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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.