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Abstract

Understanding how zooplankton traits and environmental context influence carbon export is crucial for accurately quantifying biological sequestration, particularly in the dynamic and under-sampled Southern Ocean. Among these organisms, Salpa thompsoni plays a disproportionate role due to its high fecal pellet production (FPP) and widespread diel vertical migration (DVM). However, the influence of Life Cycle Stage (LCS) and body size on salp-mediated carbon flux has remained poorly resolved. This dissertation investigates S. thompsoni carbon export across seven Southern Ocean locations spanning 1989 to 2022, integrating behavioral analyses, mechanistic modeling, and methodological comparisons to provide a stage- and size-specific framework for salp carbon cycling. Chapter 2 analyzes 13 paired day-night net tows to show that DVM behavior varies strongly with LCS and body size. Contrary to traditional assumptions, small blastozooids frequently migrated deeper and more extensively than larger reproductive oozoids, which often remained shallow or exhibited reverse DVM patterns. Chapter 3 builds on these insights by modeling salp FPP and export using a mechanistic framework parameterized by body size, temperature, and prey availability. While LCS and size class were not model inputs, stratifying outputs post hoc revealed that depth of residence, rather than body size per se, governed export efficiency. Body size indirectly influenced export outcomes by affecting vertical distribution, which in turn shaped individual FPP and carbon fate. Larger, surface-residing salps produced more but were prone to remineralization, whereas smaller, deeper-residing individuals contributed disproportionately to long-term sequestration. Chapter 4 compared net and imaging methods, demonstrating that conventional nets severely under-sample fragile taxa such as salps and misrepresent their vertical structure. In situ imaging revealed higher abundances, finer-scale distributions, and more accurate estimates of DVM dynamics. Together, these findings suggest that accurately quantifying salp-mediated carbon flux requires resolving vertical habitat, trait structure, and methodological biases. This dissertation reframes salps as structured, behaviorally complex actors whose export contributions depend on the interplay of LCS, body size, and depth. The trait- and depth-resolved framework developed here provides transferable tools and parameterizations for improving salp representation in biogeochemical models and for reducing uncertainty in Southern Ocean carbon sequestration projections.