Abstract

Green turtles (Chelonia mydas) once played an important role in structuring Caribbean seagrass ecosystems, but today that role has been diminished due to its overexploitation, which has reduced Caribbean abundance to only 0.33% of pre-Colombian levels (McClenachan et al. 2006). Reduced C. mydas grazing in Caribbean seagrass beds has implications for the productivity (Zieman et al. 1984, Moran and Bjorndal 2005, Molina Hernández and Van Tussenbroek 2014, Holzer and McGlathery 2016) and benthic community composition (Molina-Hernández and van Tussenbroek 2014) of these habitats. Caribbean seagrass productivity has shown variable responses to grazing leading Fourqurean et al. (2010) to theorize that seagrass productivity should respond favorably to grazing when the energetic benefits of reduced self-shading are greater than the energetic costs of having to replace lost biomass. Holzer and McGlathery (2016) proposed that grazing should enhance seagrass production when seagrasses are not limited by phosphorous and to reduce seagrass production when plants are phosphorous-limited. These explanations can be combined to theorize that seagrass production should be enhanced by grazing when the energetic benefit of reduced self-shading is greater than the energetic cost of replacing lost biomass and plants are not phosphorous limited, but grazing should be expected to lower seagrass production if plants are phosphorous limited or if the cost of replacing lost biomass is greater than the benefit of reduced self-shading. To assess how the lack of grazing may have affected these variables in Bocas del Toro, Panamá we conducted a 168-day simulated grazing experi-ment to assess benthic community and seagrass productivity under varying levels of grazing pressure.

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