Oxygen Minimum Zones Impact Bentho-Pelagic Coupling on Upwelling Margins with Implications for Demersal Fisheries Productivity

Diel vertically migrating (DVM) organisms link shallow pelagic and deep-sea benthic habitats because they are an important food source for demersal fish on continental margins up to depths of 1000 m. On continental margins in upwelling regions, the presence of a midwater oxygen minimum zone (OMZ) can compress the daytime depth of the DVM community, potentially reducing this trophic linkage at deeper depths. With the use of ROVs and otter trawls, we examine how the reliance of the demersal fish community on pelagic prey resources changes spatially along the southern California margin (100-1200 m) from a shallower well-oxygenated habitat to a deeper severely hypoxic habitat based on changes in demersal fish community composition, gut content and stable isotope analysis. Consistent with our hypothesis, we find that a shift in dominant feeding mode from bentho-pelagic to benthic occurs at the upper boundary of the OMZ (~450 m, O₂ < 22.5 μmol kg^-1), and is associated with a shift in demersal fish community composition. Fishes living within the OMZ, are predominantly benthic feeders and have enriched δ¹⁵N signatures, suggesting they are feeding at a higher trophic level than species living outside of the OMZ. In contrast, at depths shallower than the OMZ between 300-350 m, 75-90% of the fish community can be composed of predominantly bentho-pelagic feeders with lighter δ¹⁵N signatures; fish densities are also higher at these depths. Along with physiological tolerance, results suggest that changes to bentho-pelagic connectivity may also affect the ranges of demersal fish species, such as rockfish, on the US West Coast with relation to oxygen levels. In the Southern California Bight, only ~20% of the margin seafloor is at depths shallower than the upper boundary of the OMZ and this area may shrink as the OMZ shoals over time. Since feeding on DVM organisms is a more efficient trophic pathway than reliance on benthic prey items, this oxygen-driven disruption in bentho-pelagic coupling may also negatively impact demersal fisheries productivity.