Low Pcrit but no hypoxia tolerance? Hypoxia compensation in the Arctic keystone species Boreogadus saida.

Abstract

Global warming has already caused a loss of almost 50% Arctic sea-ice coverage since the 1980s. Sea-ice loss strengthens summer stratification of the oceans water column and, consequently, hypoxic zones in the deep-water layers may form. The present study investigated the response of an Arctic keystone species, the Polar cod, Boreogadus saida, to hypoxia and warming. We measured the respiratory capacity (standard, routine and maximum metabolic rates, SMR, RMR, MMR, aerobic scope, critical oxygen saturation (Pcrit)) and swimming performance of Polar cod under progressive hypoxia at 2.4 {degrees}C and after warm acclimation to close to the species thermal limit (10.0 {degrees}C) via flow-through and swim tunnel respirometry. We observed clear and stable patterns that were similar in both thermal regimes: Polar cod displayed oxygen-regulating behaviour under progressive hypoxia, with SMR never below aerobic baseline metabolism and a very stable AS. Our study revealed that Polar cod can handle exceptionally low oxygen saturations down to a Pcrit of 5.9 % air saturation at typical habitat temperatures. Closer to critical temperatures (10.0 {degrees}C), Pcrit rose to 21.6 % air saturation. However, the pertinent question remains whether the observed behaviour can be summarized under classic hypoxia tolerance, as we a) did not observe any metabolic downregulation and b) no anaerobic component of the hypoxia response in Polar cod, which are usually put forward in the definition of hypoxia tolerance. Therefore, we describe the observed metabolic response to hypoxia rather as metabolic hypoxia compensation than hypoxia tolerance as the mechanisms involved here actively seek to improve oxygen supply instead of (anaerobically) tolerating hypoxia through metabolic depression.