Testing the Effects of Recovering Sea Otters on Seagrass Ecosystems in Southeast Alaska

• Main Author: Jessica J. Saavedra
• Other Authors: Brent B. Hughes, PhD, Ginny L. Eckert, PhD, Daniel E. Crocker, PhD
• Format: Master Thesis
• Language: English
• Published: Sonoma State University 2021
• Subjects: Zostera marina; Sea otter; Eelgrass; Alaska > Alexander Archipelago; Enhydra lutris; Alaska > Prince of Wales Island; Prince of Wales Island; Saavedra; Archipelago; Alaska
• Online Access: https://hdl.handle.net/20.500.12680/k0698d786

Over time, the loss of a top predator can alter communities, ecosystem function and resilience. The purpose of this study is to investigate how the recovery of sea otters (Enhydra lutris), as toppredators, affect their invertebrate prey within a seagrass (Zostera marina) ecosystem in southeast Alaska. Previous work in southeast Alaska has determined that sea otters and seagrass are positively associated, but what is unknown are the trophic mechanisms underlying this relationship. We conducted a reciprocal transplant experiment to test for the effects of sea otters on eelgrass communities. By moving seagrass in areas with low sea otter abundance to high sea otter abundance, and vice versa, we aim to determine the drivers of associations between sea otters and eelgrass. This experimental design allowed for a cage-free design in order to minimize cage artefacts. We also conducted benthic invertebrate surveys of crab and clams in eelgrass communities within our transplant experiments and adjacent seagrass beds to quantify trophic interactions. The results suggest that sea otters in southeast Alaska are not initiating a trophic cascade linking crab to grazers to epiphytes to eelgrass. Instead, the results do show positive association in the removal of belowground clam competitors by sea otters generating a growth in eelgrass in the high sea otter sites in southeast Alaska. The findings imply that sea otter initiatedtrophic cascades in eelgrass ecosystems may not be common across their overlapping ranges but will depend on the local biotic and abiotic local conditions. Future efforts will aim to synthesize sea otter impacts to seagrass systems across their entire range. Saavedra, Jessica J. 2021. Testing the Effects of Recovering Sea Otters on Seagrass Ecosystems in Southeast Alaska. Department of Biology, Sonoma State University.