Characterizing deepwater oxygen variability and seafloor community responses using a novel autonomous lander

Abstract

Studies on the impacts of climate change typically focus on changes to mean conditions. However, animals live in temporally variable environments that give rise to different exposure histories that can potentially affect their sensitivities to climate change. Ocean deoxygenation has been observed in nearshore, upper-slope depths in the Southern California Bight, but how these changes compare to the magnitude of natural O2 variability experienced by seafloor communities at short timescales is largely unknown. We developed a low-cost and spatially flexible approach for studying nearshore, deep-sea ecosystems and monitoring deepwater oxygen variability and benthic community responses. Using a novel, autonomous, hand-deployable Nanolander® with an SBE MicroCAT and camera system, high-frequency environmental (O2, T, estimated pH) and seafloor community data were collected at depths between 100 and 400 m off San Diego, CA, to characterize timescales of natural environmental variability, changes in O2 variability with depth, and community responses to O2 variability. Oxygen variability was strongly linked to tidal processes, and contrary to expectation, oxygen variability did not decline linearly with depth. Depths of 200 and 400 m showed especially high O2 variability; these conditions may give rise to greater community resilience to deoxygenation stress by exposing animals to periods of reprieve during higher O2 conditions and invoking physiological acclimation during low O2 conditions at daily and weekly timescales. Despite experiencing high O2 variability, seafloor communities showed limited responses to changing conditions at these shorter timescales. Over 5-month timescales, some differences in seafloor communities may have been related to seasonal changes in the O2 regime. Overall, we found lower-oxygen conditions to be associated with a transition from fish-dominated to invertebrate-dominated communities, suggesting this taxonomic shift may be a useful ecological indicator of hypoxia. Due to their small size and ease of use with small boats, hand-deployable Nanolanders can serve as a powerful capacity-building tool in data-poor regions for characterizing environmental variability and examining seafloor community sensitivity to climate-driven changes.