The roles of benthic diversity and environmental factors in nutrient and macrofaunal dynamics within the oxygen minimum zone of the British Columbia continental slope

Global warming has expanded oxygen minimum zones (OMZs), or oceanic “dead zones”. Despite their ecological significance, the diversity, and functional roles of benthic organisms in nutrient cycling and carbon (C) uptake in OMZ remain understudied. In this study, we focused on the Northeast Pacific continental slope OMZ (600-1200 m depth) off Vancouver Island, Canada, along an oxygen and depth transect (∼77, 40, and 10 μmol ‧ l -1 at 200-, 475-, 850-m depth, respectively). Using a multicorer we collected sediment cores and performed shipboard incubations to examine organic matter remineralization and nutrient flux rates. In parallel, we performed pulse-chase tracer incubations with isotopically labelled algae ( Phaeodactylum sp.) at the 475- and 850-m sites, to assess macrofaunal C uptake and benthic nutrient flux responses to fresh phytodetritus. Our results revealed no clear influence of decreasing oxygen on macrofaunal abundances, nutrient regeneration, or on the role of infauna in organic matter remineralization. Nontheless, organic matter quantity and quality strongly influenced macrofaunal community structure. Despite differences in community composition among sites, we also observed no clear relationship between macrofaunal community composition and nutrient fluxes, with phytoplankton-based food as a key food source for oxygen-deficient-adapted infauna. The highest C uptake rate and highest net nutrient influxes at the 475 m site contrasted lower rates at 850 m site, where only one flabelligerid and one cirratulid polychaete species ingested the organic matter, followed by amphipods, cumaceans, unidentified crustaceans, and bivalves, all primarily in the upper 2 cm of sediment. Our findings highlight the complex interplay of oxygen, depth, and organic matter, suggesting that organic matter quality and quantity may obscure oxygen effects on benthic biodiversity and ecological processes. • Quantity and quality of particulate organic matter drive macrobenthic community structure. • Benthic nutrient net influxes typified the 475 m hypoxic site. • We found no clear relationship between infaunal communities and nutrient fluxes. • Infauna responded to fresh phytodetrital input in the hypoxic sampling sites. • Carbon uptake from infaunal organisms decreased in relation to hypoxia.