Abstract
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 pHT, 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 pHT 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 pHT 7.53 and 1490 μatm pCO2. The pHT 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.
Highlights
A select group of marine organisms can enter the Oxygen Minimum Zones (OMZs) and even anoxic waters, while performing diel vertical migration (DVM)
Salinity was higher in the northern region between the surface and 50 m depth, lower salinity values were observed around 29°S, associated with freshwater inputs from the Quebrada Los Choros River (Fig. 2B)
Other migrant organisms could contribute to the observed patterns; net catches confirmed that E. eximia was the dominant species in both night and day conditions
Summary
A select group of marine organisms can enter the Oxygen Minimum Zones (OMZs) and even anoxic waters, while performing diel vertical migration (DVM). In the northern Humboldt Current System (NHCS), a few euphausiid species (e.g. Euphausia mucronata) have been described as entering to OMZs during their diel vertical migration (DVM)[3]. This OMZ (upper boundary of ∼45 μM or 1 mL dissolved oxygen L−1) has become one of the shallowest and most intense OMZs in the world’s oceans[4,24], with dissolved oxygen (DO) concentrations falling below the sensor-specific detection limits (~ 3 nmol L−1) in a significant fraction of the OMZ waters. Acoustic observations have revealed that some organisms such as fish lack the ability to enter O MZs60
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