Intellectual Merit The upper ocean to mesopelagic depth range (0-1000m) encompasses a complex interwoven ecosystem characterized by intricate relationships among its inhabitants and their environment. It is a critically important zone for oceanic biogeochemical and export processes and hosts key food web components for commercial fisheries. Oxygen minimum zones (OMZs,) large midwater oceanic regions with very low oxygen, are expected to expand in extent as a result of future climate change and ocean deoxygenation (decreasing oxygen). While oxygen is known to be important in structuring midwater ecosystems, a clear mechanistic understanding of effects of low oxygen on marine zooplankton and nekton, key components of food webs and biogeochemical cycles, has been lacking. This project examined zooplankton distributional and physiological responses to oceanic low oxygen in the Eastern Tropical North Pacific OMZ and the significance of predicted OMZ expansion and ocean deoxygenation for organisms, ecosystems, and society in the future. New concepts in metabolic theory (the Metabolic Index) were evaluated to model zooplankton and nekton responses to oxygen and temperature gradients and changes. Enhanced understanding of zooplankton distributional and physiological responses to low oxygen and OMZ variability was accomplished with unique shipboard sampling strategies (horizontally sequenced and vertically stratified MOCNESS zooplankton net tows) and live experiments. A new seagoing instrument (Wire Flyer) was deployed for mesopelagic environmental profiling and transects. Metabolic models demonstrated that a species’ resting and active metabolic rates and their oxygen- and temperature-sensitivities were mechanistically and quantifiably linked and that the Metabolic Index and the Factorial Aerobic Scope (common measures of animal physiological fitness) were equivalent (Fig. 1). This provided new insight into marine biogeography relative to animals’ temperature and oxygen tolerances. Metabolic constraints associated with temperature-dependent hypoxic tolerance limit suitable habitat (Fig. 2), and animals have limited scope for physiological adaptation to future changes. The influence of metabolic trait diversity on biogeography was applied conceptually beyond this project to California Current fish and globally. Previously undescribed submesoscale and mesoscale OMZ oxygen variability affected distributions of major zooplankton groups. Despite extraordinary hypoxia tolerance, many zooplankton lived near their physiological limits and responded to very slight (≤1%) changes in oxygen (Fig. 3). For particular taxa, changes in horizontal and vertical distributions and abundances, diel vertical migration patterns, life history strategies (diapause depths), and physiological parameters indicated a variety of species-specific adaptations and behavioral responses to oxygen levels and gradients. Copepod species may track oxygen and change vertical distributions associated with the shape of the oxygen vertical profile, (Fig. 4), adjust diel vertical migration depths (surprisingly, the nighttime upper depth), shift their diapause (resting) depth, or alter depth range within a thick or thin upper aerobic zone (habitat compression). Because temperature decreases with depth, changes in depth distributions have metabolic implications that may affect food webs, the biological pump, and biodiversity. Future ocean deoxygenation may elicit major unanticipated changes to midwater ecosystem structure and function (Fig. 5). Unlike coastal "dead zones" where oxygen levels can suddenly plummet and kill marine life, zooplankton in OMZs are adapted to live where many other organisms cannot. They have adapted over millions of years to the very low oxygen of this extreme yet widespread midwater habitat. However, they are living at the limits of their physiological capability and are sensitive to very small changes in oxygen. Many species decrease in abundance when oxygen gets just a bit lower, although a few specialists thrive in the lowest oxygen water. If OMZs expand, even slight changes in lowest oxygen levels could push zooplankton beyond their extraordinary physiological limits and alter depth distributions and abundances. Among the zooplankton, there will likely be winners and losers with future ocean deoxygenation, as species cope with environmental change. Changes in individual copepod species abundances, vertical distributions, and life history strategies may create potential perturbations to intricate food webs and processes. OMZ expansion into shallower waters may disrupt zooplankton distributions, alter food links between zooplankton and commercial fisheries, and alter the functioning of the biological pump. Future ocean deoxygenation could result in some animals being unable to adapt and persist. Broader Impacts Results have been disseminated in publications, conferences, public outreach venues, and undergraduate and graduate classes. Students received training at sea and in the laboratory and completed theses. The Wire Flyer profiler is being used beyond this project. Overall, this collaborative project highlighted unexpected complexities of predicting responses of zooplankton and ecosystems to ocean deoxygenation and OMZ expansion. It illuminated ongoing challenges of meshing submesoscale physical oceanographic variability, species-specific organism responses, and species-specific physiological capabilities to global measurements and models of future ocean deoxygenation. Results are increasing understanding of potential impacts of ocean deoxygenation and OMZ expansion on important species, processes, and ecosystems. Understanding present-day variability and adaptations, including unexpected responses, can inform future predictions and provide a window into potential effects of ocean deoxygenation (Fig. 5). Last Modified: 04/06/2021 Submitted by: Karen F Wishner
©2020 Biological and Chemical Oceanography Data Management Office.