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Life in the earth’s oceans simply would not exist without the presence of dissolved oxygen. This life-giving substance is not, however, distributed evenly with (a) depth in the oceans. Oxygen levels are typically high in a thin surface layer 10–20 metres deep. Here oxygen from the atmosphere can freely diffuse into the seawater, plus there is plenty of floating plant life producing oxygen through photosynthesis. Oxygen concentration then decreases rapidly with depth and reaches very low levels, sometimes close to zero, at depths of around 200–1,000 metres. This region is referred to as the oxygen (b) minimum zone. This zone is created by the low rates of oxygen diffusing down from the surface layer of the ocean, combined with the high rates of consumption of oxygen by decaying organic matter that sinks from the surface and accumulates at these depths. Beneath this zone, oxygen content (c) increases again with depth. The deep oceans contain quite high levels of oxygen, though not generally as high as in the surface layer. The higher levels of oxygen in the deep oceans reflect in part the origin of deep-ocean seawater masses, which are derived from cold, oxygen-rich seawater in the surface of polar oceans. That seawater sinks rapidly down, thereby (d) exhausting its oxygen content. As well, compared to life in near-surface waters, organisms in the deep ocean are comparatively scarce and have low metabolic rates. These organisms therefore consume (e) little of the available oxygen.
*dissolve: 용해시키다 **diffuse: 퍼지다