Doctor of Philosophy in Oceanography (PhD)
Structures and drivers of zooplankton communities of the British Columbia coastal ocean
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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.
The cultural and ecological contributions of salmon cannot be understated, as these keystone species have underpinned coastal ecosystems and societies from time immemorial. Despite this millennia-long intimate relationship with Pacific salmon, returns of stocks have become unpredictable and difficult to manage from overfishing and multiple complex stressors. Research has shown that juvenile salmon feeding is a crucial factor for growth and recruitment, and the ocean conditions driving prey availability are tightly coupled with survival of salmon. Pink and chum are abundant co-migratory species of salmon that may exert competitive pressure for food resources during their vulnerable early marine phase. However, competition research on juvenile pink and chum salmon is limited, especially within the complex British Columbia coast. This research aimed to fill gaps in understanding of juvenile pink and chum foraging strategies and interactions in areas of good and poor foraging conditions during their coastal outmigration. In the Discovery Islands and Johnstone Strait regions, there were foraging deserts and oases, where juvenile salmon mean stomach fullness values ranged from 6% body weight. In good foraging conditions, juvenile pink and chum both consumed the same high-quality crustacean prey with limited competition, but under poor foraging scenarios, salmon diets differed. Chum salmon consistently consumed gelatinous prey and pink salmon relied more heavily on copepods and nearshore zooplankton, differing in niche in response to competitive interactions. There was a match between predators and prey in 2015, when salmon fed on larger prey, and were in healthier condition (K = 1.0). There was a potential mismatch in 2016, when small prey taxa may have caused poorer condition for juvenile salmon (K = 0.94). Chum salmon had a stronger relationship to prey size than pink, when larger chum successfully consumed the largest prey. These foraging strategies of opportunistic specialization may indeed provide salmon with resilience to face the challenges of shifting climates. Pink and chum salmon can be monitored as indicators for ecosystem health and zooplankton availability. Salmon reflect the health of socio-ecological systems and require our understanding and care to view them holistically as they migrate through diverse, challenging habitats.
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