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Faunal relationships in Caribbean seagrass beds

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... Ninety-five percent of all respondents from Ifaty (n = 60) and 98% of the ones from Toliara (n = 60) stated to eat sea urchins, all of which specifying the striped sea urchin Tripneustes gratilla, which feeds in seagrass beds (Ogden, 1980). The respondents' reasons for sea urchin consumption were that sea urchins were delicious (34%; n = 74), that it was a Vezo tradition to eat sea urchins (11%), that sea urchins were nutritious (9%) and that sea urchins were available during times when fish were scarce or bad weather (strong winds) hindered them from fishing (8%). ...
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Small-scale fisheries (SSF) are crucial for food security and poverty alleviation. Many SSF are however under pressure, and in need of better management paying special attention to the key seascape ecosystems which are supporting them. This study investigates the importance of seagrass beds for SSF households and their food security in southwestern Madagascar. The specific aims of this study were to: i) analyze if and how seagrass-associated fish contributes to subsistence and/or the economy of local fishing households, ii) identify and compare seagrass ecosystem goods and services valued by local fishers in a rural and an urban setting, and iii) analyze links between local people and seagrasses in terms of local ecological knowledge, use and traditions. The results showed that seagrasses were the most important fishing habitats for most fishers. Seagrass-associated fish species were both the economically most important and most commonly fished species, and are a major source of protein in the region. Further, seagrass-derived sea urchins are important complements to local people’s diets. The findings illustrate that seagrasses contribute both through subsistence and income generation to food security and wellbeing of coastal people in southwestern Madagascar. This highlights the need to consider seagrass ecosystems in management towards sustainable SSF and their ability to sustain food security for future generations.
... The traditional paradigm that herbivory on living seagrass tissue is at most modest due to low palatability and nutritional quality (e.g., Ogden 1980, Klumpp et al. 1993) has shifted. Improved methodologies for estimating consumption of seagrass production indicate that rates vary greatly over time and space, from negligible (< 5%) to nearly 100% of leaf production (Cebrián and Duarte 1998, Heck and. ...
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The sea urchin Lytechinus variegatus is a known grazer of both living and dead tissue of turtlegrass, Thalassia testudinum, occasionally denuding large areas of seagrass. Field studies have attempted to assess effects of herbivory on seagrass by enclosing urchins at various densities. However, it is unclear how unrestricted urchins affect seagrass at lower densities more typically observed in the field. This study describes movement, feeding, and distribution of L. variegatus within beds of T. testudinum in St. Joseph Bay, Florida (USA) to quantify this urchin’s impact as a seagrass grazer. Urchins were absent from portions of seagrass beds closest to shore, present at low densities midway across the bed, and at highest densities (up to ~5 individuals/m2) at the offshore edge of the bed. Urchins tended not to aggregate, moved twice as rapidly where seagrass cover was reduced, and moved > 20X faster when placed in areas of open sand. Dead seagrass tissue occurred 4—30X more frequently on oral surfaces than living seagrass tissue. Fecal pellets with dead seagrass tissue were > 3X more common than pellets with live seagrass tissue. Injury to seagrass leaves was more common along dead leaf sections than live sections (> 2—10X). Overall, spatial distributions, movement, and diet indicate that L. variegatus at densities observed in this study would tend to have minimal effects on living seagrass. Episodic periods of denuding grassbeds reported in the literature suggest L. variegatus switches to live seagrass tissue as dead tissue becomes scarce during times of high urchin density.
... Simulated grazing was initiated in clipped plots by clipping all seagrass blades at the bladesheath junction (~2 cm above the sediment surface) with stainless steel scissors to mimic natural green turtle grazing (Bjorndal, 1980;Moran and Bjorndal, 2005). All blades within experimentally clipped plots were re-clipped every time mean blade length in the plot reached 5 cm, consistent with natural green turtle grazing behavior in the Caribbean region (Bjorndal, 1980;Ogden, 1980). Intervals between clipping events varied between 12 and 37 days, as blade growth varies with temperature (Moran and Bjorndal, 2005), and clipped plots were maintained for the entire 16-month duration of the study. ...
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Seagrass meadows host diverse invertebrate faunal communities. Infaunal organisms residing in the sediments of meadows play important roles in the functioning of these ecosystems, such as the breakdown of organic matter. Disturbance to the benthic environment through grazing by megaherbivores, such as dugongs (Dugong dugon), can reduce infauna abundance within localized areas in a meadow. However, it is not known how removal of the aboveground canopy of a meadow without severe physical disturbance to the benthic habitat, such as through grazing by green turtles (Chelonia mydas), affects seagrass meadow infaunal communities. Increasing green turtle abundance will likely lead to greater areas of grazed seagrass with implications for the invertebrate infaunal communities within meadows. We experimentally simulated green turtle grazing for 16 months in a Thalassia testudinum seagrass meadow in The Bahamas to test effects of grazing on meadow infaunal communities. Total abundance of the infaunal community was reduced by 59% within six months of simulated grazing and thereafter remained lower throughout the experiment. Six out of eleven individual infaunal groups present (e.g. nematodes) also decreased in abundance following simulated grazing, but temporal abundance dynamics varied among groups. Simulated grazing had no effect on Simpson's Diversity Index of the infaunal community inhabiting the meadow at any point during the 16-month experiment. Though diversity was not affected, relative abundance of individual groups varied over time, and simulated grazing led to a significant change in infaunal community composition. These results demonstrate how green turtle grazing may affect the infaunal communities of shallow seagrass meadows with potential implications for the ecosystem services provided by these important habitats.
