Thalassia Testudinum Research Articles

Effect of habitat complexity on predation success: re-evaluating the current paradigm in seagrass beds

The complex structure of seagrass beds provides increased protection for prey com- pared to unvegetated substrates. Additionally, most studies support the notion that predation inten- sity is negatively correlated with seagrass density. However, virtually every study to previously test this idea has used the same number of predators and prey across a gradient of seagrass density treat- ments. This design ignores another well-established fact: invertebrate and fish abundance increase with increasing seagrass density. Therefore, it makes sense to carry out predation experiments that increase the number of both predators and prey as seagrass density increases. Outdoor tank experi- ments (7 density treatments: 0 to 3000 leaves m -2 ) were carried out using artificial seagrass units modeled after turtlegrass Thalassia testudinum and field mesocosm experiments (4 density treat- ments: 0 to 3000 leaves m -2 ) in living T. testudinum in Big Lagoon (Florida, USA). In these experi- ments the absolute number of predators (the pinfish Lagodon rhomboides) and prey (grass shrimp Palaemonetes pugio) were increased with increasing seagrass density. Both experiments yielded similar results: the only significant difference in predation rate was between the unvegetated treat- ment and the seagrass treatments as a group. These results suggest that dense seagrass does not nec- essarily provide increased protection for prey organisms.

Clonal diversity and structure related to habitat of the marine angiosperm Thalassia testudinum along the Atlantic coast of Mexico

The clonal structure of the tropical seagrass Thalassia testudinum was studied at 16 sites along the Mexican Atlantic coast, situated in back-reef, shallow coastal and lagoon habitats. Thalassia testudinum was highly clonal, with an overall average clonal richness ( R) of 0.55. The largest genet found in this study extended throughout the sampling area (∼230 m), with an estimated max age almost reaching 600 years. Lagoons with higher nutrient availability reflected by nutrient content of leaves (mean leaf C:N ratio 11.4) and lower hydrodynamic regimes reflected by the percentage of fine sediments (on average 23%), sustained larger genets of T. testudinum (mean of the largest genets over populations was 167.3 m) than the shallow coastal areas (C:N 12.3, 6.2% fine sediment, mean largest genet 10.3 m) and the more oligotrophic back-reefs (C:N 16.3, 2.7% fine sediment, mean largest genet 6.5 m). Population genetic analysis showed different levels of clonality, genotypic diversity and spatial genetic relatedness for this seagrass per habitat, with the lagoons presenting much lower levels of clonal diversity than the other two habitats.

Importance of seagrass as a carbon source for heterotrophic bacteria in a subtropical estuary (Florida Bay)

A stable carbon isotope approach was taken to identify potential organic matter sources incorporated into biomass by the heterotrophic bacterial community of Florida Bay, a subtropical estuary with a recent history of seagrass loss and phytoplankton blooms. To gain a more complete understanding of bacterial carbon cycling in seagrass estuaries, this study focused on the importance of seagrass-derived organic matter to pelagic, seagrass epiphytic, and sediment surface bacteria. Particulate organic matter (POM), seagrass epiphytic, seagrass ( Thalassia testudinum) leaf, and sediment surface samples were collected from four Florida Bay locations with historically different organic matter inputs, macrophyte densities, and primary productivities. Bulk (observed and those reported previously) and compound-specific bacterial fatty acid δ 13C values were used to determine important carbon sources to the estuary and benthic and pelagic heterotrophic bacteria. The δ 13C values of T. testudinum green leaves with epiphytes removed ranged from −9.9 to −6.9‰. Thalassia testudinum δ 13C values were significant more enriched in 13C than POM, epiphytic, and sediment samples, which ranged from −16.4 to −13.5, −16.2 to −9.6, and −16.7 to −11.0‰, respectively. Bacterial fatty acid δ 13C values (measured for br14:0, 15:0, i15:0, a15:0, br17:0, and 17:0) ranged from −25.5 to −8.2‰. Assuming a −3‰ carbon source fractionation from fatty acid to whole bacteria, pelagic, epiphytic, and sediment bacterial δ 13C values were generally more depleted in 13C than T. testudinum δ 13C values, more enriched in 13C than reported δ 13C values for mangroves, and similar to reported δ 13C values for algae. IsoSource mixing model results indicated that organic matter derived from T. testudinum was incorporated by both benthic and pelagic bacterial communities, where 13–67% of bacterial δ 13C values could arise from consumption of seagrass-derived organic matter. The IsoSource model, however, failed to discriminate clearly the fraction of algal (0–86%) and mangrove (0–42%) organic matter incorporated by bacterial communities. These results indicate that pelagic, epiphytic, and sediment surface bacteria consumed organic matter from a variety of sources. Bacterial communities incorporated consistently seagrass-derived organic matter, the dominant macrophyte in Florida Bay, but seagrass δ 13C values alone could not account fully for bacterial δ 13C values.

