Advancing acoustic studies of pelagic fish and zooplankton at the western Atlantic-Arctic gateway

The patterns of distribution among fish and zooplankton in the Arctic and their deep ocean habitats remain poorly described. Climatic disruptions to natural variability in temperature, light, and nutrient supply can modify species distributions, impacting the ecosystem at various levels. Across spatial and vertical domains, these environmental factors form complex relationships with species assemblages, which are challenging to measure with conventional sampling methods. Hydroacoustics is a promising approach to studying the distribution of organisms in hard-to-reach ecosystems. This thesis aims to advance new hydroacoustic methodologies for the study of pelagic fish and zooplankton and identify the structural drivers of their communities and distribution along the boundaries of Arctic-Atlantic modulated ecosystems. In my thesis, I used several emerging technologies, including lowered acoustic probes and broadband acoustic measurements, to assess the distributional patterns of pelagic fish and zooplankton at meter-level to ocean basin scales. I found that measurements of lanternfish inhabiting deep-scattering layers require detailed analysis that includes the identification and removal of anomalous signals, which can arise from a combination of physical and biological processes. To deal with this, I introduced a machine learning approach that helps identify anomalous signals and improves the precision of acoustic density measurements. When examining similar lanternfish dominated communities at the basin scale, I report that temperature-driven water mass boundaries act as a barrier to the dispersal of mesopelagic communities at high-latitudes, in both the northern and southern hemispheres. Furthermore, this same boundary appears to impact the vertical distribution of pelagic life, with likely impacts the vertical transport of nutrients and carbon. In the high Arctic, I report mixing processes in glacial fjord ecosystems can impact the vertical distribution of copepods and the morphological patterns of marine snow. This work highlights contrasting conditions outside of two adjacent marine terminating glaciers, which each offer a glimpse of what future Arctic conditions may bring. Overall, my work contributes to a greater understanding of the structural drivers of Arctic pelagic communities and can be used to understand their sensitivities to future change.