Carbon flow and food web structure in the mesopelagic zone of the North Atlantic Ocean

Abstract

Mesopelagic ecosystems are vital habitats that link the euphotic zone and the deep ocean through food web interactions and carbon flow pathways. In this dissertation, I use carbon compound-specific stable carbon isotope analysis of amino acids (CSIA-AA) and DNA gut metabarcoding methodologies to provide a broad ecological outlook on mesopelagic carbon flow coupled with finer scale taxonomic details. In Chapter 2, I analyze the diets of seven abundant mesopelagic fish species by combining the integrative power of CSIA-AA with the instantaneous, taxonomic aspects of DNA gut metabarcoding. Three primary diet types were identified: copepod-based, fish-based, and generalist. Additionally, carbon sources were variable across the two years, but cyanobacteria were consistently an important carbon source - evidence that mesopelagic fish are essential exporters in weaker biological pump systems. Finally, this chapter includes cyanobacteria CSIA-AA signature data that was previously missing from the literature. In Chapter 3, I augment the CSIA-AA data by adding genus-level zooplankton data and samples from the winter season. Zooplankton were more dispersed among all the end members than fish, particularly in the winter. Fish, however, still relied the most on cyanobacteria-sourced carbon. This chapter supplies the first zooplankton carbon CSIA-AA data set at such a fine taxonomic resolution. In Chapter 4, I examine the effect of phytoplankton community structure on fish and zooplankton carbon sources by sampling before and during diatom a diatom bloom. Zooplankton, and to a lesser extent fish, showed a shift to diatom-based carbon sources during the bloom. As a whole, this dissertation advances our knowledge of mesopelagic food webs by providing a baseline carbon-CSIA-AA data set for key zooplankton and fish species across several seasons that will inform ecological models to understand how the mesopelagic will react to anthropogenic pressure.