Midwater anoxia disrupts the trophic structure of zooplankton and fish in an oxygen deficient zone

Abstract Anoxic waters in the ocean's oxygen deficient zones (ODZs) limit the vertical migrations of zooplankton and mesopelagic fish impacting their ecology and influence on biogeochemical processes. Using an oxypleth‐tracking, nighttime‐only sampling protocol, this research reconstructed the trophic interactions of fish larvae and adults, and zooplankton, across the Eastern Tropical North Pacific ODZ. Bulk zooplankton δ 15 N increased latitudinally by ~ 3.3‰ from Costa Rica to Baja California due to anoxia‐derived denitrification and consequent enrichment of nitrogen sources for producers. Zooplankton δ 15 N also increased with depth, with an abrupt 3.4‰ increase below the anoxic core (~ 900 m depth), indicating a distinct trophic structure in the resident zooplankton community. Above the anoxic core, δ 15 N was similar for fish larvae (10.1‰) and zooplankton (10.5‰), reflecting a shared food source. An exception was the hypoxia‐tolerant myctophid Diogenichthys laternatus (δ 15 N = 7.5‰) that possibly feeds on chemoautotrophy‐derived material at the oxic‐anoxic interface. The δ 15 N of fish adults residing below the anoxic core, like the meso‐bathypelagic Notolychnus valdiviae (17.11‰) and Cyclothone spp. (15.89‰), was, on average, 4.8‰ higher than larval stages sampled at shallower depths, and 1.2‰ higher than zooplankton below the anoxic core. This stark increase in fish and zooplankton δ 15 N directly below the anoxic core suggests that anoxic waters act as a barrier for the downward trophic transfer by vertical migrants into the deep sea. Considering the current trends of ocean deoxygenation, this anoxia‐derived disruption of the migrant pump could limit the carbon sequestration potential of ODZs.