What happens in meadows after populations of natural grazers rebound following centuries of low abundance? Many seagrass ecosystems are now experiencing this phenomenon with the recovery of green turtles (Chelonia mydas), large-bodied marine herbivores that feed on seagrasses. These seagrass ecosystems provide a rare opportunity to study ecosystem-wide shifts that result from a recovery of herbivores. We evaluate changes in regulation of seagrass productivity in a naturally grazed tropical ecosystem by: 1) comparing Thalassia testudinum productivity in grazed and ungrazed areas, and 2) evaluating potential regulating mechanisms of T. testudinum productivity. We established 129 green turtle exclusion cages in grazed and ungrazed areas to quantify T. testudinum growth (linear, area, mass, productivity:biomass [P:B]). In each exclosure, we recorded temperature, irradiance, water depth, nitrogen:phosphorus ratio (N:P) of blade tissue, grazing intensity before cage placement, and T. testudinum structural and nutrient characteristics. Thalassia testudinum exhibited compensatory growth in grazed areas via stimulated blade linear growth, blade area growth, and P:B across seasonal high and low growth periods and in shallow (3-4m) and deep (9-10m) seagrass meadows. Irradiance, depth, and N:P ratios had significant roles in regulating mass growth and P:B of T. testudinum in ungrazed areas. Depth was a significant regulating factor of mass growth and P:B in grazed areas; rates were higher and more variable in shallow meadows than in deep meadows. Grazing intensity was also a significant regulating factor for P:B, stimulating tissue turnover with increasing grazing pressure. This study provides important insights into how recovery of a large marine herbivore can result in dramatic, sustainable changes in the regulation of seagrass productivity. We also highlight the need for a historical perspective and use of appropriate indicators, including P:B and grazing intensity, when evaluating seagrass response to green turtle grazing as meadows are returned to a natural grazed state. In an age of green turtle recovery and global seagrass decline due to anthropogenic threats, a thorough understanding of green turtle-seagrass interactions at the ecosystem level is critical to ensure the restoration of seagrass ecosystems and continued recovery of green turtle populations.

中文翻译：

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大型食草动物的恢复改变了加勒比自然放牧生态系统中海草生产力的调节

在经历了几个世纪的低丰度之后，自然食草动物的数量反弹之后，草地上会发生什么？随着绿海龟（Chelonia mydas）的恢复，许多海草生态系统现在正在经历这种现象，绿海龟是一种以海草为食的大型海洋食草动物。这些海草生态系统提供了一个难得的机会来研究由食草动物恢复导致的整个生态系统的变化。我们通过以下方式评估自然放牧热带生态系统中海草生产力调节的变化：1) 比较放牧和未放牧地区的海草生产力，以及 2) 评估 T. testudinum 生产力的潜在调节机制。我们在放牧和未放牧区域建立了 129 个绿海龟排除笼，以量化睾丸炎的生长（线性、面积、质量、生产力：生物量 [P:B]）。在每个披露中，我们记录了温度、辐照度、水深、叶片组织的氮磷比 (N:P)、笼子放置前的放牧强度以及 T. testudinum 结构和营养特征。Thalassia testudinum 在放牧区通过刺激的叶片线性生长、叶片面积生长和 P:B 在季节性高低生长期以及浅 (3-4m) 和深 (9-10m) 海草草甸中表现出补偿性生长。辐照度、深度和 N:P 比率在调节未放牧地区的 T. testudinum 的质量增长和 P:B 方面具有重要作用。深度是放牧区质量增长和P:B的重要调节因子；与深草甸相比，浅草甸的发病率更高且变化更大。放牧强度也是 P:B 的重要调节因素，随着放牧压力的增加刺激组织更新。这项研究提供了重要的见解，了解大型海洋食草动物的恢复如何导致海草生产力调节的显着、可持续变化。在评估海草对绿海龟放牧的反应时，随着草地恢复到自然放牧状态，我们还强调需要从历史角度和使用适当的指标，包括 P:B 和放牧强度。在由于人为威胁导致绿海龟恢复和全球海草减少的时代，彻底了解生态系统层面绿海龟-海草的相互作用对于确保海草生态系统的恢复和绿海龟种群的持续恢复至关重要。在评估海草对绿海龟放牧的反应时，随着草地恢复到自然放牧状态，我们还强调需要从历史角度和使用适当的指标，包括 P:B 和放牧强度。在由于人为威胁导致绿海龟恢复和全球海草减少的时代，彻底了解生态系统层面绿海龟-海草相互作用对于确保海草生态系统的恢复和绿海龟种群的持续恢复至关重要。在评估海草对绿海龟放牧的反应时，随着草地恢复到自然放牧状态，我们还强调需要从历史角度和使用适当的指标，包括 P:B 和放牧强度。在由于人为威胁导致绿海龟恢复和全球海草减少的时代，彻底了解生态系统层面绿海龟-海草的相互作用对于确保海草生态系统的恢复和绿海龟种群的持续恢复至关重要。