Project: Trophic linkages in eelgrass ecosystems

NSF Award Abstract:
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.