1. Increasing green turtle abundance will lead to increased grazing within seagrass habitats-ecosystems that are important for carbon sequestration and storage.However, it is not well understood how carbon dynamics in these ecosystems respond to grazing and whether a response differs among meadows or locations.2. We measured seagrass ecosystem metabolism in grazed and ungrazed areas of Thalassia testudinum meadows with established green turtle foraging areas across the Greater Caribbean and Gulf of Mexico. We sampled meadows from five locations that differed in seagrass and environmental characteristics. Established meadows of the invasive seagrass Halophila stipulacea were also present at two of these locations, and we measured ecosystem metabolism in these meadows for comparison to grazed and ungrazed areas of the native T. testudinum.3. Across all individual sites, rates of net ecosystem production (NEP) ranged from 56% to 96% lower in grazed areas than ungrazed areas of T. testudinum meadows.Rates of NEP were also strongly, positively correlated with above-ground seagrass biomass across sites. While metabolic carbon capture rates were lower in grazed areas, heterotrophic respiration was not stimulated, and grazing therefore did not result in significant metabolic remineralization of carbon in these meadows. NEP in H. stipulacea meadows was similar to rates in T. testudinum meadows at all three sites, suggesting that metabolic carbon capture may remain similar in Caribbean meadows where this invasive seagrass is replacing native species. Synthesis.Our results show that there is a consistent response in metabolic carbon dynamics to green turtle grazing in T. testudinum meadows across the Greater Caribbean region. An increase in grazing will not likely stimulate remineralization of carbon as these important habitats are returned to a natural grazed state. K E Y W O R D Scarbon dynamics, grazing, green turtle, Halophila stipulacea, invasive, metabolism, plantherbivore interactions, Thalassia testudinum
Concerns over the sustainability of green turtle grazing have become a central topic in discussions of seagrass protection. Understanding grazing plot dynamics and aging is critical to evaluating the sustainability of grazing and understanding the role green turtles play in structuring seagrass foraging habitats. We investigated grazing plot dynamics and aging at 2 naturally grazed seagrass meadows in the Caribbean: one experiencing increasing grazing and one experiencing decreasing grazing. By integrating in-water characterization of grazing plots with aerial imagery of naturally grazed meadows, we show that green turtle grazing plots are dynamic, with establishment of new plots, expansion and merging of existing plots, and abandonment and regrowth of plots occurring within 3 and 13 mo periods. We also document that green turtles contribute to a component of habitat complexity by increasing edge habitat in seagrass meadows, and that they can maintain grazing plots for more than 9 yr. By July 2019, green turtles had been maintaining grazing plots for at least 6.65 yr where grazed area was decreasing and for at least 9.22 yr where grazed area was increasing. We offer new insight into the life cycle of grazing plots and show that green turtle grazing plot use is sustainable over longer periods than previously documented. Future studies investigating grazing plot dynamics and evaluating the sustainability of green turtle grazing over longer periods, across seasons, and in different regions, as well as evaluating cumulative effects of anthropogenic stressors, will be key to effective co-management of green turtles and globally declining seagrasses.
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