Why Is the Ocean Blue?

Have you ever wondered why the ocean is blue or why it is sometimes another color, like green, instead? Here's the science behind the color of the sea.​

The Answer Is in the Light

There are a few reasons why the ocean is blue. The best answer is that the ocean is blue because it is mostly water, which is blue. Water most strongly absorbs light in the 600 nm to 800 nm range. You can see the blue color even in a glass of water by observing it against a sheet of white paper in sunlight.

When light strikes water, like sunlight, the water filters the light so that red is absorbed and some blue is reflected. Blue also travels farther through water than light with longer wavelengths (red, yellow, and green), though very little light reaches deeper than 200 meters (656 feet), and no light at all penetrates beyond 2,000 meters (6,562 feet). So, deeper water appears darker blue than shallow water, following Beer's law.

Another reason the ocean appears blue is because it reflects the color of the sky. Tiny particles in the ocean act as reflective mirrors, so a large part of the color you see depends on what is around the ocean.

Dissolved minerals in water also contribute to its color. For example, limestone dissolves in water and gives it an overall turquoise color. This color is noticeable in the Caribbean and off of the Florida Keys.

Green Oceans

Sometimes the ocean appears other colors besides blue. For example, the Atlantic off the East Coast of the United States usually appears green. This is due to the presence of algae and plant life. Photosynthetic organisms contain chlorophyll, which not only appears green, but also absorbs red and blue light. Depending on the type of phytoplankton, the water may appear more blue-green to emerald green.

Yellow, Brown, and Gray Oceans

The ocean may appear gray under a cloudy sky or brown when the water contains a lot of sediment, as when a river empties into the sea or after the water has been stirred up by a storm.

The chemical composition of the sediment plays a part in the resulting water color. Tannins turn water black, brown, or yellow. Lots of sediment in the water makes it opaque instead of translucent.

Red Oceans

Some oceans appear red. This happens when a specific type of phytoplankton reaches a high enough concentration to produce a "red tide." Sometimes the algae also releases toxins into the water, but not all red tides are harmful. Examples of red algae and places where the ocean is red include Karenia brevis in the Gulf of Mexico, Margalefadinium polykroides and Alexandrium monilatum in the Chesapeake Bay, and Mesodinium rubrum in Long Island Sound.

Related Science

For more on the color blue in science, check out these articles:

• "Why Blood Isn't Blue"
• "Why Babies Have Blue Eyes"
• "Why Veins Look Blue"
• "Why Ice Is Blue"

Sources

• Braun, Charles L.; Sergei N. Smirnov (1993). "Why is water blue?". J. Chem. Educ. 70 (8): 612. doi:10.1021/ed070p612
• Filipczak, Paulina; Pastorczak, Marcin; et al. (2021). "Spontaneous Versus Stimulated Surface-Enhanced Raman Scattering of Liquid Water". The Journal of Physical Chemistry C. 125(3): 1999-2004. doi:10.1021/acs.jpcc.0c06937
• Mishchenko, Michael I; Travis, Larry D; Lacis, Andrew A (2002). Scattering, Absorption, and Emission of Light by Small Particles. Cambridge, UK: Cambridge University Press.
• Morel, Andre; Prieur, Louis (1977). "Analysis of variations in ocean color". Limnology and Oceanography. 22 (4): 709–722. doi:10.4319/lo.1977.22.4.0709
• Vaillancourt, Robert D.; Brown, Christopher W.; Guillard, Robert R. L.; Balch, William M. (2004). "Light backscattering properties of marine phytoplankton: relationships to cell size, chemical composition and taxonomy". Journal of Plankton Research. 26 (2): 191–212. doi:10.1093/plankt/fbh012