Turtle Grass (Thalassia testudinum) is an important seagrass found from Bermuda and southern Florida south to the Gulf of Mexico, the West Indies, Central America, and Venezuela. It can form very extensive beds in protected shallow waters that serve as both habitat and a food source for a tremendous diversity of organisms, among them sea turtles, which graze on T. testudinum and are the source of its common name. (Dineen 2001 and references therein)
Thalassia testudinum is a perennial grass growing from a long, jointed rhizome that may be buried 25 centimetres (9.8 in) deep in the substrate but is more usually found 5 to 10 cm (2 to 4 in) below the surface. Some nodes are leafless but others bear a tuft of several erect, linear leaf blades. These are up to 30 centimetres (12 in) long and 2 cm (0.8 in) wide and have rounded tips. The flowers grow on short stalks in the axils of the leaves and are greenish-white, sometimes tinged pink, and are followed by seed pods.
Turtle grass is found growing in meadows in calm shallow waters throughout the Caribbean Sea and the Gulf of Mexico and as far north as Cape Canaveral in Florida. Extensive meadows can be formed on muddy sand, coarse sandy and clayey seabeds, especially those with a calcareous content. This grass favours high-salinity waters with low turbidity such as calm lagoons. It cannot grow in fresh water but some growth is possible at a salinity of 10 parts per thousand. The plant's preferred range is 25 to 38.5 parts per thousand with a temperature range of 20 to 30 °C (68 to 86 °F). It is found from low-tide mark down to depths of 30 metres (98 ft) depending on water clarity. It often grows in meadows with other seagrasses where it is the climax species.
Turtle grass can reproduce both vegetatively and by sexual reproduction. The main propagative method is by increase in length of the rhizomes. This mainly takes place in spring and early summer but can happen at any time of year and results in an increase in the size of the turtle grass bed. It has been found that where plants have been damaged mechanically, such as by the propellers of boats, the cut ends of rhizomes are unable to grow and holes may develop in the turtle grass meadow.
Unusually for the marine environment, turtle grass is a flowering plant. In the spring and early summer, many turtle grass plants produce small flowers at the base of the leaves. Male and female flowers grow on separate plants. Fruits develop in two to four weeks and became detached and float away after about eight weeks.
It has been recently discovered that zooplankton acts as a pollinator for turtle grass. Plankton is drawn to the seagrass’s nutritious mucilage—a carbohydrate-rich substance that houses pollen. As the plankton feeds on the mucilage, excess pollen grains stick to their bodies. Plankton then moves from seagrass to seagrass feeding and spreading the pollen.
Turtle grass and other seagrasses form meadows which are important habitats and feeding grounds. Associated seagrass species include Halophila engelmannii and Syringodium filiforme. Many epiphytes grow on the grasses, and algae, diatoms and bacterial films cover the surface of the leaf blades. The grass is eaten by turtles, herbivorous parrotfish, surgeonfish and sea urchins while the leaf surface films are a food source for many small invertebrates. Decaying turtle grass leaves are responsible for the majority of detritus in meadow areas. This grass is subject to periodic dieback episodes in the Florida Bay area. One such episode in 1987 killed off a large proportion of the plants and the resulting increased sedimentation and greater growth of epiphytes on the remaining plants caused a secondary dieback event. The areas affected have since been reseeded and planted with rhizomes and have recovered. In general, the population of this grass is stable.
Rhizomatous green algae in the genus Caulerpa often live among the grasses and many animals make seagrass meadows their home. These include bivalves and other molluscs, polychaete worms, amphipods and juvenile fish which hide among the leaf blades, sea urchins, crabs and caridean shrimps.
Thalassia testudinum is restricted to the Gulf of Mexico and the Caribbean and has been recorded from Bermuda (the other species in the genus, T. hemprichii, is widely distributed in the coastal waters of the Indian Ocean and the western Pacific) (Larkum et al. 2006).
