There is one thing about the ocean that we all know for sure, and that is that the ocean and the sea appear to be blue. Why is the ocean blue to our eyes?
The ocean contains more than 97% of the Earth’s water and supports 99% of the global biosphere. Therefore, the ocean is extremely important for life on our planet.
Due to its vastness, only 5% of the ocean has been explored and charted by humans. The rest, especially its deepest parts, remain unexplored.
In this article, we will explain why the ocean is blue, so you can finally understand how it works.
Is the ocean blue?
Of course, we all know that clean water is colorless, it is transparent. So, why does the ocean – and other bodies of water – appear blue?
For many years, we have believed that the ocean and other bodies of water are a shade of blue because they reflect the blue sky… but this is not entirely true! Sure, the surface of the water reflects the sky: from the coast, it may appear blue on a sunny day, gray when it is cloudy or stormy, or even show shades of pink during sunset. But if we go below the surface, the blue color remains, and it also remains when we look at the ocean from space. In these cases, the water does not reflect the sky.
The reason why the ocean appears blue is indeed a reflection, but it is not simply a mirror of the sky.
In fact, sunlight contains the entire spectrum of colors, from red to violet, as we see in the rainbow. Each color has a specific wavelength, the color red has longer wavelengths. The wavelengths of the other colors become progressively shorter, with blue and violet at the end of the range, having the shortest wavelengths of all.
When light hits the ocean, the water absorbs the longer wavelengths first, reflecting the colors with the shorter wavelengths back to our eyes. Then, like a filter, the water molecules absorb the red parts of the light spectrum and leave behind the colors of the blue spectrum, which our eyes see.
As soon as we reach a depth of a few meters, most of the red and orange light disappears completely, absorbed by the water molecules. Shortly after, the yellow and green wavelengths are absorbed, leaving only blue and purple, which are able to penetrate deeper.
However, this phenomenon only occurs up to a certain depth. In fact, most of the ocean is completely dark, as almost no wavelengths penetrate deeper than 200 meters.
Do all bodies of water appear blue?
Everything absorbs light at a different wavelength, and then reflects the remaining colors back to the observer.
When light penetrates the water completely, like in a glass or a very shallow body of water, we see the water as colorless, because not enough photons (the molecules made from light radiation) are absorbed. The light simply shines through.
This is mainly why different bodies of water can have different shades of blue. The deeper the body of water, the darker and deeper the color, because the more water there is, the more light is absorbed.
You may have noticed that, at times, water can also appear to be colors other than blue. This is often a result of the characteristics of the water or the soil.
Some rivers or ponds, for example, can appear a muddy brown rather than blue, which is often due to the presence of sediment in the water, especially after the water has been agitated.
The crystal clear waters of the Caribbean, which are usually closer to shades of light green than blue, you might have thought that it is because the water is cleaner. Sure, this has an impact, but often the colors are given by the plant life that exists in many Caribbean waters, helping to reflect more green light. Often, the most beautiful Caribbean waters also tend to be shallow, and the composition of the ocean floor, given mainly by coral, can be responsible for a different way of reflecting light.
Why do scientists study the color of the ocean?
Some types of particles (for example, phytoplankton cells, also called microalgae) also contain substances that absorb different wavelengths of light, changing the color reflected by the water. There are many substances in the water that absorb light and therefore change the colors reflected. Usually, these substances are composed of organic carbon, and scientists generally refer to them as CDOM, short for colored dissolved organic matter.
One essential substance that absorbs light in ocean water is chlorophyll, which phytoplankton use during the process of photosynthesis. Chlorophyll is a green pigment, and so phytoplankton preferentially absorb the red and blue portions of the light spectrum for photosynthesis, thus reflecting green light.
Thus, regions of the ocean with high concentrations of phytoplankton appear with shades other than the usual blue: they range from teal to green, depending on the density and type of phytoplankton population they contain.
The principle behind studying the color of the oceans from space is simple: the more phytoplankton there is in the water, the greener it appears; the less phytoplankton, the bluer it will be.
So, scientists are studying the color of the ocean to better understand phytoplankton and how they affect the ocean and the Earth.
It turns out that these tiny organisms can have a big impact on a large-scale system, such as climate change. For example, phytoplankton use carbon dioxide for photosynthesis, providing nearly half of the oxygen we breathe on the planet. So, a large and thriving global population of phytoplankton means that more carbon dioxide is being pulled out of the atmosphere, mitigating the effects of pollution and lowering the effects of global warming and climate change.
Scientists have found that a given population of phytoplankton can double in number about once a day, meaning they are able to respond very quickly to changes in their environment.
Examining the color of the ocean helps researchers understand and monitor phytoplankton, which can be a step toward predicting environmental changes.
Changes in any phytoplankton population, such as changes in its density, distribution, and rate of population growth or decline, will alert scientists to changing environmental conditions.
Bibliography
https://oceanservice.noaa.gov/facts/oceanblue.html
https://www.scientificamerican.com/article/why-does-the-ocean-appear/
https://science.nasa.gov/earth-science/oceanography/living-ocean/ocean-color
https://www.wonderopolis.org/wonder/why-is-the-ocean-blue
