Thalassia testudinum productivity and grazing by green turtles in a highly disturbed seagrass bed

Williams, Susan L.

doi:10.1007/bf00391121

Cited by 137 publications

(114 citation statements)

References 19 publications

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“…Williams 1988, Makowski et al 2006, MacDonald et al 2013. This is corroborated by results from a study involving lavage sampling of green turtles in DRTO, which found many turtles had eaten seagrass during the day (K.M.…”

Section: Discussionmentioning

confidence: 55%

Trading shallow safety for deep sleep: juvenile green turtles select deeper resting sites as they grow

Hart¹,

White²,

Iverson³

et al. 2016

Endang. Species. Res.

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To better protect endangered green sea turtles Chelonia mydas, a more thorough understanding of the behaviors of each life stage is needed. Although dive profile analyses obtained using time-depth loggers have provided some insights into habitat use, recent work has shown that more fine-scale monitoring of body movements is needed to elucidate physical activity patterns. We monitored 11 juvenile green sea turtles with tri-axial acceleration data loggers in their foraging grounds in Dry Tortugas National Park, Florida, USA, for periods ranging from 43 to 118 h (mean ± SD: 72.8 ± 27.3 h). Approximately half of the individuals (n = 5) remained in shallow (overall mean depth < 2 m) water throughout the experiment, whereas the remaining individuals (n = 6) made excursions to deeper (4 to 27 m) waters, often at night. Despite these differences in depth use, acceleration data revealed a consistent pattern of diurnal activity and nocturnal resting in most individuals. Nocturnal depth differences thus do not appear to represent differences in behavior, but rather different strategies to achieve the same behavior: rest. We calculated overall dynamic body acceleration (ODBA) to assess the relative energetic cost of each behavioral strategy in an attempt to explain the differences between them. Animals in deeper water experienced longer resting dives, more time resting per hour, and lower mean hourly ODBA. These results suggest that resting in deeper water provides energetic benefits that outweigh the costs of transiting to deep water and a potential increased risk of predation.

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Section: Discussionmentioning

confidence: 55%

Trading shallow safety for deep sleep: juvenile green turtles select deeper resting sites as they grow

Hart¹,

White²,

Iverson³

et al. 2016

Endang. Species. Res.

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“…Larger species with thicker rhizomes, such as T. testudinum, may also be better at translocating their stored carbohydrates longer distances and are therefore better able to compensate for loss than those species with thinner rhizomes, e.g., H. wrightii and S. filiforme (Marba, Santiago, Diaz-Almela, Alvarez, & Duarte, 2006). Within the timeframe of our study, the reduced productivity and leaf biomass for all three species in grazed patches when compared to ungrazed plants suggest an increased reliance on aboveground biomass and increases the plant's reliance on belowground biomass as a energy reserve (Williams, 1988). As these reserves are depleted to replace leaves lost to grazers, the lower soluble carbohydrate content of the rhizomes reduces the ability of the plant to counter prolonged or future disturbances until the reserves can be replaced (KuiperLinley et al, 2007).…”

Section: Discussionmentioning

confidence: 68%

Morphological and physiological responses of seagrasses (Alismatales) to grazers (Testudines: Cheloniidae) and the role of these responses as grazing patch abandonment cues

Lacey

Collado‐Vides

Fourqurean

2014

RBT

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Green sea turtles, Chelonia mydas, are grazers influencing the distribution of seagrass within shallow coastal ecosystems, yet the drivers behind C. mydas patch use within seagrass beds are largely unknown. Current theories center on food quality (nutrient content) as the plant responds to grazing disturbances; however, no study has monitored these parameters in a natural setting without grazer manipulation. To determine the morphological and physiological responses potentially influencing seagrass recovery from grazing disturbances, seagrasses were monitored for one year under three different grazing scenarios (turtle grazed, fish grazed and ungrazed) in a tropical ecosystem in Akumal Bay, Quintana Roo, Mexico. Significantly less soluble carbohydrates and increased nitrogen and phosphorus content in Thalassia testudinum were indicative of the stresses placed on seagrasses during herbivory. To determine if these physiological responses were the drivers of the heterogeneous grazing behavior by C. mydas recorded in Akumal Bay, patches were mapped and monitored over a six-month interval. The abandoned patches had the lowest standing crop rather than leaf nutrient or rhizome soluble carbohydrate content. This suggests a modified Giving Up Density (GUD) behavior: the critical threshold where cost of continued grazing does not provide minimum nutrients, therefore, new patches must be utilized, explains resource abandonment and mechanism behind C. mydas grazing. This study is the first to apply GUD theory, often applied in terrestrial literature, to explain marine herbivore grazing behavior. Rev. Biol. Trop. 62 (4): 1535-1548. Epub 2014 December 01.