Stable Isotopes Reveal Complex Changes in Trophic Relationships Following Nutrient Addition in a Coastal Marine Ecosystem

Complex links between the top-down and bottom-up forces that structure communities can be disrupted by anthropogenic alterations of natural habitats. We used relative abundance and stable isotopes to examine changes in epifaunal food webs in seagrass (Thalassia testudinum) beds following 6 months of experimental nutrient addition at two sites in Florida Bay (USA) with different ambient fertility. At a eutrophic site, nutrient addition did not strongly affect food web structure, but at a nutrient-poor site, enrichment increased the abundances of crustacean epiphyte grazers, and the diets of these grazers became more varied. Benthic grazers did not change in abundance but shifted their diet away from green macroalgae + associated epiphytes and towards an opportunistic seagrass (Halodule wrightii) that occurred only in nutrient addition treatments. Benthic predators did not change in abundance, but their diets were more varied in enriched plots. Food chain length was short and unaffected by site or nutrient treatment, but increased food web complexity in enriched plots was suggested by increasingly mixed diets. Strong bottom-up modifications of food web structure in the nutrient-limited site and the limited top-down influences of grazers on seagrass epiphyte biomass suggest that, in this system, the bottom-up role of nutrient enrichment can have substantial impacts on community structure, trophic relationships, and, ultimately, the productivity values of the ecosystem.

High levels of gene flow and low population genetic structure related to high dispersal potential of a tropical marine angiosperm

Marine ecosystems are traditionally considered to be highly connected, caused by long- distance movements of propagules in an open fluid medium. But this principle is not universal, and populations of many marine organisms have restricted gene-flow, including ones with drifting propagules. Direct tracking of marine free-floating propagules over large distances is virtually impossible, but long-distance dispersal (LDD) can be deduced from their drifting time and transport rates. Alternatively, LDD can be estimated as genetic exchange with the use of allelic frequency data obtained with genetic markers. The present study compares both approaches of LDD estimation for the tropical seagrass Thalassia testudinum. Fruit dispersal potential was measured in Puerto Morelos reef lagoon, Mexico. Between 10 and 15% of the fruits floated to the surface (floating potential <1 to 10 d), with a derived potential dispersal of <1 to 360 km. The remainder of the fruits dehisced in situ followed by limited seed dispersal (<1 to 10 m). Genetic diversity was characterized using 6 microsatellite loci for 16 populations distributed along a 1350 km long stretch of the Mexican coast, revealing a total of 58 alleles. AMOVA showed that only 9.75% of the total variation was accounted for by genetic differences between the populations. Isolation by distance was significant, and revealed a panmictic area of 350 km. The results of both approaches of LDD estimation coincided well and the maximum displacement of the fruits of this marine angiosperm exceeded, by 1 to 2 orders of magnitude, the reported seed dispersal by terrestrial seed plants.

Commensal Foraging Between Double-crested Cormorants and a Southern Stingray

Abstract We describe a commensal relationship between Double-crested Cormorant, (Phalacrocorax auritus) and a southern stingray (Dasyatis americana). Three cormorants were observed closely following a large southern stingray in a shallow turtle grass (Thalassia testudinum) bed near Long Key, Florida and feeding upon a small fish disturbed by the stingray's benthic foraging. This represents an example of a commensal relationship in which one participant benefits while the other is unaffected.

