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Geosciences LibreTexts

19.5: Oceanic Zone

  • Page ID
    13152
  • As previously stated, the oceanic zone encompasses the ocean that lies beyond the continental shelf and exists where the water depth exceeds 200 meters. At the surface of this zone is the euphotic zone, or the zone that receives enough sunlight to support photosynthesis. Here, the primary producers are phytoplankton. 

    phytoplankton-Diatoms-ocean-food-chains-foundations-fishes.jpg

    An example of an oceanic food chain is shown above. Phytoplankton are eaten by zooplankton, which are eaten by small fish, which are eaten by mackerel, which are eaten by tuna, which are eaten by large sharks. 

    An interesting event occurs everyday in the oceanic zone. At night zooplankton migrate toward the surface to feed. They are followed by larger predators. When the sun rises, they all sink back to deeper, darker waters, to avoid predation. This migration is referred to as Diel Vertical Migration, as it happens on a daily scale. This migration occurs all of the world, including in some lakes. As these organisms return to deeper waters, they defecate, producing food sources for deep sea organisms.