system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is the ocean sometimes really dark, like in the north Atlantic yet in some places a turquoise colour like in the Caribbean or South Pacific?
gpt
It’s because dead animals and plants sink in water. That might seem obvious, but it means that any nutrients in sea water are rapidly used up by animals and plants and when those organisms die they sink to the bottom taking the nutrients with them. The only way those lost nutrients can be replaced is if water from the ocean bottom, where all those dead organisms go, can be moved to the surface again. 

That cycling can’t happen in warmer regions because the surface water is warm and warm water floats on the cold water of the ocean bottom. As a result warmer waters very rapidly become nutrient deficient. 

Because they are nutrient deficient very little actually lives in tropical waters. Very few algae, very little plankton and so forth. And because nothing much lives in tropical waters they remain clear. That means that light can get go waayyy down into the depths, and as light passes through the water the blue wavelengths get scattered. That produces a vivid turquoise blue ocean. 

In contrast cold waters are dirty and full of life due to the upwelling of water from the ocean bottom. All that dirt and all the life it supports absorbs sunlight very fast. Light will only penetrate a few meters in cold waters. With very little penetration the light also doesn’t have much chance to scatter. That produces an ocean that is very dark colored, and where the blue wavelengths are muted by the reds and greens to produce a muddy blue-gray. 

So what it all comes down to is that tropical waters are clear, and they are clear because they are sterile and nutrient deficient. Coral reefs thrive in tropical waters precisely because they are clear and sterile and that allows enough sunlight for the photosynthetic symbiotic algae that live in the coral and that provide most of their food.