Abstract
Anoxic waters in the ocean’s Oxygen Minimum Zones (OMZs) limit the vertical migrations of zooplankton and mesopelagic fish impacting their development, trophic interactions, and influence on biogeochemical processes. Using an oxypleth-tracking, nighttime-only sampling protocol, this research reconstructed the distribution and trophic interactions of fish larvae and adults, and zooplankton, across the Eastern Tropical North Pacific OMZ. Results revealed a wide midwater anoxic core, extending from Costa Rica to Baja California, that was almost devoid of fish larvae (<1 larvae/1000 m³) and zooplankton (5 mL/1000 m³) at night. Epipelagic species (e.g., Auxis sp.) were predominantly found in the surface oxic level, whereas meso-bathypelagic species (e.g., Diogenichthys laternatus, Cyclothone spp.) were present throughout the study area, including below the anoxic core as transformation larvae and juveniles. Bulk zooplankton δ¹⁵N increased latitudinally by ~3.3‰ from Costa Rica to Baja California associated with anoxia-derived denitrification and consequent enrichment of nitrogen sources for producers. Zooplankton δ¹⁵N also increased with depth, with an abrupt 3.4‰ increase below the anoxic core, indicating a ‘trophic jump’ in the resident zooplankton community. Fish adults residing below the anoxic core had a mean δ¹⁵N 4.8‰ higher than larval stages sampled at shallower depths, and 1.2‰ higher than zooplankton below the anoxic core. This trophic jump indicates that anoxic waters act as a barrier for the trophic transfer into the deep sea by vertical migrants, known as the migrant pump. These findings illustrate how a vertical expansion of anoxic waters in OMZs could further constrain the habitat of epipelagic species and the vertical movements of meso-bathypelagic species, decreasing their contribution to the carbon flux into the deep ocean. International collaboration, increased sampling efforts, and precautionary approaches on fisheries policies should be implemented to assess and respond to the effects of deoxygenation.