From earth and ocean: Investigating the importance of cross-ecosystem resource linkages in estuaries of the Pacific Northwest

Similar to how political boundaries do not reflect the cultural ties and ancestral lineages of human history, classical ecological perspectives often do not account for the complex relationships amongst ecosystems at local, regional or global scales. Cross-ecosystem resource linkages provide crucial subsidies to many ecosystems on Earth. Resource subsidies can contribute to the productivity, form, and function of recipient ecological communities. However, a subsidy’s importance can vary widely among landscapes as a result of resource availability, ecosystem characteristics and consumer traits. Estuaries are composed of highly connected habitats that reside at the interface between terrestrial, freshwater and marine environments. Consequently, they are ideal systems to explore the importance of resource subsidies and how their role can vary spatially. This thesis examines the assimilation of, and response to resource subsidies in estuaries of the Northeastern Pacific. I focus on two spatial subsidies: terrestrial resources delivered to estuaries via the movement of freshwater, and salmon resources that enter coastal watersheds during fall spawning seasons. First, I show that species-specific distributions of live spawning salmon, their associations with terrestrial predators, and physical characteristics of individual systems drive salmon subsidies to riparian forests and estuaries. I then focus on subsidy responses in two estuarine consumers; soft-shell clam (Mya arenaria) and Dungeness crab (Metacarcinus magister). Through the use of stable isotopes, I demonstrate that landscape-level traits such as watershed size and salmon density drive the assimilation of subsidies in both species and that location within an estuary can mediate responses in sedentary consumers. However, terrestrial-derived subsidies also influence the size of individuals, suggesting this resource may have farther-reaching effects. Finally, I compare the dietary composition of three consumers and find that subsidy contributions increase with availability while accounting for other estuarine resources. Mobile consumers may benefit most, by being better able to exploit heterogeneous resource pools. This thesis demonstrates that terrestrial- and salmon-derived resource subsidies contribute to the resource base in estuarine ecosystems and that terrestrial subsidies may have the most pronounced effects. Ecosystems are connected, but the strength of these connections varies. It is therefore crucial to place resource dynamics within the context of specific landscapes and species to properly evaluate subsidy importance.