Sulfide intrusion in the tropical seagrasses Thalassia testudinum and Syringodium filiforme

Sulfur and oxygen dynamics in the seagrasses Thalassia testudinum and Syringodium filiforme and their sediments were studied in the US Virgin Islands (USVI) in order to explore sulfide intrusion into tropical seagrasses. Four study sites were selected based on the iron concentration in sediments and on proximity to anthropogenic nutrient sources. Meadow characteristics (shoot density, above- and below-ground biomass, nutrient content) were sampled along with sediment biogeochemistry. Sulfide intrusion was high in T. testudinum, as up to 96% of total sulfur in the plant was derived from sediment-derived sulfides. The sulfide intrusion was negatively correlated to the turnover of sulfides in the sediments regulated by both plant parameters and sediment sulfur pools. Sediment iron content played an indirect role by affecting sulfide turnover rates. Leaf production was negatively correlated with sulfide intrusion suggesting that active growth reduced sulfide intrusion. Sulfide intrusion was lower in S. filiforme (up to 44%) compared to T. testudinum consistent with a higher internal nighttime oxygen concentrations found for S. filiforme. When S. filiforme can take advantage of its ability to maintain high internal oxygen concentrations, as was the case on the USVI, it could increase its success in colonizing unvegetated disturbed sediments with potentially high sulfide concentrations.

Phosphorus release with carbonate dissolution coupled to sulfide oxidation in Florida Bay seagrass sediments

We hypothesized that CaCO3 dissolution, coupled to sulfide oxidation, is an important mechanism by which solid‐phase inorganic P (iP) becomes available to seagrass in tropical carbonate sediments. To examine this supposition, we measured field sulfate reduction rates and simulated the acidity (10‐50 µmol H2SO4 cm−3 sediment) generated by subsequent sulfide oxidation from high (western) and low (eastern) total P (TP) sediments in Florida Bay. Dissolution experiments were conducted using sediment slurries at field pH (pore‐water pH ~6.5‐7.5). While CaCO3 dissolution (maximum 1.3% of sediment dry weight) was evidenced at all sites by leaching of Ca2+ into slurries, at three of the four sites PO43‐ was primarily recovered in the sediment exchangeable pools and at a lower P : Ca ratio than observed in source sediments. In contrast, no PO43‐ was recovered from an eastern bay site with the lowest TP and finest‐grained sediments, suggesting that PO43‐ was either tightly adsorbed or incorporated into the carbonate matrix post‐acidification. The potential for tight PO43‐ resorption by sediment from the low‐TP site was also supported by the rapid rate of isotopic exchange of 33PO43‐ into the sediment matrix. These adsorptive and incorporation processes may explain the low PO43‐ in pore waters in the eastern vs. western regions of Florida Bay, even upon dissolution of carbonates. Carbonate dissolution coupled to sulfide oxidation could potentially provide 1.8 and 23.5 µmol iP m−2 d−1 at eastern and western bay sites, respectively, meeting 5% of eastern and 29% of western bay seagrass (Thalassia testudinum) P requirements.

Evidence of negative impacts of ecological tourism on turtlegrass (Thalassia testudinum) beds in a marine protected area of the Mexican Caribbean

Many marine protected areas (MPAs) have been established in recent years. Some MPAs are open to tourists to foster environmental education and generate revenue for the MPA. This has been coined “ecological tourism”. Here, we examine the impact of ecological tourism on turtlegrass (Thalassia testudinum) health in one area of the “Costa Occidental de Isla Mujeres, Punta Cancun y Punta Nizuc” MPA in the Mexican Caribbean. A heavily visited location was compared with an unvisited location. Turtlegrass leaves at the visited location were sparser, shorter, grew more slowly, and had more epiphytes than at the unvisited location. Vertical and horizontal rhizomes of turtlegrass also grew more slowly at the visited than at the unvisited location. There is reasonable evidence to suggest that the observed differences are likely due to the deleterious impacts of novice and careless snorkelers. If continuing, these impacts could cause severe degradation of the visited areas in this MPA and, thus, changes in management policies seem in order.

