Mapping organic matter supply pathways to mesopelagic zooplankton food webs using stable isotope analysis of amino acids

Abstract

The biological pump represents the main pathway by which carbon is exported from the atmosphere to be sequestered in the deep sea. Zooplankton play a central role by mediating multiple pathways of carbon export and thus are key players in the ocean carbon cycle. This dissertation assesses the role of zooplankton in the biological pump by analyzing a large set of amino acid stable isotope data from zooplankton and particles collected at multiple sites in the Pacific and Atlantic Oceans.To facilitate this work, Chapter 2 describes the development and testing of a novel Bayesian mixing model tailored for use with δ15N values of individual amino acids. This model simultaneously estimates trophic relationships between consumers and organic matter sources at the base of the food web, determines the relative contributions of these basal organic matter sources to consumers, and accounts for trophic discrimination affecting amino acid δ15N values during protozoan and metazoan trophic steps. We demonstrate that this new model can enhance our understanding of the roles of small particles and diel vertical migration in deep-sea organic matter supply pathways and provide a new framework for exploring the foundational role of heterotrophic protists in marine ecosystems. Chapter 3 then employs this model to assess mesopelagic organic matter supply and trophic ecology at sites with varying levels of particle export and primary productivity. Our findings suggest that earlier studies likely overestimated the role of microbially degraded detritus in supporting deep-sea food webs. Instead, we identify a substantial contribution from vertically migrating zooplankton transporting undegraded surface organic matter to mesopelagic depths, particularly in regions with higher levels of net primary productivity and low particle export. At the same time, our results reaffirm the importance of microbially degraded particles in the lowest productivity environments. We contrast these patterns with the high productivity, high-export conditions of the North Atlantic spring bloom, where passive sinking of large, partially degraded aggregates dominates organic matter delivery to the mesopelagic zone. This reanalysis affirms that vertically migrating zooplankton play a critical role in regulating upper ocean elemental cycling, while also delineating high- and low-productivity end-member systems where passive flux predominates. Finally, in Chapter 4, we apply essential amino acid δ13C fingerprinting to assess the extent to which mesopelagic zooplankton rely on nutrients derived from diatomaceous biomass, versus other phytoplankton taxa present during bloom decay. Our analysis reveals a marked shift in zooplankton nutrition from reliance on essential amino acids produced predominantly by diatoms and bacteria during peak bloom, to those produced by dinoflagellates during bloom decay. This transition was consistent across all zooplankton size fractions from the surface to 750 m, suggesting a tight coupling between mesopelagic organic matter supply and surface phytoplankton succession, and highlighting the selectivity of zooplankton grazing in favor of recently produced organic matter over partially degraded detritus. Together, these findings help paint a more refined picture of the specific pathways supplying organic matter to the mesopelagic zone across a range of oceanographic settings, highlighting the preference of zooplankton for high-quality, protein-rich organic matter against a backdrop of resource scarcity in the deep sea.