Mass-specific respiration of mesozooplankton and its role in the maintenance of an oxygen-deficient ecological barrier (BEDOX) in the upwelling zone off Chile upon presence of a shallow oxygen minimum zone Katty Donoso a,b, ⁎, Ruben Escribano b a Graduate Program in Oceanography, Department of Oceanography, University of Concepción, Chile b PLAMZ, Department of Oceanography, University of Concepción, Chile abstract article info Article history: Received 28 May 2012 Received in revised form 17 May 2013 Accepted 23 May 2013 Available online 30 May 2013 Regional index terms: Humboldt Current Chile Eastern South Pacific Keywords: Respiration BEDOX layer Oxygen Oxygen minimum zone Upwelling Mesozooplankton A shallow oxygen minimum zone (OMZ) in the coastal upwelling zone off Chile may vertically confine most zooplankton to a narrow (b 50 m) upper layer. From laboratory experiments, we estimated oxygen consump- tion of the mesozooplankton community obtained in Bay of Mejillones, northern Chile (23°S) in May 2010, December 2010 and August 2011. Mass-specific respiration rates were in the range of 8.2– 24.5 μmol O 2 mg dry mass -1 day -1 , at an average temperature of 12 °C. Estimates of the mesozooplankton biomass in the water column indicated that its aerobic respiration may remove daily a maximum of about 20% of oxygen available at the base of the oxycline. Since previous work indicates that zooplankton aggregate near the base of the oxycline, the impact of aerobic respiration on oxygen content might be even stronger at this depth. Mesozooplankton respiration, along with community respiration by microorganisms near the base of the oxycline and a strongly stratified condition (limiting vertical flux of O 2 ), are suggested as being critical factors causing and maintaining a persistent subsurface oxygen-deficient ecological barrier (BEDOX) in the upwelling zone. This BEDOX layer can have a major role in affecting and regulating zooplankton distribution and their dynamics in the highly productive coastal upwelling zone of the Humboldt Current System. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Presence of a shallow (b 50 m) and intense (b 5 μmol O 2 L -1 ) oxygen minimum zone (OMZ) constitutes a critical condition for ecological and biogeochemical processes in the highly productive upwelling region of the eastern South Pacific. The shallow OMZ strongly affects vertical distribution of pelagic organisms (Escribano et al., 2009; Judkins, 1980; Morales et al., 1999), restricting their habitat to a rather narrow (0–50 m) well-oxygenated layer for most dominant species (Escribano et al., 2009). This OMZ system also allows the development of particular microbial communities and their associated metabolisms (e.g. Stevens and Ulloa, 2008), indicating that the OMZ system is a key factor for nutri- ent cycling below the photic layer and hence fuels new production in the photic zone by recycling inorganic N within the OMZ which is then returned to the photic layer through upwelling (Farías et al., 2009). The upper boundary of the OMZ has been referred to in several works as the depth at which oxygen levels fall as low as 1 mL L -1 or 0.5 mL L -1 (b 50 μmol O 2 L -1 )(Fuenzalida et al., 2009). This depth also coincides with the base of the oxycline, and usually with the base of the thermocline (Escribano et al., 2009). At this depth, oxygen concentration changes abruptly to extremely low values (b 0.1 mL O 2 L -1 or b 25 μmol O 2 L -1 ), and as a consequence this thin layer can act as an ecological barrier for most aerobic organisms. We propose naming this layer the Oxygen-deficient Ecological Barrier (BEDOX is the Spanish acronym). Most of the metazooplankton ap- pear strongly constrained by this BEDOX layer, and therefore its var- iability may exert a strong control on distribution and dynamics of epipelagic plankton (upper 200 m). Vertical mixing and ventilation are some of the physical processes that maintain or regulate the OMZ system, while upwelling intensity is the major driver of changes in depth of the upper limit of the OMZ in the coastal area (Morales et al., 1999). However, there are also some bi- ological processes which can also help maintain the OMZ and its upper limit, such as oxygen consumption by bacterial metabolism (Farías et al., 2009), and community respiration in general (Daneri et al., 2000). The importance of aerobic respiration by metazoans, which are not nor- mally included in incubation bottles, has not yet been evaluated. Metazooplankton in the upwelling zone tend to concentrate within Journal of Marine Systems 129 (2014) 166–177 ⁎ Corresponding author at: Department of Oceanography, University of Concepción, P.O. Box 160-C, Concepción, Chile. Tel.: +56 41 2661175. E-mail address: kdonoso@udec.cl (K. Donoso). 0924-7963/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmarsys.2013.05.011 Contents lists available at ScienceDirect Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys