The ecological functioning of Southern Ocean pteropods and the use of novel field- and laboratory-based approaches

Southern Ocean (SO) thecosome pteropods are considered sentinels of anthropogenic ocean acidification (OA), and mounting evidence shows that rapid changes in ocean chemistry will affect their functional roles within marine ecosystems. However, there are major gaps in understanding of their current functional roles, along with the nature and magnitude of ongoing changes to these roles. There is urgency to address these gaps to establish a benchmark against which ongoing species-to-ecosystem scale responses may be assessed. The research in this thesis draws together state-of-the-art methodologies to provide new insights on the intra- and interspecific ecological roles of co-occurring SO pteropod species, focusing on the Indian sector of the Southern Ocean. In the first two data chapters of this thesis, I used analysis of tissue stable isotope signatures to derive new insights into trophodynamics of among 3 key species of Southern Ocean pteropods. In Chapter 2, I first used isotopic niche metrics and a comparative analysis of several normalization models to show that it is important to account for lipids, either chemically or mathematically, before applying stable isotopes analysis (SIA) to estimate ecological niches for polar pteropods with moderate to high lipid content. In Chapter 3, I then extended the isotopic analysis to assess interspecific variability in feeding behaviour, including examining relationships between trophic position and body size. I found that Clio pyramidata f. sulcata occupied the lowest trophic position and smallest niche breadth relative to two gymnosomes, Clione limacina antarctica and Spongiobranchaea australis. This analysis also revealed dichotomous feeding and diet behaviour in gymnosomes, with C. limacina antarctica exhibiting either a more generalist behaviour than expected or a niche breadth reflective of food limitation. In my third data chapter, I drew upon data from sediment traps to assess the contribution of pteropods to particle flux, which is a major (but poorly understood) mechanism by which pteropods contribute to carbon recycling in and export from Southern Ocean ecosystems. Two sediment traps were deployed to measure pteropod swimmer abundance and composition on two time scales. In both experiments, highest abundances were measured for Limacina helicina antarctica veligers. Fluorescence and sinking particulate organic and inorganic carbon possessed the most explanatory power for abundances of thecosome compositions, while gymnosome abundances were largely influenced by increasing adult L. helicina antarctica counts. In my final data chapter, I examined fecundity, which is a key gap in knowledge of pteropod life histories (and information for their representation in population and ecosystem models). I assessed fecundity as the number of fertilized eggs per spawning event. I assessed egg and egg mass morphometrics, potential drivers of production, and presented a comprehensive description of the embryogenetic stages. Overall, egg dimensions change very little through successive embryonic stages, however egg areas decrease through the spawning season. Significant differences in egg and egg mass morphology between voyage samples were also discussed. Together, these chapters represent a synthesis of emerging methods and broader ecological implications. Understanding of the key roles of pteropods in the Indian Sector of the Southern Ocean estimated here can enable better assessments of their impacts throughout the Southern Ocean and global marine ecosystems more broadly.