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It is well known that baleen whales migrate to the polar seas to feed on plankton and krill (marine crustaceans). The question why are the polar seas rich with krill and plankton? If the answer "it is because they are rich with nutrients" then I ask why are they rich with nutrients?

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Water density and temperature is very important for understanding water ecosystems. Water is most dense a little above freezing temperature; when surface water is warmer than that, it's less dense and sits on top of cooler water. When surface water is chilled to freezing and increases in density, it can sink and mix with deeper water.

Photosynthesis can only occur in the top of the water column because light doesn't penetrate very deep. When the surface water stays warm and doesn't mix, that top layer gets depleted of nutrients. Mixing brings up anything that has settled and allows life to use the "full column" of nutrients.

See for example https://www.nature.com/scitable/knowledge/library/the-biological-productivity-of-the-ocean-section-70631438/ :

In the high latitude ocean, surface water is cold and therefore the vertical density gradient is weak, which allows for vertical mixing of water to depths much greater than the sunlit "euphotic zone" as a result, the nutrient supply is greater than the phytoplankton can consume, given the available light

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