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“…Mar Ecol Prog Ser 419: [223][224][225][226][227][228][229][230][231][232] 2010 (Williams 1988, Bjorndal 1997. Whether to reject the relatively low nutrient content of old seagrass leaves (Bjorndal 1980) or to avoid the calcareous encrusting epiphyte communities on those old blades , green sea turtles often clip seagrass shoots to within approximately 1 cm of the sediment surface, allowing the leaves to float away; they then repeatedly graze the newly-produced, unepiphytized, high nutrient-content seagrass leaves that grow after the clipping (Bjorndal 1997).…”

Section: Resale or Republication Not Permitted Without Written Consenmentioning

confidence: 99%

“…In other locations with more limited distribution of seagrass resources, sea turtle populations and their demands exceed the ability of the seagrass meadows to provision them. Williams (1988) found that the combined effects of sea turtle feeding and anchor damage were leading to the decline of the few seagrass meadows in 2 bays in the US Virgin Islands, and predicted negative consequences for the turtle population if this loss were to continue. In Bermuda, the distribution of tropical seagrass meadows is restricted in terms of area compared to similar coastal environments in more tropical locations.…”

mentioning

confidence: 99%

Effects of excluding sea turtle herbivores from a seagrass bed: Overgrazing may have led to loss of seagrass meadows in Bermuda

Fourqurean

Manuel²,

Coates³

et al. 2010

Mar. Ecol. Prog. Ser.

135

128

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Protecting a Thalassia testudinum-dominated seagrass meadow from grazing by sea turtles for 1 yr caused an increase in the biomass of seagrasses and an increase in the structural complexity of the seagrass canopy, as the length and width of the seagrass blades increased in comparison to grazed plots. Plots from which turtles were excluded had higher rates of primary production on a per-shoot or areal basis, but the relative growth rate was not affected. The leaves of seagrasses protected from grazing had lower concentrations of nitrogen and phosphorus than grazed blades, but the storage of soluble carbohydrates in the rhizomes increased markedly in the protected plots, suggesting that reduced carbon fixation caused by the removal of photosynthetic leaves is the mechanism for seagrass decline in heavily grazed meadows, not nutrient limitation as has been suggested in the literature. The continued grazing of sea turtles in our plots did not lead to significant changes in seagrass shoot density or nutrient content over the 1 yr duration of our experiments. The decreased canopy cover and the shorter, thinner seagrass leaves induced by sea turtle grazing in our experimental plots suggest that the progressive narrowing and thinning of seagrasses observed before the collapse of 2 offshore seagrass beds in Bermuda during the 1990s may have been in response to repeated and intense grazing of those seagrass beds. KEY WORDS: Caribbean Coastal Marine Productivity Program · CARICOMP · Thalassia testudinum · Nutrient content · Soluble carbohydrates · Productivity Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 419: [223][224][225][226][227][228][229][230][231][232] 2010 (Williams 1988, Bjorndal 1997. Whether to reject the relatively low nutrient content of old seagrass leaves (Bjorndal 1980) or to avoid the calcareous encrusting epiphyte communities on those old blades , green sea turtles often clip seagrass shoots to within approximately 1 cm of the sediment surface, allowing the leaves to float away; they then repeatedly graze the newly-produced, unepiphytized, high nutrient-content seagrass leaves that grow after the clipping (Bjorndal 1997). Thayer et al. (1984) surmised that these repeated grazing events would stress the seagrasses, so that leaf widths would decrease, short shoot density would decline, leaf production would decline, and recycling of nutrients through a local detritus cycle would be interrupted. Further, they suggested that internal stores of carbohydrates and proteins would be mobilized from the rhizomes to grow new leaves, and that the repeated cropping of new leaves would eventually lead to a decline in the nutrient content of the seagrass as stores were depleted, resulting in poor food quality and decreased food production for the turtles. These declines would then cause the turtles to abandon that grazing patch and begin the cultivation of a new grazing site. However, recent experimental work has shown that growth rates (Moran & Bjorndal 2...

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Thalassia testudinum productivity and grazing by green turtles in a highly disturbed seagrass bed

Cited by 137 publications

References 19 publications

Trading shallow safety for deep sleep: juvenile green turtles select deeper resting sites as they grow

Trading shallow safety for deep sleep: juvenile green turtles select deeper resting sites as they grow

Morphological and physiological responses of seagrasses (Alismatales) to grazers (Testudines: Cheloniidae) and the role of these responses as grazing patch abandonment cues

Effects of excluding sea turtle herbivores from a seagrass bed: Overgrazing may have led to loss of seagrass meadows in Bermuda

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