Abstract
Oxygen Minimum Zones prevail in most of the world’s oceans and are particularly extensive in Eastern Boundary Upwelling Ecosystems such as the Humboldt and the Benguela upwelling systems. In these regions, euphausiids are an important trophic link between primary producers and higher trophic levels. The species are known as pronounced diel vertical migrators, thus facing different levels of oxygen and temperature within a 24 h cycle. Declining oxygen levels may lead to vertically constrained habitats in euphausiids, which consequently will affect several trophic levels in the food web of the respective ecosystem. By using the regulation index (RI), the present study aimed at investigating the hypoxia tolerances of different euphausiid species from Atlantic, Pacific as well as from Polar regions. RI was calculated from 141 data sets and used to differentiate between respiration strategies using median and quartile (Q) values: low degree of oxyregulation (0.25 < RI median < 0.5); high degree of oxyregulation (0.5 < RI median < 1; Q1 > 0.25 or Q3 > 0.75); and metabolic suppression (RI median, Q1 and Q3 < 0). RI values of the polar (Euphausia superba, Thysanoessa inermis) and sub-tropical (Euphausia hanseni, Nyctiphanes capensis, and Nematoscelis megalops) species indicate a high degree of oxyregulation, whereas almost perfect oxyconformity (RI median ≈ 0; Q1 < 0 and Q3 > 0) was identified for the neritic temperate species Thysanoessa spinifera and the tropical species Euphausia lamelligera. RI values of Euphausia distinguenda and the Humboldt species Euphausia mucronata qualified these as metabolic suppressors. RI showed a significant impact of temperature on the respiration strategy of E. hanseni from oxyregulation to metabolic suppression. The species’ estimated hypoxia tolerances and the degree of oxyconformity vs. oxyregulation were linked to diel vertical migration behavior and the temperature experienced during migration. The results highlight that the euphausiid species investigated have evolved various strategies to deal with different levels of oxygen, ranging from species showing a high degree of oxyconformity to strong oxyregulation. Neritic species may be more affected by hypoxia, as these are often short-distance-migrators and only adapted to a narrow range of environmental conditions.
Highlights
Oxygen concentration and water temperature are two important abiotic factors influencing several physiological processes, such as metabolic rate, energy expenditure, as well as the horizontal and vertical distribution of animals living in the world’s oceans (Torres and Childress, 1983; Claireaux and Lagardère, 1999; Ekau et al, 2010)
The overall view of the euphausiids’ respiration rates over decreasing dissolved oxygen concentration at in situ temperature shows different magnitude and patterns (Figure 3). Comparing this magnitude by area, the highest respiration rates were observed in Euphausia pacifica (NCCS), Euphausia lamelligera (ETP), and Nematoscelis megalops (BCS)
Median regulation index (RI) values ≥0.5 of the polar (E. superba, T. inermis), temperate (E. pacifica), and sub-tropical (E. hanseni, N. megalops, and N. capensis) species indicated a high degree of oxyregulation, whereas the neritic temperate (T. spinifera) and tropical (E. lamelligera) species showed a low regulation ability as RI values fluctuated between −0.25 and 0.25 (Figure 6A and Table 2)
Summary
Oxygen concentration and water temperature are two important abiotic factors influencing several physiological processes, such as metabolic rate, energy expenditure, as well as the horizontal and vertical distribution of animals living in the world’s oceans (Torres and Childress, 1983; Claireaux and Lagardère, 1999; Ekau et al, 2010). Both factors are not evenly distributed and temperature and oxygen levels at the surface area are usually higher, compared to deeper water layers. The ecosystems in the world’s oceans are characterized by distinct oxygen and temperature regimes shaping the different species’ behavior, distribution and physiological processes
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