Ecological indicators for assessing and communicating seagrass status and trends in Florida Bay

Abstract A suite of seagrass indicator metrics is developed to evaluate four essential measures of seagrass community status for Florida Bay. The measures are based on several years of monitoring data using the Braun-Blanquet Cover Abundance (BBCA) scale to derive information about seagrass spatial extent, abundance, species diversity and presence of target species. As ecosystem restoration proceeds in south Florida, additional freshwater will be discharged to Florida Bay as a means to restore the bay's hydrology and salinity regime. Primary hypotheses about restoring ecological function of the keystone seagrass community are based on the premise that hydrologic restoration will increase environmental variability and reduce hypersalinity. This will create greater niche space and permit multiple seagrass species to co-exist while maintaining good environmental conditions for Thalassia testudinum , the dominant climax seagrass species. Greater species diversity is considered beneficial to habitat for desired higher trophic level species such as forage fish and shrimp. It is also important to maintenance of a viable seagrass community that will avoid die-off events observed in the past. Indicator metrics are assigned values at the basin spatial scale and are aggregated to five larger zones. Three index metrics are derived by combining the four indicators through logic gates at the zone spatial scale and aggregated to derive a single bay-wide system status score standardized on the System-wide Indicator protocol. The indicators will provide a way to assess progress toward restoration goals or reveal areas of concern. Reporting for each indicator, index and overall system status score is presented in a red–yellow–green format that summarizes information in a readily accessible form for mangers, policy-makers and stakeholders in planning and implementing an adaptive management strategy.

Survival and Expansion of Mechanically Transplanted Seagrass Sods

Abstract Although planting seagrass is not technically complex, the ability to plant large areas is limited by the time‐consuming nature of manual methods. Additionally, manual methods use small, spatially isolated planting units (PUs; shoot bundles or plugs/cores) that are often highly susceptible to disturbance. The likelihood for harvesting intact apical meristems may be higher with large sods compared to smaller units, thus increasing survival and expansion rates. Here, we examined the survival and expansion of large units (1.5 × 1.2 m) of seagrass transplanted using a mechanized planting boat (Giga Unit Transplant System; GUTS). Twenty‐seven units of seagrass (18 Halodule wrightii and 9 Thalassia testudinum) were transplanted and monitored for survival, shoot density, and expansion. After 3 years, 74.1% of the units had survived (66.7% H. wrightii and 88.9% T. testudinum) with 12 H. wrightii units having expanded substantially beyond the bounds of the original PU, merging with adjacent units to form spatially continuous patches of seagrass. High survival rates for T. testudinum should be interpreted in light of concomitant declines in density and lack of significant expansion after 3 years. In its tested configuration, the GUTS was a viable method for transplanting H. wrightii where donor and receiver sites were in close proximity (&lt;2 km; a current limitation of the GUTS design used here). However, based on the reduced density and lack of significant expansion of T. testudinum that has persisted 3 years post‐transplant, the GUTS cannot yet be fully recommended for transplanting this species.

Laboratory investigation of the acoustic response of seagrass tissue in the frequency band 0.5–2.5 kHz

Previous in situ investigations of seagrass have revealed acoustic phenomena that depend on plant density, tissue gas content, and free bubbles produced by photosynthetic activity, but corresponding predictive models that could be used to optimize acoustic remote sensing, shallow water sonar, and mine hunting applications have not appeared. To begin to address this deficiency, low frequency (0.5-2.5 kHz) acoustic laboratory experiments were conducted on three freshly collected Texas Gulf Coast seagrass species. A one-dimensional acoustic resonator technique was used to assess the biomass and effective acoustic properties of the leaves and rhizomes of Thalassia testudinum (turtle grass), Syringodium filiforme (manatee grass), and Halodule wrightii (shoal grass). Independent biomass and gas content estimates were obtained via microscopic cross-section imagery. The acoustic results were compared to model predictions based on Wood's equation for a two-phase medium. The effective sound speed in the plant-filled resonator was strongly dependent on plant biomass, but the Wood's equation model (based on tissue gas content alone) could not predict the effective sound speed for the low irradiance conditions of the experiment, in which no free bubbles were generated by photosynthesis. The results corroborate previously published results obtained in situ for another seagrass species, Posidonia oceanica.

