Diel vertical migration into anoxic and high-pCO₂ waters: acoustic and net-based krill observations in the Humboldt...

A select group of marine organisms can enter the Oxygen Minimum Zones (OMZs) and even anoxic waters, while performing diel vertical migration (DVM). DVM of the euphausiid Euphausia eximia off northern Chile in the spring of 2015 was documented based on acoustic measurements using an echo sounder along with net samplings. Dissolved oxygen (DO) concentrations were obtained using a vertical profiler, and water samples were collected to obtain in situ nitrite (NO₂^-) concentrations as well as pH_T, total alkalinity (AT), and therefore carbon dioxide partial pressure (pCO₂) was estimated. Krill were found to migrate up to the surface (0-50 m) during the night and returned to ca. 200-300 m depth during the day, spending between 11 and 14 h at these layers. At the surface, DO and NO₂^- concentrations were 208 and 0.14 μM respectively, while pH_T was 8.04 and 405 μatm pCO₂. In contrast, at the deeper layers (200-300 m), DO and NO₂^- were < 3 and 6.3 μM respectively, with pH_T 7.53 and 1490 μatm pCO₂. The pH_T and high pCO₂ values at depths represent the conditions predicted for open ocean waters in a worst-case global warming scenario by 2150. The acoustic scatter suggested that > 60% of the krill swarms enter the OMZ and anoxic waters during the daytime. These frequent migrations suggest that krill can tolerate such extreme conditions associated with anoxic and high-pCO₂ waters. The inferences drawn from the observation of these migrations might have strong implications for the current oceanic carbon pump models, highlighting the need for understanding the molecular and physiological adaptations allowing these migrations.