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Calanoid copepods and euphausiids are key components of marine zooplanktoncommunities worldwide. Most euphausiids and several copepod species performdiel vertical migrations (DVMs) that contribute to the export of particulateand dissolved matter to midwater depths. In vast areas of the global ocean,and in particular in the eastern tropical Atlantic and Pacific, the daytimedistribution depth of many migrating organisms corresponds to the core of theoxygen minimum zone (OMZ). At depth, the animals experience reducedtemperature and oxygen partial pressure (pO2) and an increased carbondioxide partial pressure (pCO2) compared to their near-surfacenighttime habitat. Although it is well known that low oxygen levels caninhibit respiratory activity, the respiration response of tropical copepodsand euphausiids to relevant pCO2, pO2, and temperatureconditions remains poorly parameterized. Further, the regulation of ammoniumexcretion at OMZ conditions is generally not well understood. It was recentlyestimated that DVM-mediated ammonium supply could fuel bacterial anaerobicammonium oxidation – a major loss process for fixed nitrogen in the oceanconsiderably. These estimates were based on the implicit assumption thathypoxia or anoxia in combination with hypercapnia (elevated pCO2) doesnot result in a down-regulation of ammonium excretion. We exposed calanoidcopepods from the Eastern Tropical North Atlantic (ETNA; Undinulavulgaris and Pleuromamma abdominalis) and euphausiids from theEastern Tropical South Pacific (ETSP; Euphausia mucronata) and theETNA (Euphausia gibboides) to different temperatures, carbon dioxideand oxygen levels to study their survival, respiration and excretion rates atthese conditions. An increase in temperature by 10 °C led to anapproximately 2-fold increase of the respiration and excretion rates ofU. vulgaris (Q10, respiration = 1.4;Q10, NH4-excretion = 1.6), P. abdominalis(Q10, respiration = 2.0; Q10, NH4-excretion = 2.4) and E. gibboides (Q10, respiration = 2.0;Q10, NH4-excretion = 2.4; E. mucronata nottested). Exposure to differing carbon dioxide levels had no overallsignificant impact on the respiration or excretion rates. Species from theETNA were less tolerant to low oxygen levels than E. mucronata fromthe ETSP, which survived exposure to anoxia at 13 °C. Respirationand excretion rates were reduced upon exposure to low oxygen levels, albeitat different species-specific levels. Reduction of the excretion andrespiration rates in ETNA species occurred at a pO2 of 0.6 (P.abdominalis) and 2.4 kPa (U. vulgaris and E. gibboides)at OMZ temperatures. Such low oxygen levels are normally not encountered bythese species in the ETNA. E. mucronata however regularly migratesinto the strongly hypoxic to anoxic core of the ETSP OMZ. Exposure to lowoxygen levels led to a strong reduction of respiration and ammonium excretionin E. mucronata (pcrit  respiration = 0.6,pcrit NH4-excretion = 0.73). A drastic reduction ofrespiratory activity was also observed by other authors for euphausiids,squat lobsters and calanoid copepods, but was not yet accounted for whencalculating DVM-mediated active fluxes into the ETSP OMZ. Current estimatesof DVM-mediated active export of carbon and nitrogen into the ETSP OMZ aretherefore likely too high and future efforts to calculate these export ratesshould take the physiological responses of migratory species to OMZconditions into account

Ammonium excretion and oxygen respiration of tropical copepods and euphausiids exposed to oxygen minimum zone conditions