Phytoplankton bloom status: Chlorophyll a biomass as an indicator of water quality condition in the southern estuaries of Florida, USA

Altered freshwater inflows have affected circulation, salinity, and water quality patterns of Florida Bay, in turn altering the structure and function of this estuary. Changes in water quality and salinity and associated loss of dense turtle grass and other submerged aquatic vegetation (SAV) in Florida Bay have created a condition in the bay where sediments and nutrients have been regularly disturbed, frequently causing large and dense phytoplankton blooms. These algal and cyanobacterial blooms in turn often cause further loss of more recently established SAV, exacerbating the conditions causing the blooms. Chlorophyll a (CHLA) was selected as an indicator of water quality because it is an indicator of phytoplankton biomass, with concentrations reflecting the integrated effect of many of the water quality factors that may be altered by restoration activities. Overall, we assessed the CHLA indicator as being (1) relevant and reflecting the state of the Florida Bay ecosystem, (2) sensitive to ecosystemdrivers (stressors,especially nutrient loading), (3)feasible to monitor, and (4) scientifically defensible. Distinct zones within the bay were defined according to statistical and consensual information. Threshold levelsof CHLA for each zonewere defined using historical data and scientific consensus. A presentation template of condition of the bay using these thresholds is shown as an example of an outreach product.

Seagrass Loss in Belize: Studies of Turtlegrass (Thalassia testudinum) Habitat Using Remote Sensing and Ground-Truth Data

Spatial and temporal change in turtlegrass (Thalassia testudinum) habitat of the South Water Caye Marine Reserve (SWCMR) in Belize were analyzed using satellite images backed up with ground-truth data. We had two primary objectives. First, we wanted to determine areal expanse of seagrass across a large area (~12 km by 3 km) of the SWCMR, and address its change over time. We used paired satellite images taken during 2001 and 2005 to determine coverage by seagrass and measure temporal variables. These analyses recorded an overall seagrass loss of 1.8% (52.3 ha) during the 4 yr period. Secondly, we wanted to determine whether seagrass gains or losses were consistent across the study area. Replicate sampling was used as a statistical basis and confi rmed a signifi cant loss of seagrass across the region. It also helped identify two regions of signifi cant seagrass loss; one 600 ha area lost 12.4% of its seagrass; another 240 ha area lost nearly 40%. These components helped us assess seagrass habitat in an area perceived as critical to Belize fi sheries, and provided the scale and statistical rigor necessary to adequately assess a broad region of study. The salient results from our study were not the magnitude of seagrass loss per se, but the loss in seagrass habitat from an area that is thought to be relatively pristine. Seagrass-habitat loss in this region of the Caribbean Sea may be evidence that even near-pristine areas can be impacted by anthropogenic factors. Determining the causes of habitat loss may help prevent loss of productivity, habitat, and livelihood for the associated human and nonhuman communities. INTRODUCTION Seagrass ecosystems are among the most productive on earth, and their ecological and economic importance is becoming obvious as they diminish worldwide. Seagrass habitats are vital primary producers that improve water quality, promote sedimentation, recycle nutrients, and provide structure that serves as refuge and nursery ground for fisheries species (e.g., Moriarty and O’Donohue 1994, Hall et al. 1999, Short and Wyllie-Echeverria 2000, Gillanders et al. 2003, Green and Short 2003, Corlett and Jones 2005). The interest in seagrass is global, as is the research effort to assess changes in seagrass habitat. Many studies centered around areas of long-term loss of seagrass (e.g., Short and Short 2003, Duarte et al. 2008), while others documented regions of seagrass recovery (e.g., Virnstein et al. 2007). Turtlegrass (Thalassia testudinum) is a common seagrass species in waters of the tropical western Atlantic from Venezuela to eastern Florida and the Bahamas (den Hartog 1970), and is one of many species adversely affected by natural and anthropogenic factors. Researchers contend that protecting seagrass habitat may prevent loss of commercial fisheries, improve water quality, and help maintain healthy interrelated communities (Ward 1998). The purpose of this study was to discern the stability of the turtlegrass-dominated seagrass community in the South Water Caye Marine Reserve (SWCMR) of Belize by using satellite imagery. This reserve is generally perceived to be pristine, far from the coastal influences that adversely affect most other seagrass communities, and an area of interest to Belize fisheries and tourism. We had two primary objectives. First, we wanted to determine areal expanse of seagrass, and address its change over time by analyzing satellite images taken 4 years apart. Secondly, we wanted to determine whether seagrass gains or losses were consistent across the study area, and used ground truthing to confirm our observations by visiting sites of concern identified by satellite images. The multiple components of our study helped assess seagrass habitat in an area perceived as critical to Belize fisheries, and provided the scale and statistical rigor necessary to adequately assess a broad region of study.