... Green turtles are prominent megaherbivores in meadows in many areas (Heithaus and others 2014) and consume seagrass throughout their circumglobal range (Bjorndal 1997). They graze aboveground seagrass biomass by cropping blades near the sediment surface and repeatedly re-grazing the same areas (Bjorndal 1980;Ogden 1980), thereby maintaining a short meadow canopy. Green turtles have been recorded consuming belowground rhizome biomass in addition to aboveground tissues when they reached hyper-abundance within a protected area in Indonesia (Christianen and others 2014); however, to our knowledge, this grazing behavior has not been reported elsewhere. ...
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Seagrass meadows buffer sediments against resuspension and erosion by reducing water velocity and attenuating wave energy, thereby promoting accumulation of sediment and associated carbon. Grazing by green turtles (Chelonia mydas) can significantly reduce the aboveground canopy in meadows. Increasing green turtle population sizes will return more seagrass areas to a naturally grazed state; however, it is not well understood how green turtle grazing will affect sediment processes in seagrass meadows. To evaluate effects of grazing, we measured sediment erosion following a clipping experiment in a shallow Caribbean Thalassia testudinum seagrass meadow and rates of sediment resuspension in an area naturally grazed by turtles. Following removal of the seagrass canopy, erosion of surface sediments did not increase compared to unclipped reference plots during the clipping experiment. We provide the first estimates of particle deposition and resuspension rates from a seagrass meadow grazed by green turtles. Rates did not differ between areas naturally grazed for at least one year and ungrazed areas. On average, 51% of the total sediment flux was comprised of resuspended sediments in the area grazed by turtles, and 52% in the ungrazed area of the meadow. Green turtle grazing also did not affect the carbon content of sediment particles or the downward carbon flux in the meadow. Our results demonstrate that grazing did not increase the vulnerability of surface sediments to loss in this system, and as green turtles recover, their natural grazing regime may not directly affect sediment processes contributing to carbon accumulation in shallow, coastal meadows.
... Responses of T. testudinum blade morphometry followed expected patterns based on previous studies of green turtle grazing (Bjorndal, 1980;Moran and Bjorndal, 2005;Ogden, 1980). Blade lengths were shorter and blade widths were narrower in grazed areas compared to ungrazed areas in both bays (p < 0.05 for all comparisons; Table 2). ...
Article
Seagrass meadows are often comprised of diverse assemblages of seagrasses and algae. Green turtles (Chelonia mydas) are a prominent megaherbivore in seagrass meadows—capable of consuming large amounts of aboveground seagrass biomass and driving shifts in seagrass species dominance. Previous green turtle grazing studies have focused on diversity and dynamics of seagrasses, but there has been little focus thus far on diversity of other primary producers within meadow communities. We investigated the effects of grazing on seagrasses and benthic macroalgae composition by surveying a series of seagrass meadows across two separate bays in Little Cayman, Cayman Islands, in which green turtles had established foraging areas. Effects of grazing on diversity were spatially variable among meadows. Diversity of the seagrass and macroalgae community (Simpson's Index) was not significantly different between areas grazed by turtles compared to ungrazed areas in one surveyed bay but was significantly lower among grazed areas in the other bay. Lower diversity was the result of lower densities of all seagrass and macroalgae species other than Thalassia testudinum among grazed areas in one bay. Diversity was also positively correlated with densities of total seagrass and total macroalgae across all meadows. Density of the dominant seagrass, T. testudinum, did not differ between the two bays or between grazed and ungrazed areas. As green turtle abundance increases and more seagrass meadows return to a naturally grazed state, it is important to understand the effects grazing has in these ecosystems and how effects may vary among locations.
... Seagrasses are considered the main dietary component in several life stages of green turtles (Bjorndal 1980). In fact, the green turtle is the most abundant large vertebrate consumer of seagrasses in the world (Ogden 1980). In northwest of Mexico, eelgrass (Zostera marina) was found in diet samples at canal del infiernillo (Felger & Moser 1973) Like other sea turtle species Chelonia mydas shows a slow grow and delayed sexual maturity which, along with the intrinsic characteristics of its life history, allows it to spend several decades in different habitats where it uses and modifies the environment ). ...
