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| Description: Physical Environment: In the open ocean, sometimes called the pelagic zone, water depth averages 4,000 m and nutrient concentrations are typically low, though the waters may be periodically enriched by ocean upwellings, which carry mineral nutrients from the bottom waters to the surface. Pelagic waters are mostly cold, only warming near the surface. Location: Across the globe, covering 70% of the Earth's surface. Plant Life: Where light levels are high at the surface, many microscopic, photosynthetic organisms (phytoplankton) grow and reproduce. Phytoplankton account for nearly half the photosynthetic activity on Earth and produce much of the world's oxygen. Animal Life: Open-ocean organisms include zooplankton, minute animal organisms consisting of some worms, copepods (tiny shrimplike creatures), tiny jellyfish, and the small larvae of invertebrates and fish that graze on the phytoplankton. The open ocean also includes free-swimming animals collectively called nekton, which can swim against the currents to locate food. The nekton include large squids, fish, sea turtles, and marine mammals that feed on phytoplankton, zooplankton, or each other. Only a few of these organisms live at any great depth. In some areas, there exists a unique assemblage of animals associated with deep-sea hydrothermal vents that spew out hot (350°C) water rich in hydrogen sulphide. In this dark, oxygen-poor environment, large polychaete worms exist together with other organisms that are chemoautotrophs (see Figure 57.3). Effects of Humans: Oil spills and a long history of garbage disposal have polluted the ocean floors of many areas. Overfishing has caused many fish populations to crash, and the whaling industry has greatly reduced the numbers of most species of whales.
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