Posidonia oceanica (Linnaeus) Delile was included in the Texas flora (D. S. Correll and M. C. Johnston 1970; D. S. Correll and H. B. Correll 1972) because of specimens washed ashore along the Gulf of Mexico. The specimens were later determined to be Thalassia testudinum, based upon comparative growth studies and upon flavonoid chemistry profiles (C. McMillan et al. 1975).
Thalassia testudinum grows in shallow coastal waters that are protected from strong wave surge. In clearer water it can be found at greater depths than in murky water. (Dineen 2001 and references therein)
Seagrass beds provide food and shelter, both directly and indirectly, to many ecologically and economically important fish and shellfish species. Over 100 species of fishes and over 30 crustacean species are found in Florida Bay, including both permanent residents and temporary residents using seagrass habitat as a nursery ground, such as spotted seatrout, redfish, snook, tarpon, snappers, and grunts. Important shellfish species include pink shrimp from the Tortugas bank, blue crabs, and spiny lobsters. (Robblee 1991)
Kirsch et al. (2002) studied grazing by smaller herbivores (e.g. the bucktooth parrotfish Sparisoma radians) on Thalassia tedudinum in Hawk Channel, in the northern Florida Keys (USA). They found that that seagrass grazing varied greatly both spatially and seasonally but, on average, grazers consumed virtually all of the aboveground production at 2 of the 3 sites studied. When experiments were repeated in the summer of a second year at 6 sites, seagrass grazing again varied greatly among sites, but at 3 of the sites most of the daily production of seagrass shoots was consumed by small herbivorous fishes. These results suggest that while it is undoubtedly true that modern day grazing by manatees, turtles, and waterfowl on seagrass is reduced relative to historical levels due to declines in populations of these large grazers, small vertebrate grazers nevertheless consume a substantial fraction of seagrass production in the northern Florida Keys.
A variety of sea urchins may graze heavily on Thalassia testudinum, sometimes even overgrazing (i.e., grazing at a rate that exceeds the seagrass growth rate), which may dramatically reduce seagrass biomass, leading to a restructuring of the local ecosystem (Eklof et al. 2008 and references therein).
Tussenbroek and Brearley (1998) found a burrowing isopod, Limnoria simulata, in sheaths of Thalassia testudinum in the Puerto Morelos reef lagoon, Mexican Caribbean, and this isopod likely has this habit across the Caribbean (Tussenbroek and Brearley 1998).
The widespread decline of seagrass, in particular Thalassia testudinum, in Florida Bay (Florida, U.S.A.) in 1987 was followed by a cascade of ecological effects. By 1992, frequent phytoplankton blooms began to appear in the central and western bay where none had been recorded previously. Negative impacts extended to higher trophic levels as well, including 100% mortality of some sponge species. Spiny lobster and pink shrimp catches at Tortugas Banks plunged in 1988 to their lowest levels in decades and game fish catch also declined. Algae blooms persist and the bloom ‘‘footprint’’ has expanded to include the eastern bay. (Madden et al. 2009 and references therein)
Under favorable conditions, Thalassia testudinum can grow several centimeters per day (Dineen 2001 and references therein).
