dcsimg
509.39952810.130x130
» Archaeplastida »

Thalassia testudinum Banks & Sol. ex K. D. Koenig

Brief Summary

Comprehensive Description

    Comprehensive Description
    provided by EOL staff

    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
    provided by wikipedia

    Thalassia testudinum, commonly known as turtlegrass,[3] is a species of marine seagrass. It forms meadows in shallow sandy or muddy locations in the Caribbean Sea and the Gulf of Mexico.[4]

    Description

    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.[5][6] 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.[7]

    Distribution and habitat

    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.[6]

    Reproduction

    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.[6] 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.[5]

    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.[6]

    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.[8]

    The seeds are viviparous,[9] and can start new patches of seagrass, but it is thought that the main method of reproduction is vegetative.[6]

     src=
    Seagrass bed with dense Thalassia testudinum and an immature queen conch (Eustrombus gigas), Rice Bay, San Salvador Island, Bahamas

    Ecology

    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.[6] 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.[9]

    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.[6]

    References

    1. ^ den Hartog, C. (2005). "Taxonomy and biogeorgraphy of seagrasses". In Larkum, Anthony W.D.; Duarte, Carlos; Orth, Robert J. Seagrasses: Biology, Ecology and Conservation. Springer-Verlag New York, LLC. ISBN 978-1-4020-2942-4..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"""""'"'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}
    2. ^ Guiry, Michael D. (2012). Guiry MD, Guiry GM, eds. "Thalassia testudinum Banks ex König, 1805". AlgaeBase. National University of Ireland, Galway. World Register of Marine Species. Retrieved 2012-11-07.
    3. ^ "Thalassia testudinum". Natural Resources Conservation Service PLANTS Database. USDA. Retrieved 9 December 2015.
    4. ^ Guiry, M. D.; Guiry, G. M. (2012). "Thalassia testudinum Banks ex König". AlgaeBase. Retrieved 2012-11-08.
    5. ^ a b Colin, Patrick L. (1978). Marine Invertebrates and Plants of the Living Reef. T.F.H. Publications. pp. 474–475. ISBN 0-86622-875-6.
    6. ^ a b c d e f g Dineen, J. (2001-07-25). "Thalassia testudinum (Turtle grass)". Smithsonian Marine Station at Fort Pierce. Retrieved 2012-11-07.
    7. ^ "Turtle-grass (Thalassia testudinum)". Interactive Guide to Caribbean Diving. Marine Species Identification Portal. Retrieved 2012-11-08.
    8. ^ "Scientists Discover an Underwater Pollinator | Hakai Magazine". Hakai Magazine. Retrieved 2017-02-01.
    9. ^ a b Short, F. T.; Carruthers, T. J. R.; van Tussenbroek, B.; Zieman, J. (2010). "Thalassia testudinum". IUCN Red List of Threatened Species. Version 2012.2. Retrieved 2012-11-08.
     title=

Distribution

    Distribution
    provided by EOL staff

    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).

    Distribution
    provided by eFloras
    Fla., La., Tex.; Mexico; West Indies; Central America; South America (Colombia).

Morphology

    Comments
    provided by eFloras
    Thalassia testudinum is possibly the most important marine spermatophyte along the coasts of the Caribbean and Gulf of Mexico (C. den Hartog 1970). The species grows from the low-water mark to nearly 10 m depth in very clear water. Establishment occurs on a wide variety of substrates, including organic matter, rocky matter, coral sand, and dead reef-platforms. Once the species is established, the substrate type becomes less important, especially in areas of low current. Dead leaves and rhizomes accumulate among the erect living leaves for considerable periods of time. The beds are important not only in substrate development but also in substrate stabilization. Massive amounts of substrate are lost in areas without turtle-grass colonies during hurricanes, but only minimal loss occurs in turtle-grass beds. Substrate loss is lessened by roots and rhizomes binding the substrate, as well as by the leaves lowering water velocity.

