system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is seawater green in some places, and clear/blue in other places? And why dont these colors mix?
gpt
Green sea water is usually the presence of floating algae, diatoms and other plankton that feed on the nutrients in the water. Green water has more stuff alive in it, usually colder water. Colder water can hold more dissolved oxygen in it. Often colder because of upwelling of deeper nutrient rich water that's being pushed to the surface by underwater geological features. 

Tropical water tends to be clear/blue because it's warmer and therefore has less oxygen and have less nutrients, therefore less microscopic aquatic life.