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Our planet's seas are constantly recycling themselves. Showers of marine snow send organic matter cascading down to the sea floor, and zooplankton excrete poop full of nitrogren, phosphorus, and iron in deep water as they go about their regular up-and-down migration through the water column. This is a downward "pump" of resources. But other organisms can also bring some of these elements back from the deep. Whales and other marine mammals, some researchers have hypothesized, replenish the surface waters with their excrement. [Source: National Geographic]
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