in 1987, a mass die-off of Thalassia testudinum began in Florida Bay (Robblee et al. 1991). Robblee et al. estimated that over 4,000 hectares of seagrass beds had been denuded and an additional 23,000 hectares were affected to a lesser degree. About a third of the dense seagrass beds of western Florida were impacted over a period of just a year. Until the 1980s, Florida Bay was widely viewed as a healthy and stable ecosystem, with clear water, lush seagrass beds, and highly productive fish and shrimp populations. By 1992, the ecosystem appeared to have changed from a clear water system, dominated by benthic primary production, to a turbid water system, with algae blooms and resuspended sediments in the water column. (Rudnick et al. 2005 and references therein)
A defining feature of Florida Bay is its shallow depth, which averages just one to two meters. Light sufficient to support photosynthesis can reach the sediment surface in almost all areas of the bay, resulting in dominance of seagrass beds as both a habitat and a source of primary production (i.e., capturing energy from the sun through photosynthesis). In some portions of the bay, salinity can rise rapidly during drought periods due to water loss from evaporation exceeding input from precipitation and freshwater inflow. Following observations of Florida Bay’s dramatic ecological changes in the 1980s, it was commonly assumed that a direct cause of these changes was a longterm increase in salinity, which in turn was caused by the diversion of freshwater away from Florida Bay via South Florida Water Management District canals. However, subsequent research has indicated that these ecological changes may not be attributable to a single cause. While decreased freshwater inflow and resultant increased salinity have been part of the problem, it appears that other human activities, as well as natural forces, may have also played a role. (Rudnick et al. 2005 and references therein) Duarte (2002) notes that the causes of this die-off continue to be debated, and may include, among others, increased anthropogenic (i.e., human-caused) nutrient loading, the effects of climatic changes involving a long time interval without hurricanes affecting the area also causing unusually low freshwater discharge, and the effects of the increased accumulation of detritus derived from loss of large grazers. Difficulties in experimenting at the appropriate scale of entire seagrass meadows to test these hypotheses have made it difficult to assess to what degree the decline was due to human or natural causes, or a combination of both. (Rudnick et al. 2005 and references therein)
Regardless of the cause of the mass-mortality event, once it was initiated, the ecology of Florida Bay changed. Continued seagrass mortality results in increased sediment resuspension and increased nutrient (nitrogen and phosphorus) release from sediments, stimulating phytoplankton growth in the water column. The presence of phytoplankton and suspended sediment results in decreased light penetration to seagrass beds. This decreased light can limit seagrass growth and sustain the feedback loop. Dynamics of this feedback loop are probably not independent of the salinity regime. Seagrass wasting disease, caused by a slime mold (Labyrinthula sp.) infection, is more common at salinities close to or greater than seawater than at low salinities. High salinity may have played a role in the initial seagrass mass mortality event, but more likely has served to promote seagrass re-infection since that event. Incidence of this disease may therefore be directly affected by water management actions. (Rudnick et al. 2005 and references therein)
On a global scale, seagrasses--marine flowering plants that include the widely distributed genera Zostera, Thalassia, and Posidonia--in general appear to be in trouble (Waycott et al. 2009). Seagrasses form some of the most productive ecosystems on earth, rivaling even crops of corn and sugar cane. Seagrass meadows provide ecosystem services such as supporting commercial fisheries worth as much as $3500 per hectare per year, subsistence fisheries that support entire communities, nutrient cycling, sediment stabilization, and globally significant sequestration of carbon. Seagrasses and the services they provide are threatened by the immediate impacts of coastal development and growing human populations as well as by the impacts of climate change and ecological degradation. Seagrass losses also disrupt important linkages between seagrass meadows and other habitats, and their ongoing decline is likely producing much broader and long-lasting impacts than the loss of the meadows themselves. (Waycott et al. 2009 and references therein)
Rudnick et al. (2005) emphasize that if the state of the seagrass community is to be used as a criterion to guide and assess the success of environmental restoration efforts, scientists and managers must specify the desirability of alternative states. Based on studies of historic changes of seagrass communities in Florida Bay and anecdotal information. it is likely that the Florida Bay of the 1970s and early 1980s, with lush T. testudinum and clear water, was probably a temporary and atypical condition. From an ecological perspective, restoration should probably strive for a more diverse seagrass community with lower T. testudinum density and biomass than during that anomalous period. (Rudnick et al. 2005 and references therein)
If efforts to restore the Everglades are successful, patterns of freshwater flow toward more natural patterns will drive Florida Bay’s seagrass community and trophic web toward its pre-drainage condition. Decreased salinity caused by increasing freshwater flow would likely have a direct effect on seagrass communities through physiological mechanisms, resulting in greater spatial heterogeneity of seagrass beds, a decrease in the dominance of T. testudinum, and an increase in coverage by other seagrass species. Decreased salinity would also likely decrease the infection of T. testudinum by the slime mold Labyrinthula. Light availability depends on phytoplankton growth and sediment resuspension, which in turn depend on nutrient availability, grazing, and stabilization of sediments by seagrass beds. (Rudnick et al. 2005)