Vertical variability of Euphausia distinguenda metabolic rates during diel migration into the oxygen minimum zone of the Eastern Tropical Pacific off Mexico

Knowledge of metabolic rates of euphausiid diel vertical migrants in the ocean is of paramount importance to understand the role of these organisms in the downward transport of carbon due to their feeding in the epipelagic zone and subsequent respiration, egestion, excretion and mortality in the mesopelagic zone. Enzymatic activities are used as proxies for the estimation of metabolism in deep waters because of the logistical problems related to measure these rates at depth. However, metabolic activities of euphausiids in the so-called oxygen minimum zones (OMZs) are still not well understood. The OMZs are expanding as the effect of the global warming. Knowledge about the metabolic response of organisms transiting these zones will help to understand the fate of zooplanktonic communities and vertical active flux. The electron transport system (ETS) and aminoacyl-tRNA synthetases (AARS) activities were used as proxies for potential respiration and growth in planktonic organisms. Here, we measured these enzymatic activities in Euphausia distinguenda in the well-oxygenated epipelagic layer and in the OMZ of the northern boundary of the Eastern Tropical Pacific off Mexico. E. distinguenda was found in the mixed layer at night (similar to 3.5 mL O-2 L-1) and near the core of the OMZ (0.2 mL O-2 L-1) during daytime between 200 and 350 m depth. We found higher ETS and AARS specific activities in the warm mixed layer at night and significantly lower in the colder OMZ during daytime because of temperature differences. However, when the effect of temperature was subtracted in both enzyme activities, higher values in the mesopelagic layer were observed. This result is tentatively explained by the increase in substrates as the effect of feeding at night in the upper layers and their oxygen consumption at depth during day in order to compensate for the colder temperature and low dissolved oxygen concentration. This physiological mechanism allows E. distinguenda diel vertical migrants to partly face the physical constraints of vertical migration imposed to avoid predators.