An experimental and in situ application of stable isotopes and fatty acids to investigate the trophic ecology of moon jellyfish (Aurelia spp., Linnaeus 1758)

Scyphozoan jellyfish are important components of marine ecosystems as generalist feeders with complex trophic interactions. These interactions can be investigated using biomarkers, like stable isotope (SI) ratios and fatty acid (FA) profiles. However, the absence of reliable estimates for SI and FA turnover time and modification in jellyfish limits the accuracy of these approaches for investigation of jellyfish trophic ecology. In this thesis, I conducted a controlled feeding experiment for two scyphozoan predators (Aurelia aurita and Chrysaora pacifica) and two prey types (crustacean zooplankton and gelatinous A. aurita) to provide quantitative estimates for SI and FA turnover time and modification between trophic levels. I estimated SI trophic enrichment factors for jellyfish feeding on crustacean zooplankton (Δδ¹³C = 1.19‰ and Δδ¹⁵N = 2.09‰) and jellyfish feeding on interspecific jellyfish (Δδ¹³C = 1.59‰ and Δδ¹⁵N = 1.35‰). I found some similarities between both predators when consuming the same prey, which suggests some metabolic pathways that are conserved for jellyfish. Specifically, 18-carbon FAs decreased in proportion in the predators compared to their prey, while 20-carbon FAs increased, which implies a 2-carbon elongation pathway in jellyfish. By providing estimates for turnover time and modification of SIs and FAs for jellyfish, I have advanced the utility of SIs and FAs for investigating jellyfish trophic ecology. After establishing SI and FA turnover time and modification parameters, I applied these parameters to investigate the trophic ecology of Aurelia labiata in a temperate coastal food web. Using SIs and FAs for 152 jellyfish 19-225 mm in size, I documented a shift in diet, where the proportion of zooplankton in the diet of A. labiata increased as bell diameter increased. I also documented a size-based shift in the nutritional quality of A. labiata, where C:N decreased with size, arachidonic (ARA) and docosahexaenoic (DHA) acid increased with size, and eicosapentaenoic (EPA) acid was unaffected by size. Only changes in C:N and DHA were apparently related to changes in the diet. Marine food webs are highly size structured, so these size-specific results will have implications for the flow of energy and nutrients through jellyfish in marine food webs broadly.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat