The interplay between trophic ecology, environmental variability, and an endangered marine species
A grand challenge of the 21st century is to understand the response of ecosystems and populations of species to environmental variability and intensifying climate change. My dissertation focuses on the potential for changing environmental conditions to influence marine food webs, foraging ecology, and ultimately population success of consumers. I combined biogeochemical tools (stable isotope analyses) of zooplankton and endangered leatherback turtles with measures of oceanography and environmental conditions to evaluate changes in foraging ecology and food web dynamics over time. My research specifically focuses on long-term trends in the foraging ecology and habitat use of Atlantic and Pacific leatherback turtles and how environmental variability in the Pacific may alter food web dynamics in a critical foraging area for a declining leatherback population. My first two chapters were focused on leatherback turtles, a cosmopolitan species with populations inhabiting tropical and temperate regions throughout the global ocean. In Chapter 1, I examined the trophic ecology of North Atlantic leatherbacks over an eighteen-year period to test the hypothesis that shifts in foraging ecology or environmental conditions in the North Atlantic have contributed to leatherback population recovery. In Chapter 2, I focused on a subgroup of the critically endangered Western Pacific leatherback population that forages in the California Current. Here, I addressed questions about their diet, habitat use, and the trophic structure of leatherback prey in the California Current Large Marine Ecosystem (CCLME). These two chapters allowed me to better understand whether the continuing decline of Pacific leatherbacks was related to dietary differences potentially driven by variability in environmental conditions between ocean basins as the North Atlantic population of turtles are steadily increasing. In Chapter 3, I investigated ecosystem responses to a multi-year, warm water anomaly (a marine heatwave and strong El Niño event) in the CCLME, which is a productive upwelling system that supports the biomass of many commercially and ecologically important species, including the leatherback population that Chapter 2 focused on. My findings illustrate mechanisms through which the amount of energy transferred to higher trophic level consumers is altered by environmental variability in the CCLME. In my first three chapters, I used stable isotope analyses, which can be a valuable tool for reconstructing patterns of trophic or foraging ecology over time. However, archived tissues that are used for analyses are often stored in chemical preservatives, which may affect their potential for use in isotope ecology. In Chapter 4, I conducted laboratory experiments to test the effects of common chemical preservatives on stable isotope values to better understand how we can best use preserved and archived tissues in future studies. My research provides insight into the trophic ecology and habitat use of an endangered marine consumer. Although I found no differences in trophic position between leatherback conspecifics, environmental conditions in the North Atlantic may have contributed to the recent increases in this population. My research elucidates the effects of a strong environmental perturbation on the California Current food web, which is a productive upwelling region used by many commercially important and protected species. This work provides trophic position estimates for two leatherback populations, several gelatinous zooplankton species, and calanoid copepods in the California Current, which can be incorporated into future ecosystem or habitat models and used for ecosystem-based management of marine resources. Furthermore, my results contribute to our understanding of temporal trends in foraging ecology and food web responses to environmental variability and anomalous warming, which is useful for predicting ecosystem responses to future climate change scenarios.