Life in the Open Ocean. Joseph J. Torres
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on Units
Oxygen is expressed in a variety of (nearly) interchangeable ways and those most commonly encountered in the literature have changed over the years. A few basics will help.
1. The total pressure of a gas mixture such as atmospheric air equals the sum of pressures exerted by each constituent gas. This is Dalton’s law.
2. Total (standard) atmospheric pressure is expressed as760 mm Hg (also known as “Torr”) or as 101 kilopascals (kPa), which is the SI unit for pressure.
3. The oxygen partial pressure in a standard atmosphere is the product of the mole fraction of oxygen in air × the barometric pressure:
PO2 = (0.2095) × (760 mm Hg) = 159.2 mm Hg (usually rounded up to 160 mm Hg)
PO2 = (0.2095) × (101 kPa) = 21.2 kPa (usually rounded down to 21 kPa)
4. Water in equilibrium with a gas mixture will have the same partial pressures as the gas mixture above it. This is Henry’s law. The snag is that the molar concentrations are a function of the partial pressures and an additional factor, the solubility coefficient. Thus: [A] = pA · αA
where [A] = mol l−1 of A, pA = partial pressure of A (kPa), α A = solubility coefficient (mol l−1 kPa−1).
5. Solubilities of oxygen in seawater decrease with increasing temperature and salinity. They are normally determined empirically (with exacting care!) at air saturation (e.g. Murray and Riley 1969) and compiled in a table for researchers to use. For Oregon waters (Chapter 11), the oxygen concentration at 12 °C and 33‰ was used. It was reported as 6.14 ml l−1 in Murray and Riley (1969). Knowing that a mole of oxygen is 22.4 l or 32 g, we can convert 6.14 ml l−1 to 8.77 mg l−1 (6.14/22.4) × 32. For future reference, to interchange ml l−1 with mg l−1 simply divide by 0.7. From there, it is a simple matter to convert to millimoles: 8.77/32 = 0.272 millimoles or 272 μmol kg−1, since the mass of a liter is a kg.
6. When comparing Pcs across pelagic systems with widely varying temperatures and oxygen concentrations, it is easiest to use PO2 because the mole fractions do not change, but relative solubilities do.
Severity of Oxygen Minima, “Dead Zones,” and the Intertidal
It is important to appreciate the difference between oceanic oxygen minima, which are persistent, year‐round features located predictably in certain regions of the global ocean for millennia, and the seasonally appearing regions of hypoxia or “dead zones” like that at the mouth of the Mississippi River in the Gulf of Mexico (Rabalais et al. 1994) or in the deeper water of some Scandinavian fjords during summer (Diaz and Rosenberg 1995). The persistence and predictability of oceanic oxygen minima have allowed for adaptations in their resident fauna that enable a primarily aerobic existence. In contrast, the seasonally or episodically appearing anoxia of dead zones, as well as their duration and severity, precludes such adaptation. Infaunal species and any others with limited mobility experience 100% mortality in dead zones (Diaz and Rosenberg 1995). Life in both the oceanic oxygen minima and the episodic dead zones differs from the situation in the intertidal, where high intertidal species may experience anoxia twice daily but a return to normoxia is assured with the incoming tide.
At what concentration does oxygen become limiting in oxygen minima? A good indication is a reduction in the biomass or the diversity of the fauna inhabiting them. Childress and Seibel (1998) observed that oxygen has little influence on the biomass or species composition of midwater organisms inhabiting minima down to a level of 0.20 ml l−1 or 0.63 kPa. At oxygen concentrations of 0.15 ml l−1 and below, such as that in the Eastern Tropical Pacific or Arabian Sea, oxygen minimum zones (OMZ’s) exhibit reduced biomass and diversity. In those cases, anaerobiosis while resident in the minimum accompanied by a vertical migration out of the layer to more highly oxygenated waters at night is the most likely strategy (Childress and Seibel 1998).
Intertidal species, including those that dwell in burrows as well as epifaunal species such as bivalves that are exposed for extended periods, are usually highly competent anaerobes (Hochachka 1980). Their situation as intertidal dwellers is fundamentally different from that of oxygen‐minimum‐layer species. Their anoxia and normoxia are cyclic, varying with tidal exposure. Thus, it is adaptive to be able to extract oxygen efficiently, but only down to the point where a large investment in the systems involved in oxygen uptake and transport is unnecessary. For most species, that point lies in the partial pressure range of 20–30 mm Hg oxygen or 2.7–4.0 kPa (Torres et al. 1979, 1994; Childress and Seibel 1998). For intertidal species, oxygen availability will continue to decline to or near zero oxygen as the tide recedes and anaerobiosis inevitably will become necessary.
In the deep sea, it is the stability of oxygen minima in concert with the limited food resources that have allowed the highly efficient respiratory systems of oxygen‐minimum‐layer fauna to arise.
Adaptations to Oxygen Minima
The Aerobic Strategy
Species that are able to maintain an aerobic existence in oxygen minima do so by having a highly effective system for removing oxygen from seawater, allowing them to consume oxygen in sufficient quantities to sustain life at very low oxygen concentrations. The ability to regulate oxygen consumption down to very low levels of external oxygen is defined in physiological terms as having a low critical oxygen partial pressure, or Pc. Figure 2.22 shows the relationship of oxygen consumption rate and external PO2 for an oxygen‐minimum‐layer species, the shrimp‐like lophogastrid Gnathophausia (now Neognathophausia) ingens. The species is able to regulate its oxygen consumption (VO2) down to the lowest oxygen level it experiences in its environment in the oxygen minimum layer off the coast of California (8 mm Hg oxygen or 0.8 kPa). The Pc is the point on the curves where the oxygen consumption declines precipitously toward 0 and it varies with activity level as shown. A study on the relationship of species’ critical oxygen partial pressures vs. their minimum environmental PO2 shows that for most oxygen minimum dwellers, species’ Pcs are equivalent to the lowest oxygen concentrations encountered in nature (Childress and Seibel 1998). What is surprising is that all pelagic species living in normoxic waters that have been examined can also regulate their oxygen consumption down to a low PO2: 4–6 kPa, or 20–30% of air saturation (Childress 1975; Donnelly and Torres 1988; Cowles et al. 1991; Torres et al. 1994). Thus, even at very much higher habitat O2 levels, pelagic species maintain a Pc near 4 kPa. It is tempting to conclude that 4 kPa reflects a global ocean oxygen minimum that had to be accommodated by most pelagic species at some point during geological history. While possible, evidence does not support it (Childress and Seibel 1998). What is more likely is that the occasional high oxygen demands of increased activity and the respiratory machinery it requires coincidentally equip most species with the ability to take up and transport oxygen down to 4 kPa.
Figure 2.22 Oxygen consumption rate of the lophogastrid Gnathophausia ingens as a function of oxygen concentration (in milliliters of oxygen per liter). Open circles represent a single very active animal; closed circles represent the mean of 23 runs with 8 individuals; closed triangles represent a single non‐swimming animal. The vertical lines represent plus or minus one standard deviation.
Source: J. J. Childress, Oxygen minimum