Abstract

Mesozooplankton can suffer a high degree of non-predatory mortality within the Oxygen Minimum Zone (OMZ), resulting in a high abundance of carcasses. Copepod carcasses are subject to bacterial decomposition with unclear consequences for nitrogen cycling, microbial biodiversity, and ecosystem functioning. We investigated the bacterial decomposition of carcasses of the copepod Acartia tonsa from Mejillones Bay within the Humboldt Current System under different dissolved oxygen concentrations. Using molecular methods, we followed the dynamics of bacterial composition and functional marker genes for nitrogen cycling. Decomposition of carcass tissues was observed earlier under oxic conditions than under Microxic/Suboxic conditions. Aerobic bacteria colonized the carcasses, and the associated functional gene for ammonia oxidation (amoA) was detected. Under Microxic/Suboxic conditions, decomposition was dominated by anaerobic bacteria and the related denitrification functional genes, specifically nitrite reductases (nirK) and of nitrous oxide reduction (nosZ), consistent with an estimated high denitrification rate. Based on these microscale observations, our study suggested that copepod carcasses were anaerobic microenvironments that potentially supported denitrification activity in the upper and lower oxyclines. Carcass driven denitrification may be an important loss of fixed N in the OMZ. © 2025 Elsevier Ltd