    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).

    Description
    provided by eFloras
    Rhizomes elongate, 3--6 mm thick. Leaves 10--60 cm ´ 0.4--1.2 cmm, margins entire proximally, minutely serrulate near apex; veins 9--15. Inflorescences: staminate inflorescences 1--3-flowered; peduncles 3--7 cm; margins of spathes connate on 1 side; pistillate inflorescences 1-flowered; peduncles 3--4 cm; spathes connate on both sides. Flowers: staminate flowers: pedicels 1.2--2.5 cm; stamens 9; pistillate flowers nearly sessile, styles 7--8. Fruits bright green to yellow-green or red, 1.5--2.5 cm diam., dehiscing in 5--8 valves; beak 4--7 mm.

Habitat

    Habitat
    provided by EOL staff

    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)

    Habitat
    provided by eFloras
    Ocean floor consisting of organic matter, rock matter, coral sand, or dead reefs; -10--0m.

Associations

    Associations
    provided by EOL staff

    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).

General Ecology

    Ecology
    provided by EOL staff

    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)

Cyclicity

    Flowering/Fruiting
    provided by eFloras
    Flowering spring--summer.

Growth

    Development
    provided by EOL staff

    Under favorable conditions, Thalassia testudinum can grow several centimeters per day (Dineen 2001 and references therein).

Conservation Status

    Conservation Status
    provided by EOL staff

    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)

    Conservation Status
    provided by EOL authors
    Although the IUCN lists the conservation status of Turtle Grass as "Not Evaluated", there are significant conservation issues known to be associated with this marine plant. Notably the Manatee and other marine fauna are dependent on relationships with Turtle Grass, and correspondingly a number of protected areas have been established within the range of the Turtle Grass species. As an example, The Chetumal Bay Manatee Sanctuary is home to one of the largest remaining populations of Mexican Caribbean manatees, approximately 150 to 200 individuals, an animal that early Spanish explorers thought were mermaids. The sanctuary is situated in Chetumal Bay a national protected area of the Quintana Roo province of Mexico. The bay is a mixture of terrestrial and aquatic environments creating a landscape of exceptional beauty. The protected area also harbours other threatened and endangered species such as crocodiles (Crocodylus moreleti), the river white turtle (Dermatemys mawii) and jaguar (Pantera onca).