A Land-Based Thalassia testudinum Nursery Near Tampa Bay, Florida

An experimental, land-based seagrass nursery, built near Tampa Bay, FL, produced cultivars of Thalassia testudinum Banks and Solander ex König (turtle grass). The nursery was a rectangular basin measuring 6.1 X 12.2 m that graded from 1.5 to 2.0 m in depth constructed inside a fiberglass-covered pole building. The basin had a vinyl liner covered with 10–20 cm of sand and 1.0–1.3 m of water. The nursery ran for 26 mo and focused on producing cultivars of T. testudinum using differing planting techniques. Survival rates were 84% after 3–12 mo in the first planting and 91% after 4 mo in the second planting because early harvesting was required as a result of leaks in the vinyl liner. The planting method that resulted in the highest survival rate was bare rhizomes with two or more short shoots. The cultivars from the two nursery experiments were used in two mitigation projects in Tampa Bay. The nonrecurring cost of the nursery was $12,081.45 over a 3-yr period, and recurring costs are estimated at $22,280.00, with a potential production of 2,500 turf-like 20-cm2 units of T. testudinum that could be sold at $20.00 per planting unit.

UVB radiation may ameliorate photoinhibition in specific shallow-water tropical marine macrophytes

Anthropogenic stratospheric ozone depletion causes an increase in UVB radiation impinging on the earth's surface, which is a threat to plants not adapted to higher UVB irradiances. Investigations were carried out among tropical marine macrophytes, Turbinaria turbinata, Sargassum polyceratium var. ovatum, Padina sanctae-crucis, Lobophora variegate, Dictyota spec., Halimeda discoidea, Udotea flabellum, Thalassia testudinum and Syringodium filiforme collected from 0.3 to 26 m depths at the Belizean barrier reef, where ultraviolet radiation (UV)-irradiances are naturally high. Photoinhibition was induced under full solar, UV (UVA + UVB), and UVA only-depleted radiation conditions. Photosynthetic activity during high radiation stress and during recovery in reduced solar radiation was determined in vivo by measuring fluorescence changes using a PAM fluorometer device. Generally, UV caused an additional decrease of photosynthetic performance during high light stress which varies according to species, depth of growth and UV penetration at the site of collection; an observation in concordance with the conventional harmful UV-radiation effects on phototrophs. When solar radiation was reduced by 50%, significant photosynthetic recovery was observed. However, some shallow water species which are adapted to high UV were observed to recover less under treatment with depleted solar UVB radiation. Our result supports earlier reports that UVB causes not only negative effects on photosynthesis, but may also facilitate or induce recovery processes in aquatic macrophytes acclimated to high solar radiation which grow at the upper shoreline. Among the eulittoral macroalgae, e.g. Dictyota spec., P. sanctae-crucis, and H. discoidea and the seagrass T. testudinum, initiation of photosynthetic recovery processes in the presence of low irradiance of short UV-wavelengths may present an ecophysiological advantage compared to macrophytes which initiate photosynthetic recovery process during low light or in the absence of UV.

Phosphorus Availability and Salinity Control Productivity and Demography of the Seagrass Thalassia testudinum in Florida Bay

Biomass, net primary productivity (NPP), foliar elemental content, and demography of Thalassia testudinum were monitored in populations from five sites across Florida Bay beginning in January 2001. Sites were selected to take advantage of the spatial variability in phosphorus (P) availability and salinity climates across the bay. Aboveground biomass and NPP of T. testudinum were determined five to six times annually. Short-shoot demography, belowground biomass, and belowground NPP were assessed from a single destructive harvest at each site and short-shoot cohorts were estimated from leaf scar counts multiplied by site-specific leaf production rates. Biomass, relative growth rate (RGR), and overall NPP were positively correlated with P availability. Additionally, a positive correlation between P availability and the ratio of photosynthetic to non-photosynthetic biomass suggests that T. testudinum increases allocation to aboveground biomass as P availability increases. Population turnover increased with P availability, evident in positive correlations of recruitment and mortality rates with P availability. Departures from seasonally modeled estimates of RGR were found to be influenced by salinity, which depressed RGR when below 20 psu or above 40 psu. Freshwater management in the headwaters of Florida Bay will alter salinity and nutrient climates. It is becoming clear that such changes will affect T. testudinum, with likely feedbacks on ecosystem structure, function, and habitat quality.