Chapter
Although subject to several caveats, stranding data may provide information on geographic ranges, seasonal distribution and life history of marine turtle aggregations in nesting, foraging/development, and migratory areas. Strandings are the main source of biological samples for ecological studies on marine turtles. Samples are collected either from live and recovered turtles or from dead ones through necropsy. Moreover, strandings provide data on sex ratios of local aggregations, diseases, and feeding ecology, among other topics. Two methods can be used to record stranded marine turtles on beaches: (1) creating a marine stranding network or (2) organizing beach surveys over the area of interest. In order to provide valuable information, stranding networks must use replicable protocols and be able to record the majority of stranding events. However, the discovery and reporting of washed carcasses depends on observation effort and public awareness, and economic resources are critical for the effectiveness of the network. Beach surveys for stranding records need constant effort over areas of interest. Stranding data can be recorded across wide spatial and temporal ranges at high resolution due to the low cost per unit effort compared to in-water or aerial-based studies. Stranding data is often considered as non-representative of populations at sea, since the probability of stranding varies widely in space and time, as predators, scavengers, winds and sea currents may prevent carcasses from reaching the shore. However, strandings yield reasonable data on the occurrence and distribution of marine turtle species in the adjacent marine area. Thus, stranding data, while an imperfect measure of marine turtle abundance, distribution and activity, provides additional information that can complement research studies at sea. Analyses of stranding data can also contribute to a better implementation of conservation measures and management on endangered marine turtles in coastal waters through the identification of (1) the areas probably used by marine turtles, and (2) anthropogenic mortality sources in the region of interest. Strandings can also be used for public awareness on the conservation of these threatened species.
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Seagrass diversity and distribution in Southeast Asia have been mainly studied in backreef zones but have rarely been studied on intertidal mudflats in estuaries. This research article aims to study seagrass distribution (species diversity, shoot density, and biomass) and morphology (leaf length and leaf number per shoot) on intertidal mudflats in Brunei Bay, Borneo, to understand seagrass responses to water and sediment conditions along the environmental gradient in the estuary. In total, five seagrass species (Cymodocea rotundata, Halodule pinifolia, Halophila ovalis, Halophila minor, and Thalassia hemprichii) were found at five different intertidal sites, and the diversity and biomass were highest at the most upstream site. H. pinifolia was the only species found at all five sites, indicating that this species is the most tolerant of and adaptable to varying water and sediment conditions. H. pinifolia had the highest biomass and longest leaf length at the site with the highest silt and clay percentage (SC%) and organic matter content of sediment, where no other seagrass species were found, suggesting that H. pinifolia could efficiently acquire nutrients without interspecies competition. In contrast, H. ovalis and T. hemprichii were found at the upstream sites and preferred a relatively low SC% in sediment. H. minor was found only at the most downstream site, which had the lowest SC% and is likely to experience the strongest hydrodynamics and receive the least nutrients of all the sites. Based on this study, the seagrass response to environmental gradients in estuaries is proposed to be species-specific, which would have led to the unique seagrass distribution in Brunei Bay.
Chapter
“[F]or most of the past 50 My, Caribbean seagrass communities have had to withstand heavy, sustained grazing pressure from several sympatric lineages of large mammalian herbivores. This factor is almost totally absent both from these communities today (wherein manatees are scarce or absent in most areas) and from the thinking of the aquatic botanists and marine ecologistswho study these communities. Consequently, the long-established tenet that seagrass ecosystems are largely detritus-based … must be revised to recognize that the modern situation is anomalous, and that the ‘normal’ pattern throughout most of tropical seagrass history has been that much (probably most) of the primary productivity has been channeled through the guts of herbivores, particularly sirenians.” (Domning, 2001)
Chapter
Seagrasses are aquatic angiosperms that live and complete their life cycles totally submerged in a saline-to-brackish medium (Thayer et al. 1975). They are unique in that they are the only land plants that have totally returned to the sea. Although not true grasses, their habit of ribbon-like leaves results in meadows or beds resembling terrestrial grasslands. This complex habitat structure, in an otherwise relatively featureless environment, provides substrate and protection for large populations of invertebrates and serves as a nursery and feeding ground for numerous fish species, many with food and commercial value (see Zieman 1982; Phillips 1984; Thayer et al. 1984). Thayer et al. 1984). The ecological importance of seagrass beds in supporting this diverse community is generally attributed to their high productivity, which rivals that of cultivated agricultural crops (Table 1). This primary production supplies carbon to herbivores and to a complex detritus-based food web (Zieman et al. 1979; Kikuchi 1980; Ogden 1980). Additionally, other investigations (Kitting et al. 1984; Fry et al. 1987) suggest that seagrasses in more eutrophic coastal and estuarine systems provide substrate for epiphytic micro- and macroalgae, which then serve as the primary food sources for herbivores.
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