Yang, N.; Allan, E. A.; Stover, S. E.; Grassian, B. D.; Sosik, H. M.; Llopiz, J. K.; Govindarajan, A. F.

The diversity and interactions among mesopelagic organisms are difficult to study and as a result, are insufficiently unaccounted for in food web and biogeochemical models. This knowledge gap hinders our ability to model and forecast ecosystem function and formulate effective policies for conservation and management in the face of growing interest in exploiting midwater living resources. We used multi-marker metabarcoding of environmental DNA (eDNA) samples collected from Northwest Atlantic Slope Water to resolve patterns of eukaryotic community composition spanning taxonomically across protists (microbial eukaryotes), invertebrates, and vertebrates and vertically from the ocean surface to the base of the mesopelagic zone. With statistical network analyses, we explored cross-kingdom associations including trophic interactions such as food web dynamics and evaluate network robustness to biodiversity loss. We found depth-specific communities of distinct protist, invertebrate, and vertebrate assemblages. Ecological networks for the epipelagic, upper mesopelagic, and lower mesopelagic suggest that protists are keystone taxa and important mediators of trophic interactions; they increase network complexity and contribute to network stability. We also identified metazoans including copepods, gelatinous taxa (cnidarians, tunicates), and mesopelagic fish as important components of network interactions. Our study demonstrates a holistic approach to generate insights on mesopelagic biodiversity and implications for ecosystem resilience that can inform future conservation and management efforts.