The ocean is associated with many things, whether salt air, crashing waves, or a relaxing atmosphere, but perhaps most of all, it is associated with the color blue. For a long time, people have been wondering what determines the shade of our seas. Is the reason for their blue tint just that they reflect the color of the sky, or is it a bit deeper than that?
The Way Light Reflects off the Ocean Surface
You might have heard that the ocean is just reflecting the color of the sky, and while this is true to a certain degree, it is not the actual answer to why the sea is blue.
From the coast, the sea does appear to change color when the sky is grey and overcast or when orange and pink hues glitter on the water during sunrise and sunset.
But this reflection only occurs on the surface. The depths of the ocean remain blue as far as eyes can see, or better said, as far as the light penetrates them.
The Way Ocean Water Absorbs Light
Water molecules are pretty good at absorbing light. However, different colors or, better said, light rays are absorbed in various degrees. Water sort of acts like a filter that gradually absorbs all the colors except for blue, so blue is the prominent color that is scattered back and reaches human eyes.
The real reason for a blue ocean lies in the way water interacts with light. Sunlight consists of all the colors of the visible light spectrum, from red to violet. However, each of these colors of the rainbow has a different wavelength, making them act differently. Focusing only on visible light, the colors in the red part of the light spectrum have longer wavelengths, with red light having the longest one. These colors are absorbed by the water the quickest.
Credit: DrSciComm (CC BY-SA 4.0)
The colors in the blue part of the spectrum have shorter wavelengths; therefore, they have more energy and can travel to greater depths. Violet light has the shortest wavelength; however, blue is the color the least absorbed by water.
While reds, oranges, yellows, and even violets and greens are, at different rates, absorbed by the ocean, blues (and sometimes greens) are scattered back, leaving them to be the only colors left for our eyes to see.
Credit: Tomemorris (CC BY-SA 4.0) via Wikimedia Commons
This is also why everything under water appears to have a blue tint. While close to the surface, you can still recognize all the colors; the deeper you go, the more certain colors begin to fade. For example, 30 meters underwater, a red swimsuit looks black because water absorbs all the red hues.
What Affects the Color of the Ocean?
Now we know why the ocean is generally in the shades of blue or green, but what determines whether the water appears azure, turquoise, or brownish? Why does the sea have different shades in different areas?
Algae and Sediment
Perhaps the most surprising thing is that the color of the sea is, to a large degree, affected by phytoplankton floating in the water. Cells of this microscopic algae actually contain a green pigment called chlorophyll. They use for photosynthesis which allows them to produce about half of the Earth’s oxygen.
Chlorophyll is more prone to absorbing red and blue light while reflecting back green. This means that waters rich in phytoplankton will appear to be a shade of green rather than blue.
Typically the more algae there is, the greener the sea should look. But the exact hue doesn’t only depend on the density but also on the type of phytoplankton in the area. Some algae blooms, also known as red tides, can even color the ocean red.
Scientists monitor phytoplankton blooms in order to better understand their impact, but it also helps them study and predict other environmental changes.
Credit: European Space Agency (CC BY-SA 2.0)
Sand and silt stirred by waves and storms, runoffs from rivers, and other floating marine sediment increase the reflection of light. The rays bounce off of them, making the sea look murky or muddy brown.
Depth and Seafloor
Depth also plays a role in the ocean’s coloring. Shallow areas inherently have less water and, therefore, fewer water molecules that can absorb and reflect light. This results in light being able to shine through the water and reach the sea floor.
It comes as no surprise that, particularly in shallow water areas, the sea floor affects the shade of the ocean. The sand, rocks, or corals on the bottom reflect light differently than water molecules.
When the sea is shallow, the light bounces off of the ocean floor or marine plants, and the water tends to appear more turquoise or light blue if the seafloor consists of white sand, rocks, or plant life.
Areas with deeper water are typically navy blue because a large mass of water can absorb and scatter much more light and because there are no reflections off the ocean floor that could influence the sea color. Not to mention that no light penetrates below 3,280 feet (1000 m), and the ocean is entirely dark at this depth.
Why is some ocean water so clear and blue?
Ocean water is clearer when there is an absence of plankton and other floating substances. The water also appears to be crystal blue when it’s shallow, and the ocean floor consists of white sand.
Why is ocean water green, not blue?
If the sea looks green rather than blue, it is likely due to the abundance of algae and marine plants in the area.
Is ocean water blue because of the sky?
While the Ocean can reflect the sky on its surface, it is not the reason why it is blue. The sea looks blue due to water molecules mostly reflecting back blue color while absorbing other colors of the light spectrum.
Why is ocean water different colors in different places?
The Ocean’s depth, seafloor makeup, and the presence or absence of algae and floating sediment all influence what color the sea appears in a particular area.