Euphausiid respiration model revamped: Latitudinal and seasonal shaping effects on krill respiration rates

Abstract

a b s t r a c tEuphausiids constitute a major biomass component in shelf ecosystems and play a fundamental role in the rapid vertical transport of carbon from the ocean surface to the deeper layers during their daily vertical migration (DVM). DVM depth and migration patterns depend on oceanographic conditions with respect to temperature, light and oxygen availability at depth, factors that are highly dependent on season in most marine regions. Here we introduce a global krill respiration ANN (artificial neural network) model including the effect of latitude (LAT), the day of the year (DoY), and the number of daylight hours (DLh), in addition to the basal variables that determine ectothermal oxygen consumption (temperature, body mass and depth). The newly implemented parameters link space and time in terms of season and photoperiod to krill respiration. The ANN model showed a better fit (r 2 = 0.780) when DLh and LAT were included, indicating a decrease in respiration with increasing LAT and decreasing DLh. We therefore propose DLh as a potential variable to consider when building physiological models for both hemispheres. For single Euphausiid species investigated in a large range of DLh and DoY, we also tested the standard respiration rate for seasonality with Multiple Linear Regression (MLR) and General Additive model (GAM). GAM successfully integrated DLh (r 2 = 0.563) and DoY (r 2 = 0.572) effects on respiration rates of the Antarctic krill, Euphausia superba, yielding the minimum metabolic activity in mid-June and the maximum at the end of December. We could not detect DLh or DoY effects in the North Pacific krill Euphausia pacifica, and our findings for the North Atlantic krill Meganyctiphanes norvegica remained inconclusive because of insufficient seasonal data coverage. We strongly encourage comparative respiration measurements of worldwide Euphausiid key species at different seasons to improve accuracy in ecosystem modeling.

Ammonium excretion and oxygen respiration of tropical copepods and euphausiids exposed to oxygen minimum zone conditions

“…Empirical models exist to predict copepod (Ikeda, 2014) and euphausiid (Tremblay et al, 2014;Ikeda, 2014) respiration and excretion rates, but these models do not include pO 2 or pCO 2 as environmental factors. We developed here an ap- Table 3.…”

Ammonium excretion and oxygen respiration of tropical copepods and euphausiids exposed to oxygen minimum zone conditions

“…Empirical models exist to predict copepod (Ikeda, 2014) and euphausiid (Tremblay et al, 2014;Ikeda, 2014) respiration and excretion rates, but these models do not include pO 2 or pCO 2 as environmental factors. We developed here an ap- pO 2 (kPa) pO 2 (kPa) Figure 6.…”

Routine metabolism of Antarctic krill (Euphausia superba) in South Georgia waters: absence of metabolic compensation at its range edge

“…Therefore, most studies on euphausiids report what has been termed "routine" metabolism, which represents basal metabolism plus an uncontrolled, but assumed to be minor, contribution from other processes, including low levels of swimming. With certain caveats, the measurement of routine metabolism in euphausiids provides a comparative index of metabolic costs between populations, environments and species (Ikeda 2013;Tremblay et al 2014).…”

“…For a species as widely distributed as Antarctic krill, compensating metabolic rate to counter contrasting thermal conditions has distinct advantages. The Southern Ocean is extremely seasonal and there is strong evidence that Antarctic krill alters its respiration rate beyond thermal expectations, with minimum rates in mid-winter and maximum rates in mid-summer (Meyer 2012;Tremblay et al 2014). Increasing respiration rate in summer, particularly in colder higher latitude waters, may allow maximal use of the high productivity levels available, increasing development and maturation rates and allowing life-cycles to be completed (Thorpe et al 2019).…”