Trophic linkages in eelgrass ecosystems

海草草甸是浅海海洋环境中分布最广的栖息地之一。它们被称为“蓝碳生态系统”，因为它们在全球碳捕获和储存中发挥了不成比例的巨大作用。这些沿海植被栖息地在全球范围内的减少尤其令人不安，因为富含碳的生态系统为人类提供了重要的服务，如养分循环和碳封存，减少了洋流和波浪的压力，并为鱼类、鸟类和无脊椎动物物种提供了栖息地，其中许多物种具有重要的商业价值。最近的证据表明，这些群落的最高营养水平，即所谓的顶端捕食者，可以在保护植被覆盖的沿海栖息地方面发挥关键作用。顶级捕食者，如海獭，可以通过消耗螃蟹和鱼类等食草动物来促进这些生态系统的自上而下控制，这些食草动物是沿海植被的主要食草动物。因此，顶端捕食者可能通过捕食来减少食草动物的数量，从而间接地保护蓝碳储量。该项目将利用阿拉斯加东南部海獭的重新定居作为一个自然实验，以了解顶级捕食者对海草生态系统和碳储存的营养关系和间接影响。随着顶级捕食者范围的扩大和海獭捕猎变得越来越普遍，这项研究可以为社会决策提供信息，告诉他们如何管理这些生态上重要的海草群落。研究人员将与阿拉斯加东南部威尔士亲王岛上的阿拉斯加土著村庄进行接触，培训当地的实地助理，并通过与这些与海獭生活在一起的社区的各种利益相关者定期会议分享结果。这种参与将为研究人员和海洋资源使用者提供沟通渠道，以了解海獭在其生态系统中的多方面作用。顶级捕食者可以驱动生态系统结构的生态学理论是为了回答为什么世界是绿色的问题而发展起来的。简而言之，食肉动物控制食草动物，从而调节其植物猎物的丰度。通过这种营养级联，捕食者可以在维持植物储存的碳储量方面发挥关键作用。然而，这种观点仅限于对营养等级的直接影响，而没有考虑到高级捕食者的间接作用。顶端捕食者和中间捕食者之间的营养联系，比如以食草动物为食的螃蟹和鱼，研究得很少。在阿拉斯加东南部，大叶藻（Zostera marina）是软沉积近岸水生植被的主要形式，覆盖了近16000公里的海岸线，是加州、俄勒冈州和华盛顿州整个海岸线总和的1.25倍。研究人员将利用海獭（Enhydra lutris）在阿拉斯加东南部（一个比缅因州还大的地区）的地理扩张，在大的时空尺度上研究顶端捕食者对大叶藻群落结构和碳封存的作用。历史上海獭分布在整个北太平洋，在19世纪的毛皮贸易中从加利福尼亚北部到威廉王子湾灭绝。在过去的50年里，海獭在阿拉斯加东南部的重新引入和地理扩张是一个自然实验，研究人员可以利用它来更好地理解顶端捕食者在构建海洋生态系统中的作用，因为海獭的生存时间和密度随空间而变化，可以沿着海獭的梯度比较海草食物网。研究人员将严格测试连接顶级捕食者和海洋植被的营养级联，使用这个自然实验结合操纵实验，包括限制海草的其他假设，然后量化海草在碳封存中的作用。

Seagrass meadows are one of the most widespread habitats in shallow coastal marine environments. They have been dubbed "blue carbon ecosystems" due to their disproportionately large role in the global capture and storage of carbon (C). The worldwide decline of these coastal vegetated habitats is particularly troubling because C-rich ecosystems provide critical services to humans, such as nutrient cycling and sequestering of carbon, reduce current and wave stress, and supply habitat for fishes, birds and invertebrate species, many of them commercially important. Recent evidence suggests that the top trophic level in these communities, known as apex predators, can play a critical role in preserving vegetated coastal habitats. Apex predators, such as sea otters, can facilitate top-down control in these ecosystems by consuming herbivores such as crabs and fish that are dominant grazers of coastal vegetation. Thus apex predators may indirectly conserve blue carbon stocks by reducing the number of herbivores through predation. This project will use the recolonization of sea otters in Southeast Alaska as a natural experiment to understand the trophic relationships and indirect effects of apex predators on seagrass ecosystems and carbon storage. This research can inform societal decisions on how to manage these ecologically important seagrass communities as the apex predator range expands and sea otter hunting becomes more pervasive. Researchers will engage with Alaska Native villages on Prince of Wales Island in Southeast Alaska by training local field assistants and sharing results through regular meetings with various stakeholders in these communities that live with sea otters. This engagement will provide an avenue of communication for researchers and users of marine resources to understand the multifaceted role of sea otters in their ecosystem.The ecological theory that top predators can drive ecosystem structure was developed in response to the question of why the world is green. In short, predators control herbivores, thus regulating the abundance of their plant prey. Through this trophic cascade, predators can play a critical role in maintaining carbon stocks stored by plants. Yet this view is limited to direct effects in the trophic hierarchy and does not consider the indirect role of higher order predators. The trophic linkages between apex predators and intermediate predators, such as crabs and fish that eat grazers, are much less studied. In Southeast Alaska, eelgrass (Zostera marina) is the dominant form of soft sediment nearshore aquatic vegetation and covers nearly 16,000 km of shoreline, which is 1.25 times greater than the entire shoreline of California, Oregon, and Washington combined. The researchers will use the geographical expansion of sea otters (Enhydra lutris) in Southeast Alaska, a region larger than the state of Maine, to investigate the role of apex predators on eelgrass community structure and carbon sequestration at a large temporal and spatial scale. Sea otters were historically distributed throughout the North Pacific and exterminated from northern California to Prince William Sound during the 19th century fur trade. The reintroduction and geographical expansion of sea otters in Southeast Alaska over the past 50 years is a natural experiment that researchers can use to better understand the role of apex predators in structuring marine ecosystems, because sea otter duration and density vary over space, allowing comparison of seagrass food webs along this sea otter gradient. Researchers will rigorously test for a trophic cascade linking apex predators and marine vegetation using this natural experiment combined with manipulative experiments that include alternative hypotheses of what is limiting seagrass and then quantify the role of this seagrass in C sequestration.