UC San Diego

The processes influencing biogeographic distributions and community composition in the mesopelagic ocean are not well understood. In the pelagic ocean, the magnitude of dispersal potential and environmental variability vary with depth. In this dissertation, I compare mesopelagic zooplankton communities with epipelagic communities to determine the relative influence of dispersal and selection on midwater zooplankton. First, I compare DNA metabarcoding-resolved community composition against carbon biomass from digital Zooscan imaging, and find good overall correspondence between methods for more abundant taxa. This chapter provides methodological refinements and validation. I then examine zooplankton communities across a strong environmental gradient within the California Current Ecosystem. At a regional scale, I find cross-shore changes in community composition and vertical shifts in habitat use. The magnitude of change varies among zooplankton with different traits. In a space-for-time framework, these results suggest community turnover and vertical reorganization within future, more stratified oceans. I then contrast epipelagic and mesopelagic zooplankton communities across major biogeographic transitions between the Subarctic Pacific, North Pacific West Wind Drift, and North Pacific Subtropical Gyre. I find stronger biogeographic breaks in the epipelagic zone and larger distributional ranges for mesopelagic taxa. Varying widths of environmental and distributional ranges among taxa with different traits suggest trait- and depth-dependent effects of selection and dispersal. At the basin scale, I compare zooplankton distributions and community similarity of mesopelagic and epipelagic zooplankton at 28 locations spanning all major biogeographic provinces in the North Pacific Ocean. I find no differences in the extent of biogeographic ranges or levels of community dissimilarity between epipelagic and mesopelagic assemblages. I find stronger associations of community dissimilarity with dispersal potential and environmental dissimilarity in the epipelagic zone than in the mesopelagic zone. This dissertation finds greater similarity between epipelagic and mesopelagic biogeographic distributions at the basin scale than at intermediate or regional scales. Varying extents of environmental and biogeographic ranges among taxa with different vertical habitat use patterns, feeding behaviors, diets, and reproductive strategies suggest trait-dependent effects of selection and dispersal. This dissertation is a first step toward describing the ecological processes influencing the distributional boundaries of mesopelagic zooplankton.