Читать книгу Life in the Open Ocean - Joseph J. Torres - Страница 55

At all depths of the ocean, oxygen is removed from the water column as organisms respire and organic matter is biochemically degraded. Wave action, mixing, and photosynthetic processes replenish the lost oxygen in the upper mixed layer, but a zone of minimum oxygen forms at intermediate depth in all the world’s oceans. The severity of oxygen depletion in the minimum zones varies considerably, with values of dissolved oxygen ranging from 0 ml l⁻¹ (zero) in the Arabian Sea (Hitchcock et al. 1997) to about 4 ml l⁻¹ in the Antarctic (Smith et al. 1999) (see Chapter 1 for plots of O₂ vs. depth).

The major factors contributing to the persistence of oxygen minimum zones are global circulation patterns that result in the water at mid‐depths being out of contact with the atmosphere for hundreds of years. In areas where the water column is well‐stratified and rates of primary production are high, oxygen minima are especially severe. Dissolved oxygen values reach 0 ml l⁻¹ and stay near zero for hundreds of meters of water depth. Examples of such regions are the Arabian Sea, Bay of Bengal, Cariaco Basin, Philippine region, the northwest Pacific margin, and the eastern tropical Pacific. Hypoxic conditions in those areas extend from the bottom of the mixed layer to about 1500 m depth.

Although the oxygen concentration in oxygen minimum zones can be <0.2 ml l⁻¹, large populations of organisms can reside there. Those organisms include the decapods, mysids, and copepods and even large populations of midwater (mesopelagic) fishes (Gjøsaeter 1984).

Organisms that inhabit oxygen minima exhibit a range of adaptations for dealing with hypoxic conditions. These adaptations include behavior, such as diel vertical migration out of the layer, in addition to unusual morphological and physiological characteristics (Wishner et al. 2000). Childress and Seibel (1998) have proposed three modes of adaptation to the oxygen minimum: (i) development of mechanisms for efficient removal of oxygen from water; (ii) reduction of metabolic rates; and (iii) use of anaerobic metabolism to compensate for the difference between aerobic metabolism and total metabolic needs. The use of anaerobic metabolism may occur on a sustained basis, during periods of high metabolic demand, or during transient periods spent in the oxygen minimum layer by vertical migrators.

The decreased food energy available in the deep sea (Vinogradov 1970) favors an aerobic existence where there is sufficient oxygen to be utilized. However, an oxygen minimum such as that in the Arabian Sea, where oxygen drops to zero, constrains species to an anaerobic existence while resident within it. Anaerobiosis poses a special problem to vertebrates. With the exception of breath‐hold divers such as marine mammals and turtles, most vertebrates, including fish, rely exclusively on aerobic metabolism and only switch to locally active anaerobic pathways during periods of increased activity or ambient low oxygen levels. For example, tuna white muscle is very well adapted for anaerobiosis, which is used during burst swimming (Hochachka 1980).

The main anaerobic pathway used by vertebrates is glycolysis, resulting in lactate production (Withers 1992). However, two major problems arise with use of the glycolytic pathway. First, the energy yield is very low, only 2 ATP per glucose as opposed to 38 ATP per glucose from aerobic metabolism. Second, the accumulation of lactate is potentially harmful to the organism. Build‐up of lactate can lead to osmotic imbalance, acidosis, and, ultimately, inhibition of glycolysis (Withers 1992).