Life in the Open Ocean. Joseph J. Torres

up the remainder of the water column. Remembering that only 3% of the ocean’s volume is above 1000 m, light is therefore absent from the vast majority of the open sea.

Traditional Depth Zones in the Ocean

Figure 1.24 shows the traditional oceanic depth zones, their characteristics, and representative fauna that reside at those depths. The uppermost layer, the epipelagic zone, extends from the surface to a depth of 200 m and includes the euphotic zone. The next layer down is the mesopelagic zone (200–1000 m), which largely corresponds to the disphotic zone described earlier. Several important changes happen in the mesopelagic zone. Light is gradually extinguished or very nearly so (only in the clearest open‐ocean waters does light prevail below 1000 m), and the wavelengths are limited to the blue‐green (480 nm). The permanent thermocline results in a temperature change from near‐surface values to the more cold and monotonous temperatures characteristic of the very deep‐sea and the cold‐water masses that comprise it. Temperatures at 1000 m are usually between 4 and 8 °C, declining very gradually to 2 °C in the next vertical stratum, the bathypelagic zone, which extends from 1000 to 6000 m (Herring 2002). The bathypelagic zone includes the depths characteristic of the abyssal plain (4000 m) and the average depth of the ocean, but not the depths characteristic of the ocean trenches that include the ocean’s deepest points. Depths below 6000 m are sometimes referred to as hadal or hadopelagic and are associated mainly with oceanic trenches. The three major regions of the oceanic water column are the epipelagic, mesopelagic, and bathypelagic zones, and they will be our main concern in this book. The hadal regions are a minor contributor to the seafloor and sea life of the world ocean, (2%, Herring 2002) though those regions are quite important geologically.

Figure 1.24 Oceanic depth zones. Examples of typical fauna from each zone are represented along with temperature, light, and biomass profiles.

Concluding Thoughts

      Whether considered in terms of surface area or volume, the open ocean is a vast living space. It covers more than twice the surface area of the terrestrial biome, and in volume it is 99.5% of the habitable space on Earth! Most of that habitable space is in the deep sea, making the deep sea the earth’s “average environment.” The properties of water shape the characteristics of marine life. Its density confers structural support to marine species, allowing for the delicate constitutions exhibited by gelatinous species such as the jellyfishes and comb jellies and allowing marine plants such as kelp to reach great heights above the bottom without the use of a massive trunk. The negative side of water density is that it takes considerable energy to move through the aqueous medium because of friction drag and pressure drag. Rapid swimmers have a characteristic spindle (fusiform) shape that minimizes drag. The environment experienced by very small open‐ocean species is vastly different from that experienced by the larger, quicker swimmers: small marine species live in a much more viscous environment. The environment that a species experiences can be described with the Reynolds number, a dimensionless unit that relates inertial to viscous forces.

      Just as important are the internal adjustments that must be made by marine species to the gradients in temperature that vary with latitude and to the changes in temperature, pressure, and oxygen that vary with depth. Those latitudinal and vertical gradients are governed by ocean circulation. Surface currents, from the surface to 400 m of depth, are driven by the wind and the rotation of the Earth. Deep‐ocean circulation, which transports about 90% of the sea’s volume, is determined by the ocean’s density structure and is far more sluggish.

      The Earth has five major oceans: the Atlantic, Pacific, Indian, Arctic, and the recently recognized Southern Ocean. The high‐latitude Arctic and Southern Oceans are the most homogeneous with respect to temperature, salinity, and oxygen in the horizontal and vertical planes. The Atlantic, Pacific, and Indian Oceans are far larger, extend from tropical to boreal climates, and are far more heterogeneous in their temperature and salinity profiles. In particular, the Pacific and Indian Oceans have large areas in their coastal regions with well‐developed oxygen minima that in some cases are limiting to animal life.

      As with temperature, the presence of light in the ocean varies predictably with depth, with the longest and shortest wavelengths being largely attenuated in the upper 100 m. The light that penetrates most readily is that in the blue‐green range, with a wavelength of approximately 480 nm. Light penetration depends on water clarity, which in turn depends on the presence of particles that absorb and scatter light. In clear open‐ocean water, light penetrates to depths of approximately 1000 m. Vision as a sensory modality is therefore most important in the upper 1000 m of the ocean.

      Sound propagates better in water than in air, making the detection of sound and vibration important to most oceanic life. Unlike temperature and light, sound does not vary naturally with depth or location in the ocean, except with respect to noise generated by coastal surf and through proximity to man’s activities.

Traditionally, the ocean is divided into three main zones with respect to depth. The epipelagic zone, or upper ocean, extends from 0 to 200 m, roughly corresponding to the depths over which plant life can survive. The mesopelagic zone, or middle ocean, extends from 200 to 1000 m and corresponds roughly to both the permanent thermocline and the maximum penetration of light in the ocean. The bathypelagic zone, or deep ocean, extends from 1000 to 6000 m. It includes the average depth of the ocean, ~3700 m, and all depths but those in the deep ocean trenches. It is characterized by the complete absence of light and very low (2 °C) temperatures.

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