Threats

    Anthropogenic impacts on Thalassia testudinum Banks ex König
    provided by EOL authors
    Many large-scales declines of seagrass meadows, affecting several different species in a wide geographical range, have been reported in recent decades (8); in more than 70% of the cases, human-induced disturbances have been held responsible for them (8, 21). Thalassia testudinum, in particular, is a species with a well-documented history of large die-offs and local declines (2, 12, 14, 17). Human-related changes in nutrient levels, salinity, composition of the biological community and climate have been identified as sources (or potential sources) of disturbance for this species. Changes in nutrient levels: eutrophication. Population growth and the massive use of fertilizers in agriculture have led to a generalized exponential increase in nutrient inputs to the coastal zones (8), which is most likely the main cause of seagrass decline worldwide (8). The most common mechanism for seagrass decline under eutrophic conditions is light reduction through stimulation of epiphytes, macroalgae and phytoplankton overgrowth (1, 7), although direct physiological responses such as ammonium toxicity may also contribute (1). Specifically, seagrass leaves are often densely covered by epiphytic algae which can suppress seagrass productivity through the development of a thick coat as a result of nutrient enrichment (6, 7, 8); it has been proposed as a major mechanism for declines of seagrass meadows, and the importance of this factor has been experimentally proved for Th. testudinum (6, 7). However, it seems that the net effect of epiphytes on seagrass growth depends on a complex variety of ecological factors (7, 8). In a mesocosm experiment designed to test the effects of nutrient addition on the interactions among grazers, epiphytes, and Th. testudinum, it was shown that those effects depended critically on the intensity of grazing: in the presence of grazers, the turtle grass tended to produce a greater biomass in the tanks with higher nutrient load, but when grazers were absent, the direction of the effect was reversed, and plants with nutrients added grew less than the control plants (7). Obviously, the control exerted by nutrient loads on this species’ productivity can led to shifts on the community composition and even to drastic alterations in seagrass beds dominance (1). The occurrence of those shifts has been experimentally demonstrated in Florida bay, where some areas previously dominated by Th. testudinum were intentionally fertilized with bird guano for eight years. For the first two years, T. testudinum production was positively affected by the nutrient enrichment, but gradually Halodule wrightii, normally an early-successional species in subtropical habitats, colonized the beds, and by the end of the study accounted for 97% of the aboveground seagrass biomass (1, 5). This shift in species dominance persisted at least eight years after nutrients were no longer added, suggesting that once an area has been fertilized, the system can be resistant to change (1). The importance of considering watershed nutrient loads rather than water-column nutrient concentrations in assessing enriched conditions for Th. testudinum meadows (1) was shown in Sarasota Bay, Florida (24). There, the biomass and productivity of this species were negatively correlated with watershed N loads but not with water-column nutrient concentrations as assessed by routine monitoring programs (24). It demonstrates that the search for reliable early indicators of nutrient overenriched seagrass meadows is an urgent necessity in order to provide an effective management of these ecosystems (1); direct measures of water-column nutrients are generally ineffective, since in early phases of eutrophication the nutrients are rapidly taken up by plants, adsorbed to particulate sediments, or otherwise removed from the water (1). Changes in salinity. The role of human-induced changes in salinity as a possible cause of seagrass losses is not well understood (8). Some studies suggest that Th. testudinum meadows do not show an inmediate response to increased salinity; for example, a turtle grass meadow remained unaffected after more than six months exposure to the direct discharge of brine from a desalination plant (22). However, decreased freshwater inputs due to human activities have been proposed as a triggering factor for the most important die-off of Th. testudinum known to date, which occurred in Florida Bay in 1987 (17). About a third of the dense seagrass beds of western Florida were impacted over a period of just a year; 4,000 hectares were denuded and an additional 23,000 hectares were affected to a lesser degree (2, 17). This die-off was probably the result of complex processes caused by an interaction of several factors (1), including eutrophication, enhanced pathogen virulence and elevated water temperatures (1, 12, 14, 16), but several authors indicate that increased salinity was a key factor in the initiation of this phenomenon (8, 12, 16, 27). Groundwater flow into the bay decreased as a result of extraction of water from the region’s aquifers (8, 27); canal construction, which diverted drainage water, further contributed to a diminished freshwater inflow (8, 27). This decrease in freshwater inflow supposedly caused a gradual transition in the original composition of the seagrass vegetation, resulting in dense monospecific meadows of Th. testudinum (27) that suffered a sudden decline, probably as a consequence of sulfide self-poisoning and other effects (1, 8, 12, 16). Changes in composition of the biological community: invasive species. A recent literature review has revealed that at least 56 non-native species, primarily invertebrates and seaweeds, have been