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.
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We thank Ashley Meade, Karalyn Bridgman, and Rebecca Rash for their assistance with laboratory sample processing at the University of Florida, and the staff of the Central Caribbean Marine Institute for their support during this project. This study was funded by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1315138 to RAJ. Additional funding was provided by a Grant-in-Aid of Research from Sigma Xi and grants from the PADI Foundation, The Explorers Club Exploration Fund—Mamont Scholars Program, and the University of Florida International Center. Private donations from Lalita Shastry, the Melnick family through the Cynthia A. Melnick Endowment, and the Yoder family through the Carrie Lynn Yoder Memorial Scholarship supported our work. Additional funding for this project is from the Archie Carr Center for Sea Turtle Research (University of Florida) through support from the Disney Conservation Fund to protect Florida’s sea turtles.
RAJ, ABB, and KAB conceived of and designed the study. RAJ and AGG performed the research. RAJ analyzed the data and wrote the paper with contributions from all authors.
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Johnson, R.A., Gulick, A.G., Bolten, A.B. et al. Rates of Sediment Resuspension and Erosion Following Green Turtle Grazing in a Shallow Caribbean Thalassia testudinum Meadow. Ecosystems 22, 1787–1802 (2019). https://doi.org/10.1007/s10021-019-00372-y
- sediment dynamics
- green turtles