Water Color

Sea water is often very colored. Near the Blue Grotto on the Mediterranean island of Capri the water is an intense dark blue. Near by on the Amalfi Coast of Italy, there is the Green Grotto. Freshwater is widely colored ranging from a blue-green to green to brown.

• So why is water colored?

On the other hand, when you look at a glass of drinking water, it is clear and colorless.

• Why is water clear when you hold a glass of it up to the light?

There are three major factors that affect water color: 1) absorption and scattering of light, 2) dissolved substances, and 3) suspended living organisms and non-living substances like soil particles. We will briefly look at each of these factors.

The upper 1 meter of most water bodies rapidly absorbs infrared and red wavelengths of light resulting in warming of the upper layers of water bodies. The other wavelengths of light penetrate deeper into the water. The shorter wavelengths, like the blues and ultraviolet wavelengths, penetrate the deepest. The blue water you see is in part due to the scattering of light back up through the water surface. The shorter wave lengths are not absorbed as readily as longer wavelengths and therefore are more likely to be scattered back out of the lake.

If you have ever visited a limestone rock quarry that contains water, you probably noticed that water was green in color. Dissolved substances also have a profound effect on water color. A common molecule in hard water lakes such as limestone quarries contain CaCO_3. This molecule scatters light in the greens and blues and gives these bodies of water their characteristic blue-green color. In Costa Rica, there is a water fall that is blue during the dry season because of the large amount of copper found in the water. On the other hand, it you ever visited a bog, you noticed that the water is brown in color, looking more like a weak tea. This is because of humic compounds or organic matter dissolved in the water. In these bodies of water, UV, blue, and green wavelengths are absorbed within 1 meter, leaving the longer wavelengths to be scattered back out of the water.

If you have seen pictures or visited a pond receiving large amounts of nitrogen and phosphorous, you may have noted that the color of the water is an intense bluegreen color usually accompanied with mats of algae floating on the surface. In this case the water is bluegreen because of the bluegreen algae which reflect these wavelengths. Some waters receiving volcanic ash may be yellow to reddish brown because suspended materials reflect these wavelengths of light. Red waters are rare. "Red water" is associated with massive temporary blooms of dinoflagellates such as Glenodinium or Gonyaulax.

• How do rainbows form and what does this phenomenon have to do with living organisms?
• Why is the sky blue?
• Why do you think the water off the Island of Capri is a dark blue color? Be specific.
• If the ocean contains algae, why doesn't the open ocean look green?
• What allows red algae to be found as deep as 260 meters in the ocean?
• Are dinoflagellates animals or plants? Defend your answer.

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