UC San Diego

As oceans warm due to anthropogenic climate change, the global ocean oxygen inventory decreases in a process known as ocean deoxygenation. Since oxygen is fundamental for aerobic life, ocean deoxygenation can impact marine organisms across multiple levels of biological organization. This research used the steep gradients in oxygen characteristic of upwelling margins with oxygen minimum zones (OMZs), as a natural experiment to study how hypoxia affects the composition, diversity, trophic structure and dynamics of demersal fish communities. Despite the severely hypoxic conditions ([O2] < 22 μmol kg-1), demersal fish are shown to be present in OMZ systems around the world; several species exhibit remarkable hypoxia tolerance in the Gulf of California ([O2] < 2 μmol kg-1). Demersal fish diversity is sensitive to hypoxia, and diversity shows a non-linear response to low oxygen conditions, decreasing rapidly below a region-specific oxygen threshold. Density patterns are less predictable in relation to oxygen levels. Paired video and environmental samples, taken using an autonomous nanolander, show that seafloor communities on the upper margin (100-400 m) in the Southern California Bight (SCB) experience substantial natural oxygen variability at daily and weekly timescales, but communities do not respond to low-oxygen events at these timescales. Feeding relationships are also altered by hypoxia. At the upper OMZ boundary in the SCB a transition occurs from primarily pelagic-feeding to benthic-feeding fishes, which contrasts with the pattern on non-upwelling margins. The expansion of low-oxygen conditions in the E. Pacific may lead to a shift in community composition as intolerant species are excluded and hypoxia-tolerant species become competitive, an overall decrease in diversity, a reduction in bentho-pelagic coupling, and a lengthening of the food chain as demersal fish shift to utilizing benthic resources. Considering the natural variation of oxygen conditions across different timescales is important as existing modes of variability determine exposure histories of animals and likely influence sensitivity of communities to deoxygenation. Despite major ecological consequences of ocean deoxygenation, an analysis of Nationally Determined Contributions submitted under the Paris Agreement indicate that oxygen loss remains an under acknowledged topic by policy-makers in the international climate community.