Rates of Sediment Resuspension and Erosion Following Green Turtle Grazing in a Shallow Caribbean Thalassia testudinum Meadow
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.
Keywordssediment dynamics resuspension erosion seagrass green turtles grazing carbon
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.
- Bjorndal KA. 1997. Foraging ecology and nutrition in sea turtles. In: Lutz PL, Musick JA, Eds. The biology of sea turtles. Boca Raton: CRC Press. p 199–232.Google Scholar
- Christianen MJA, Herman PMJ, Bouma TJ, Lamers LPM, van Katwijk MM, van der Heide T, Mumby PJ, Silliman BR, Engelhard SL, van de Kerk M, Kiswara W, van de Koppel J. 2014. Habitat collapse due to overgrazing threatens turtle conservation in marine protected areas. Proc R Soc B Biol Sci 281:20132890.CrossRefGoogle Scholar
- de Mendiburu F. 2017. Agricolae: statistical procedures for agricultural research. R package version 1.2-6. https://cran.r-project.org/package=agricolae. Accessed 11 Feb 2019.
- Heithaus MR, Alcoverro T, Arthur R, Burkholder DA, Coates KA, Christianen MJ, Kelkar N, Manuel SA, Wirsing AJ, Kenworthy WJ, Fourqurean JW. 2014. Seagrasses in the age of sea turtle conservation and shark overfishing. Front Mar Sci 1:1–6. https://doi.org/10.3389/fmars.2014.00028/abstract.CrossRefGoogle Scholar
- Jackson JBC, Kirby MX, Berger WH, Bjorndal KA, Botsford LW, Bourque BJ, Bradbury RH, Cooke R, Erlandson J, Estes JA, Hughes TP, Kidwell S, Lange CB, Lenihan HS, Pandolfi JM, Peterson CH, Steneck RS, Tegner MJ, Warner RR. 2001. Historical overfishing and the recent collapse of coastal ecosystems. Science 293:629–37.CrossRefGoogle Scholar
- Ogden JC. 1980. Faunal relationships in Caribbean seagrass beds. In: Phillips RC, McRoy CP, Eds. Handbook of seagrass biology: an ecosystem perspective. New York: Garland STPM Press. p 173–98.Google Scholar
- Pinheiro J, Bates D, DebRoy S, Sarkar D, R Core Team. 2018. _nlme: linear and nonlinear mixed effects models. http://cran.r-project.org/package=nlme. Accessed 11 Feb 2019.
- R Core Team. 2018. R: a language and environment for statistical computing. R Foundation for Statistical Computing. http://www.r-project.org/. Accessed 11 Feb 2019.
- Thomson ACG, Trevathan-Tackett SM, Maher DT, Ralph PJ, Macreadie PI. 2018. Bioturbator-stimulated loss of seagrass sediment carbon stocks. Limnol Oceanogr 64:1–15.Google Scholar
- Valeur JR. 1994. Resuspension mechanisms and measuring methods. In: Floderus S, Heiskanen A, Olesen M, Wassmann P, Eds. Sediment trap studies in the nordic countries. Helsingor: Marine Biological Lab. p 185–203.Google Scholar
- Waycott M, Duarte CM, Carruthers TJB, Orth RJ, Dennison WC, Olyarnik S, Calladine A, Fourqurean JW, Heck KL, Ar Hughes, Kendrick GA, Wj Kenworthy, Short FT, Williams SL. 2009. Accelerating loss of seagrasses across the globe threatens coastal ecosystems. Proc Natl Acad Sci 106:12377–81.CrossRefGoogle Scholar
- Wickham H, Francois R, Henry L, Müller K. 2017. dplyr: a grammar of data manipulation. R package version 0.7.2. https://cran.r-project.org/package=dplyr. Accessed 11 Feb 2019.