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Gutiérrez-Bravo, J.G., M.A., Altabet, L., Sánchez-Velasco & S.P.A., Jiménez-Rosenberg (2025). Midwater anoxia disrupts the trophic structure of zooplankton and fish in an oxygen deficient zone. Limnology and Oceanography. 70(4): 886-898. DOI: 10.1002/lno.12813.

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

Juan Gerardo Gutiérrez-Bravo 1, Mark A. Altabet 1, Laura Sánchez-Velasco 2 y Sylvia Patricia Adelheid Jiménez-Rosenberg 3

1 School for Marine Science and Technology, Department of Estuarine and Ocean Sciences, University of Massachusetts Dartmouth, New Bedford, Massachusetts, USA
2 Instituto Politécnico Nacional, Centro Interdisciplinario de Ciencias Marinas, Departamento de Oceanología, Instituto Politécnico Nacional-Centro Interdisciplinario de Ciencias Marinas, La Paz, Baja California Sur, Mexico
3 Departamento de Plancton y Ecología Marina, Instituto Politécnico Nacional-Centro Interdisciplinario de Ciencias Marinas, La Paz, Baja California Sur, Mexico

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 d15N 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 d15N 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, d15N 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 (d15N = 7.5‰) that possibly feeds on chemoautotrophy-derived material at the oxic-anoxic interface. The d15N 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 d15N 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.

Palabras clave: Midwater anoxia; Trophic structure; zooplankton; fish

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