Life in the Open Ocean. Joseph J. Torres

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Figure 1.16 Flow patterns and mixing of water masses in the Southern Ocean. AAIW, Antarctic Intermediate Water; NADW, North Atlantic Deep Water; AABW, Antarctic Bottom Water; ACW, Atlantic Central Water.

      Source: Brown et al. (1989), figure 6.20 (p. 184). Reproduced with the permission of Pergamon Press.

Oxygen

      Oxygen is introduced into oceanic waters (and all other waters) by diffusion from the atmosphere, aided by wind‐induced turbulence and mixing, and sometimes supplemented to a small degree by photosynthetically produced oxygen. Its solubility in water is an inverse function of salinity and temperature. Once a water mass has left the surface, its dissolved oxygen is consumed through time, mainly by microorganisms but also by larger species such as fishes and crustaceans. As a consequence, oxygen content is an indication of how long the water has been away from the surface. It is a nonconservative property of a water mass that is sometimes used as a tracer. Water masses vary substantially in their time away from the surface. At the extremes, AABW in the Pacific retains its character for 1600 years (Garrison 2002), whereas the residence time for most deep water is 200–300 years. NADW takes about 1000 years to reach the surface after sinking at the northern end of the Great Ocean Conveyor.

Regions of low oxygen concentration are termed oxygen minima, and they vary widely in their severity. If an oceanic region includes a highly productive surface layer with large seasonal algae blooms, the bacterial oxidation of sinking organic matter may remove nearly all the oxygen from the deeper waters. Examples of such areas include the waters off the California coast, the Eastern Tropical Pacific, and the Arabian Sea. Compare the oxygen profiles from the Arabian Sea and the California Current in Figure 1.17 with the profiles for the Antarctic and the Gulf of Mexico. Oxygen concentrations at the surface and at 500 m of depth in the world ocean are shown in Figure 1.18. Most of the world’s severest minima are in regions where upwelling is very strong, and algae blooms are episodically very extensive.

      Oxygen minima may also occur because of topography. For example, the Black Sea and the Cariaco Basin off the coast of Venezuela have restricted communications with the rest of the open ocean because of a shallow sill restricting circulation of their deeper waters. As a consequence, their deep waters are isolated and become anoxic.

      Because deeper waters have always spent some time away from the surface, an oxygen minimum of some degree is always present. However, in most places it is not severe and not limiting to animal life. In the Gulf of Mexico, for example, the oxygen drops to about 50% of surface values at a depth of 600 m, as it does in the Antarctic. In contrast, the waters off southern California drop to about 5% of surface values, and in the Arabian Sea oxygen levels drop to zero below 200 m. Such low values pose severe challenges to animal life. Moreover, oxygen minima in the Cariaco Basin and the Black Sea include the presence of sulfides, which are metabolic poisons. How animals cope (or not) with such low levels of oxygen will be covered in the next chapter.

Pressure

The easiest physical characteristic of the ocean to understand is pressure. Pressure increases by 1 atmosphere (atm) (the barometric pressure at sea level) or 14.7 pounds per square inch (psi) with each increase of 10 m in depth. The metric unit of pressure is the pascal (Pa). One atmosphere is equivalent to 101.3 kPa. Pressure in the deepest point in the ocean, the Challenger Deep (depth: 10 916 m), is 16 046 psi or 1.11 × 10⁵ kPa. In contrast, pressure at the average depth of the ocean is 5586 psi or 3.85 × 10⁴ kPa. Pressure can be important in shaping the characteristics of species living in the deep sea and will be discussed in the next chapter.

Figure 1.17 Oxygen and temperature profiles from four oceanic regions. (a) Antarctic (Southern Ocean); (b) Arabian Sea; (c) Gulf of Mexico; (d) California Current.

      Source: Torres et al. (2012), figure 1 (p. 1909). Reproduced with the permission of The Company of Biologists.

Figure 1.18 Typical oceanic oxygen concentrations. (a) Surface; (b) 500 m.

Sound

      The ocean is sometimes characterized as a noisy place, though it may seem silent to scuba divers in the open ocean because most of the sounds are not discernible to human ears. Sound levels do vary considerably in the sea in the horizontal and vertical planes, and only a part is generated by the activities of humans. Detection of sound and other vibrations is a sensory modality shared by virtually all oceanic species. Some pelagic species, bottlenose dolphins, for example, use echolocation to locate prey, just as a bat does.

      Sound levels do not vary predictably in the ocean except in a general sense. Increasing distance from the crashing waves of a rocky shore will decrease the levels of ambient sound, as will increasing depth and distance from the wind‐induced turbulence of surface waters. However, the properties of sound do vary predictably, and a presentation of some basic concepts now will help with later discussions of hearing and mechanoreception in open‐ocean fauna. The physics of sound is quite complex; СКАЧАТЬ