"Oceans so blue"

The irrepressible Chris Hurtubise, apparently satisfied with my response to her query on the color of the sky but hungry for more science, continued undaunted with a new question written on my board: what about the ocean? Sometimes it's blue, and sometimes it's brown or green. What gives?

Just like air, water is also a blue thing. And also just like air, it is only slightly blue. A glass of clean water appears almost perfectly transparent, but a waist-deep pool takes on a characteristic blueish tint. Although air gets its blue color from scattering of different wavelengths of light, water molecules simply absorb reds, yellows, and greens while reflecting blue.

This is most apparent when scuba diving, where colors are visibly muted. For example, if you accidentally cut yourself the wound appears mud-brown instead of vibrantly red. Photos of spectacular coral reefs are usually taken at night (when the corals are active) and using super-high-powered lights.

But there's more to water than water. Almost all of the oceans are deserts, both in the sense of meteorology (no rain) and biology (no life). A blue ocean is a dead ocean, but currents can dredge nutrients and minerals from the deeps, and where there's food there's life. Brown sediments and green and red algae can overwhelm the natural color palette of water. A murky ocean is a lively ocean.

Speaking of corals, those creatures require shallow, clear water so their symbiotic algae can get sunlight. Normally these regions would be devoid of life, but the reef system provides the base of a complex ecosystem - to the delight of divers worldwide.