Abundancia, biomasa y floración de Thalassia testudinum (Hydrocharitaceae) en el Caribe de Costa Rica

Abundance, biomass and flowering of Thalassia testudinum (Hydrocharitaceae) in the Caribbean coast Costa Rica. Three Thalassia testudinum seagrass beds were studied at Cahuita National Park, from August 2003 to August 2006. Additionally, biomass samples of this species were collected at the Gandoca-Manzanillo National Wildlife Refuge, and data obtained from both localities in 1988 were compared with this study. The total average biomass within the substrate was more than double the biomass above ground. The average biomass for rhizomes was 310.90±72.59g/m2, 187.62±97.67g/m2 for roots and 67.90±84.40g/m2 for green leaves. The density, abundance and frequency of T. testudinum, showed peaks in January, May and September. The abundance of T. testudinum was highest in July, the rainiest month. In May, from 3000 short shoots, 95 (3.17%) had flower in 2004, 20 (0.67%) in 2005 and 7 (0.23%) in 2006. For both localities and every year of the study, the percentage of male flowers was greater than female flowers. In 2004, the flower sex ratio was two male to each female, but in 2005 and 2006 the proportion was four male to one female. Flowering was between April and June, with a peak in May, and fructification between May and August. T. testudinum flowering was correlated with the minimum tides of the year in the park. Rev. Biol. Trop. 56 (Suppl. 4): 175-189. Epub 2009 June 30.

Evaluation of aboveground and belowground biomass recovery in physically disturbed seagrass beds

Several studies addressed aboveground biomass recovery in tropical and subtropical seagrass systems following physical disturbance. However, there are few studies documenting belowground biomass recovery despite the important functional and ecological role of roots and rhizomes for seagrass ecosystems. In this study, we compared the recovery of biomass (g dry weight m(-2)) as well as the biomass recovery rates in ten severely disturbed multi-species seagrass meadows, after the sediments were excavated and the seagrasses removed. Three sites were located in the tropics (Puerto Rico) and seven in the subtropics (Florida Keys), and all were originally dominated by Thalassia testudinum. Total aboveground biomass reached reference values at four out of ten sites studied, two in the Florida Keys and two in Puerto Rico. Total belowground biomass was lower at the disturbed locations compared to the references at all sites, apart from two sites in the Florida Keys where the compensatory effect of opportunistic species (Syringodium filiforme and Halodule wrightii) was observed. The results revealed large variation among sites in aboveground and belowground biomass for all species, with higher aboveground recovery than belowground for T. testudinum. Recovery rates for T. testudinum were highly variable across sites, but a general trend of faster aboveground than belowground recovery was observed. Equal rates between aboveground and belowground biomass were found for opportunistic species at several sites in the Florida Keys. These results indicate the importance of belowground biomass when assessing seagrass recovery and suggest that the appropriate metric to assess seagrass recovery should address belowground biomass as well as aboveground biomass in order to evaluate the full recovery of ecological services and functions performed by seagrasses. We point out regional differences in species composition and species shifts following severe disturbance events and discuss ecological implications of gap dynamics in multi-species seagrass meadows.

Assessment of turtle grass ( Thalassia testudinum ex Banks Konig) community structure in a UNESCO Biosphere Reserve

Tropical turtle grass beds ( Thalassia testudinum) were investigated in an UNESCO Biosphere Reserve in St. Johns Island, United States Virgin Islands. Four coastal bays were studied to assess floral characteristics, benthic invertebrate density and secondary production. These bays varied with respect to oceanic exposure and anthropogenic stresses. Results indicate that Coral Bay Harbor, the most anthropogenetically impacted site, had the highest T. testudinum biomass, but the lowest floral diversity. Its faunal community was dominated by small polychaetes with significantly lower secondary production. The most protected site (Hurricane Hole) maintained the highest floral and faunal species richness, faunal density, faunal diversity, and secondary production. The other two bays, Great Lameshur and Little Lameshur, demonstrated intermediate plant biomass and species richness, faunal density and secondary production. Each of these bays, however, had high oceanic exposure due to their orientation which also demonstrated a sediment size shift to larger particles compared to the other sites. One unique finding was a significant increase in the shoot:root ratio in Little Lameshur where green turtle ( Chelonia mydas) grazing was frequently observed, suggesting a potential top-down structuring force in this bay.