Oceanographic Influences On Pelagic Community Ecology Across The Eastern Pacific Ocean: Insights From Acoustics And Combined Sampling Approaches

Abstract

Remote open-ocean habitats are globally understudied due to their inaccessibility and the costs associated with sampling so far from land. The eastern Pacific Ocean is one such habitat whose pelagic biology is largely unsampled, despite a rapidly emerging deep-sea mining industry across much of this region and the presence of an expanding oxygen minimum zone (OMZ). In this dissertation, I use active acoustics to investigate the dynamics of pelagic prey populations, namely micronekton and large zooplankton, in relation to regional and mesoscale oceanography across the eastern Pacific. At the broadest scale, I find that depth structure and vertical migration patterns of pelagic fauna (observed as scattering layers) vary greatly across the region, with the eastern tropical Pacific OMZ acting as the strongest predictor of these dynamics. Using these relationships to predict behaviors across the eastern Pacific Ocean, I find that an existing network of no-mining reserves lies outside of the core OMZ and may not represent or protect the pelagic OMZ fauna at highest risk from future mining impacts. At smaller spatial and temporal scales within the region, I find that the passage of cyclonic and anticyclonic mid-ocean eddies, as well as a strong near-surface current boundary, greatly influence the vertical structure and strength of acoustic scattering layers. In addition to broader regional patterns, this work highlights the considerable daily and weekly variability of pelagic populations in the eastern Pacific Ocean that is inherent to open ocean conditions and plays a large role in aggregation and predator-prey interactions. As acoustic techniques are limited in what they sample, I further explore pelagic community dynamics through the various perspectives and biases of several different sampling approaches in the first integrated evaluation of micronekton in the remote eastern Pacific. Using active acoustics, MOCNESS trawls, and ROV video footage, I find that trawl and video data greatly expand perceived micronekton distributions suggested by surface-based acoustic profiles. I find that while acoustic and trawl data show similar seasonal trends with greater total backscatter, abundance, and biomass, during the spring, the vertical distributions of these metrics are quite variable between survey sites and seasons. At large, this dissertation highlights the strong variability of pelagic faunal communities in remote ocean habitats and the complexities of observing these populations through different lenses. The research discussed is the first to use Saildrones to examine micronekton in the open ocean and provides some of the only remote sampling of pelagic prey communities in the eastern Pacific Ocean.