introduced to seagrass meadows all around the world, largely through shipping/boating activities and aquaculture (26). There is, however, few specific data about the effects of these invasions, particularly for seagrass community structure and function (26). Th. testudinum meadows have been locally colonized by the invasive green alga Caulerpa ollivieri Dostál, native from the Mediterranean Sea (26). To date, this alga has been reported from the Bahamas (11) and the Gulf of Mexico (9). A negative effect of C. ollivieri on Th. testudinum, by direct displacement, has been observed (11, 26). Eutrophication is considered a facilitating factor in the successful colonization of the meadows by C. ollivieri (11); in the locations were this invasive was recorded, Th. testudinum was covered by epiphytic algae, as typical under conditions of high nutrient loads, and C. ollivieri exhibited higher nutrient contents than native Caulerpa species, which was considered evidence for the role of sewage-derived nitrogen in its successful introduction (26). This proposition is consistent with the results of recent experiments in several different seagrass ecosystems, which confirmed that disturbance contributes to the invasibility of seagrass beds (26). Given the reduced knowledge about the role that introduced species, combined with nutrient pollution may play in global seagrass decline, more definitive studies and comprehensive surveys are required in this field (26). Climate change The global change in Earth’s systems and functioning which is currently taking place, mainly due to anthropogenic release of greenhouse gases into the atmosphere, is likely to alter to some degree most of the terrestrial and marine ecosystems. Seagrass meadows in general, and Th. testudinum populations in particular, may potentially be impacted by many factors related to climate change, including higher atmospheric concentration of CO2, higher temperatures, rising sea level, increase in the frequency and intensity of storms and increase in UV-B radiation (20). Rising levels of atmospheric CO2, and the consequent alterations in the CO2/HCO-3 equilibrium in marine water, is expected to have significant effects on seagrasses. Seagrass photosynthesis is frequently limited by the availability of dissolved inorganic carbon under natural conditions (20); it has been experimentally shown for Th. testudinum (3). This carbon limitation has been attributed to the thickness of the diffusion boundary layer surrounding leaf surfaces or to a relatively inefficient HCO-3 uptake system (20), factors which vary according to each species morphology and physiology (8). Differences among species in the ability to compete for carbon would be expected to lead to shifts in species abundances and distributions, but long-term experimental studies of seagrass community responses to carbon enrichment are lacking (20). In a similar way, increasing water temperature will directly affect seagrass metabolism, which may result in changes in patterns of species abundance and distribution (20). These direct effects will depend on the individual species’ thermal tolerances and their optimum temperatures for photosynthesis, respiration, and growth. Rising temperatures may also alter seagrasses through direct effects on flowering and seed germination (20); in Th. testudinum, flowering is apparently controlled by the water temperature (15), which implies that a phenological impact could be expected. As a consequence of global higher temperatures, which produce a thermal expansion of ocean water and an acceleration of polar ice cap melting (10, 20), sea level has been rising steadily along the 20th century (10). According to the IPCC 4th Assessment Report, the global average sea level rose about 3.1 mm/year over 1993 to 2003 (10); by the end of 21th century the projected level rise will go from 0.18 m to 0.59 m, depending on the emission scenarios (10), but the used model excludes future rapid dynamical changes in ice flow, which are likely to occur, resulting in even larger figures. The greatest direct impact of an increase in sea level will be an increase in the depth of water and the consequent reduction in available light to the bottom (20). A primary effect of increased water depth will be to alter the location of the maximum depth limit of plant growth, directly affecting seagrass distribution (20). Increased global temperatures are expected to increase the intensity and frequency of extreme weather events (4). A regime of increased storm disturbance may lead to a decrease in distribution of climax species like Th. testudinum and an increase in abundance of early colonizing and mid-successional species within the seagrass community (20). It is due to the fact that recolonization in seagrasses occurs primarily by means of horizontal rhizome growth and branching; in Th. testudinum the rates for both processes are slow [around 70 cm/year for horizontal rhizome growth, and a branching rate of 1 branch/1600 internodes (8)] compared with the growing rates showed by other smaller seagrasses (8). There is a general agreement that increases in the amount of UV-B radiation reaching the surface of the Earth will produce both mutagenic and physiological damage in plants, including seagrasses (20). For three seagrass species occurring in the Caribbean (Th. testudinum was not included in the study), it was shown that increased UV-B radiation negatively impacted their photosynthetic capacity (25). Under ozone depletion, increased UV-B radiation is most likely to affect tropical seagrass systems such as Th. testudinum meadows, where the greatest amount of direct UV light reaches the Earth’s surface (20).

Management

    Management
    provided by EOL staff

    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)