Response of three krill species to hypoxia and warming: an experimental approach to oxygen minimum zones expansion in coastal ecosystems

Abstract

AbstractTo understand the adaptation of euphausiid (krill) species to oxygen minimum zones (OMZs), respiratory response and stress experiments combining hypoxia‐reoxygenation exposure with warming were conducted. Experimental krill species were obtained from the Antarctic (South Georgia area), the Humboldt Current System (HCS, Chilean coast) and the Northern California Current System (NCCS, Oregon).Euphausia mucronatafrom theHCSshowed oxyconformingpO2‐dependent respiration below 80% air saturation (18 kPa). Normoxic subsurface oxygenation in winter posed a ‘high oxygen stress’ for this species. TheNCCSkrill,Euphausia pacifica, and the Antarctic krill,Euphausia superba, maintained respiration rates constant down to low criticalpO2values of 6 kPa (30% air saturation) and 11 kPa (55% air saturation), respectively. Antarctic krill had low antioxidant enzyme activities, but high concentrations of the molecular antioxidant glutathione (GSH) and was not lethally affected by 6 h exposure to moderate hypoxia. The temperate krill species (E. pacifica) had higher superoxide dismutase (SOD) values in winter than in summer, which relates to a higher winter metabolic rate. In all species, antioxidant enzyme activities remained constant during hypoxic exposure at the typical temperature for their habitat. Warming by 7 °C above habitat temperature in summer increasedSODactivities andGSHlevels inE. mucronata(HCS), but no oxidative damage occurred. In winter, when theNCCSis well mixed and theOMZis deeper, +4 °C of warming combined with hypoxia represents a lethal condition forE. pacifica. In summer, when theOMZexpands upwards (100 m subsurface), antioxidant defences counteracted hypoxia and reoxygenation effects inE. pacifica, but only at mildly elevated temperature (+2 °C). In this season, experimental warming by +4 °C reduced antioxidant activities and the combination of warming with hypoxia again caused mortality of exposed specimens. We conclude that a climate change scenario combining warming and hypoxia represents a serious threat toE. pacificaand, as a consequence,NCCSfood webs.