Trophic linkages in eelgrass ecosystems

海草草甸是浅海海洋环境中分布最广的生境之一。它们被称为“蓝色碳生态系统”，因为它们在全球碳捕获和储存中扮演着不成比例的大角色(C)。这些沿海植被栖息地在全球范围内的衰落尤其令人不安，因为富含碳的生态系统为人类提供了关键服务，如营养循环和碳的封存，减少海流和海浪压力，并为鱼类、鸟类和无脊椎动物物种提供栖息地，其中许多物种具有重要的商业价值。最近的证据表明，这些群落中的顶级营养层，即所谓的顶端捕食者，在保护沿海植被繁茂的栖息地方面可以发挥关键作用。顶端捕食者，如海獭，可以通过捕食食草动物，如螃蟹和鱼类，在这些生态系统中促进自上而下的控制，这些食草动物是沿海植被的主要捕食者。因此，顶端捕食者可以通过捕食减少食草动物的数量来间接保护蓝碳储备。该项目将利用阿拉斯加东南部海獭的重新定居作为一项自然实验，以了解顶端捕食者对海草生态系统和碳储存的营养关系和间接影响。随着顶级捕食者的范围扩大和海獭狩猎变得更加普遍，这项研究可以为如何管理这些具有生态重要性的海草群落的社会决策提供信息。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原住民村庄接触，方法是培训当地的现场助理，并通过与这些与海獭生活的社区的不同利益攸关方定期举行会议来分享结果。这种接触将为海洋资源的研究人员和使用者提供一条沟通的途径，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，捕食者控制了食草动物，从而调节了它们的植物猎物的丰度。通过这种营养级联，捕食者可以在维持植物储存的碳储备方面发挥关键作用。然而，这种观点仅限于营养层次中的直接影响，并没有考虑到更高级别捕食者的间接作用。顶端捕食者和中间捕食者之间的营养联系，如捕食食草动物的螃蟹和鱼类，研究要少得多。在阿拉斯加东南部，鳗草(Zostera Marina)是软质沉积物近岸水生植被的主要形式，覆盖近16,000公里的海岸线，是加利福尼亚州、俄勒冈州和华盛顿州整个海岸线总和的1.25倍。研究人员将利用阿拉斯加东南部比缅因州还大的地区海獭(Enhya Lutris)的地理扩张，在大的时空尺度上研究顶端捕食者对鳗草群落结构和碳固存的作用。在19世纪的皮毛贸易中，从加利福尼亚州北部到威廉王子湾，海獭在历史上分布在北太平洋各地，并被灭绝。阿拉斯加东南部海獭在过去50年里的重新引入和地理扩张是一项自然实验，研究人员可以利用它来更好地了解顶端捕食者在构建海洋生态系统中的作用，因为海獭的持续时间和密度在空间上是不同的，这使得沿着海獭梯度的海草食物网可以进行比较。研究人员将严格测试连接顶端捕食者和海洋植被的营养级联，使用这一自然实验结合操纵实验，其中包括限制海草的替代假设，然后量化这种海草在碳固存中的作用。

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.