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

Abstract

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.

Moving on up: Vertical distribution shifts in rocky reef fish species during climate‐driven decline in dissolved oxygen from 1995 to 2009

“…The distributions of fish and invertebrate species, including species with high commercial or recreational fishery value, have been associated with DO in previous studies (Chan et al, 2008 ; Gallo, Beckwith, et al, 2020 ; Gallo, Hardy, et al, 2020 ; Keller et al, 2010 , 2015 , 2017 ). However, it is difficult to disentangle ecosystem response to persistent or seasonal low DO due to local bathymetry or local‐scale oceanographic processes from multidecadal trends in DO stemming from either natural climate oscillations or anthropogenic climate change when surveys occur only once or sporadically.…”

“…The distributions of fish and invertebrate species, including species with high commercial or recreational fishery value, have been associated with DO in previous studies (Chan et al, 2008;Gallo, Beckwith, et al, 2020;Gallo, Hardy, et al, 2020;Keller et al, 2010Keller et al, , 2015Keller et al, , 2017. However, it is difficult to disentangle ecosystem re- levels found to have impacts on metabolism, swimming behaviour and predation in laboratory studies of several California rockfish species (Davis et al, 2018;Flannery, 2018).…”

“…Previous studies provide insight on the effects of low DO on fish survival, fitness and distribution in the productive California Coastal Current (e.g. Chan et al, 2008 ; Davis et al, 2018 ; Flannery, 2018 ; Gallo, Hardy, et al, 2020 ; Keller et al, 2017 ; McClatchie et al, 2010 ).…”

“…In a study by Gallo, Beckwith, et al, ( 2020 ), a remotely operated vehicle (ROV) was used to survey benthic fish communities in the Gulf of California at depths ranging from 200 m to 1400 m. Oxygen level was the best predictor of fish community composition and diversity, and declines in oxygen predicted by a global climate model are expected to drive a reduction in diversity by 2081–2100 (Gallo, Beckwith, et al, 2020 ). Observations from an autonomous lander at depths from 100–400 m off the coast of San Diego indicate that benthic communities transitioned from fish dominated to invertebrate dominated along a declining oxygen gradient (Gallo, Hardy, et al, 2020 ). West Coast Groundfish Bottom Trawl surveys conducted within a known hypoxic zone off the coast of Oregon show significantly lower weight to length ratios in five of six groundfish species in low DO regions (<1 ml/L) relative to moderate regions (>1 ml/L, Keller et al, 2010 ).…”

The National Geographic Society Deep-Sea Camera System: A Low-Cost Remote Video Survey Instrument to Advance Biodiversity Observation in the Deep Ocean

“…However, due to their size and weight, many lander vehicles still require large ships with winch capabilities to operate them (e.g., Hardy et al, 2013; but see Phillips et al, 2019;Gallo et al, 2020). In 2009, the National Geographic Society (NGS) Exploration Technology Lab developed a hand-deployable deep-sea lander and camera system that has since been deployed opportunistically across the world's ocean, largely on NGS Pristine Seas expeditions (e.g., Friedlander et al, 2019;Giddens et al, 2019; Table 1).…”