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

Abstract

Abstract To understand the adaptation of euphausiid (krill) species to oxygen minimum zones ( OMZ s), 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 mucronata from the HCS showed oxyconforming p O 2 ‐dependent respiration below 80% air saturation (18 kPa). Normoxic subsurface oxygenation in winter posed a ‘high oxygen stress’ for this species. The NCCS krill, Euphausia pacifica , and the Antarctic krill, Euphausia superba , maintained respiration rates constant down to low critical p O 2 values 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 increased SOD activities and GSH levels in E. mucronata ( HCS ), but no oxidative damage occurred. In winter, when the NCCS is well mixed and the OMZ is deeper, +4 °C of warming combined with hypoxia represents a lethal condition for E. pacifica . In summer, when the OMZ expands upwards (100 m subsurface), antioxidant defences counteracted hypoxia and reoxygenation effects in E. 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 to E. pacifica and, as a consequence, NCCS food webs.