan, 1979).
The optimum salinity range for T. testudinum has been reported to be from 24 to 35 psu (Phillips, 1960; McMillan and Moseley, 1967; Zieman, 1975a), but Thalassia tolerated a broad salinity range for very brief exposures, ranging from 3.5 to 5.0 psu (den Hartog, 1957 in Sculthorpe 1967) to 60 psu (McMillan and Moseley, 1967), although such exposures commonly result in leaf loss. Following the passage of hurricane Donna in South Florida in 1960, Thomas et al. (1961) considered the damage to T. testudinum by fresh water runoff to have been more severe than the physical effects of the high winds and water surge. Heavy freshwater releases reported in South Florida (Thomas et al., 1961) and Venezuela (Perez and Galindo, 2000) caused mortality of macro algae as well as vertebrate and invertebrate fauna associated with seagrass meadows, but although T. testudinum was heavily defoliated, the buried rhizomes remained intact, and leaf recovery was rapid.
T. testudinum requires a minimum sediment depth of ~25-50 cm to achieve lush growth (Zieman, 1975b), although meadow formation occurs at lesser sediment depths (Scoffin, 1970). Sediment composition and the proportion of finer-grained particles vary as a function of leaf density of T. testudinum . In the Bahamas, bare sediments had only 13% fine-grained material, while dense T. testudinum had more than 15% of fine-grained sands (Scoffin, 1970). Similarly, dense T. testudinum meadows raise the elevation within the meadows due to sedimentation (Ginsburg and Lowenstam, 1958; Zieman, 1975b; Durako andMoffler, 1985c), and increase the concentration of sedimentary organic matter (Wood et al., 1969) resulting in higher protein levels in rhizomes of T. testudinum from the middle of the beds (Durako and Moffler, 1985c).
T. testudinum may be found as deep as 10 m in clear water (Phillips and Meiiez, 1988), but is confined to maximum depths of 1-2 m in turbid habitats (Phillips, 1960; Zieman, 1982). For seagrasses in general, lower depth limits are set by ambient light levels (Dennison and Alberte, 1985), as the photosynthates produced in the leaves in light, are transported to subterranean tissues, where they are invested in new shoot growth (an investment on top of carbohydrate storage and respiration). Lee and Dunton (1996) reported that T. testudinum in Laguna Madre (Texas, USA), required an annual quantum flux in excess of 1628 mol m-2 in order to maintain a positive carbon balance, which corresponded with 14% of the surface irradiance.
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