Until recently, the answer to why water is blue divided experts, with some arguing that the blue color was simply a reflection of the sky above. While there is some truth to that statement, the reality is more complex.
Just like any other thing on earth, water’s color has to do with light. When sunlight hits water, red, orange, and yellow light (the long wavelengths of the spectrum) is absorbed, while shorter wavelength blue is reflected back. So, water doesn’t “reflect the sky,” but rather the ocean is blue for a similar (though slightly different) reason that the sky is.
Chlorophyll, which is produced by phytoplankton during photosynthesis, absorbs blue wavelengths and reflects green light. Because of this, parts of the ocean with high concentrations of phytoplankton appear to be more blue-green than translucent blue. Phytoplankton use carbon dioxide for photosynthesis; as such they are actually extremely important in the ocean’s role as a carbon sink (i.e., pulling carbon dioxide out of the atmosphere and storing it).
Water itself is only very slightly blue. Why, then, does water sometimes look dark green, turquoise, or even red? Perfectly pure water does not occur naturally. In lakes, rivers, and oceans, water always contains minerals and living organisms in varying quantities, which alter its color. For example, algae can turn a pond into a murky green, and in some instances red, while large amounts of mud will make it yellow, like the Yellow River in China. This is because particles suspended in water increase the scattering of light, changing the hue that our eyes perceive.
Testing the color of water is a quick and easy way to detect the amount of organic matter, particles, or even metals, present in a sample. These tests also allow us to assess how healthy a body of water is. Highly-coloured water can have a negative impact on ecosystems by blocking the penetration of light, making underwater life difficult.