Seagrass meadows are some of the most productive ecosystems in the world and are important sites of blue carbon storage. Green sea turtles (Chelonia mydas) are marine megaherbivores that consume seagrasses as a part of their diet across much of their global range. Caribbean seagrass meadows supported extensive grazing by green turtles before humans disrupted this coevolved system by overexploiting green turtle populations. With successful conservation efforts leading to increasing green turtle abundance in some areas, an increasing area of seagrass habitat may be returned to its naturally grazed state. To better conserve these ecosystems, it is necessary to understand how ecosystem processes, such as carbon sequestration, operate within seagrass meadows in their naturally grazed state. We simulated green turtle grazing in a seagrass meadow in Little Cayman (Caribbean) by clipping seagrass to investigate how green turtle grazing affects carbon dynamics in coastal habitats. Using benthic incubation chambers we measured rates of ecosystem production and respiration in both clipped (simulated grazing) and unclipped plots over twelve weeks, as well as in nearby areas naturally, actively grazed by green turtles.
We found that seagrass meadow carbon balance is sustainable under green turtle grazing. Following both short- and long-term grazing, meadows were autotrophic, representing a net carbon uptake from the environment. Following simulated grazing, the ecosystem exhibited carbon capture rates of 24.7 mmol C m-2 d-1 compared to rates of 119.5 mmol C m-2 d-1 in ungrazed areas, showing that under natural grazing regimes, meadows were capable of carbon uptake, albeit slowly. Metabolic rates were similar between clipped plots and naturally grazed areas. All areas exhibited high production to respiration ratios (range 1.9 – 2.4), and these did not differ among treatments (clipped, unclipped, naturally grazed, naturally ungrazed), suggesting that remineralization of carbon stored in the sediments via respiration is not stimulated by grazing. Therefore, green turtle grazing is not likely to cause a release of carbon already stored in these systems.