Buzz
The current movements in the southwestern Indian Ocean as captured by the SeaWiFS satellite. Together with nutrients from volcanic island soil and the runoff of seabird and seal droppings after rain, we observe elevated chlorophyll and phytoplankton levels, which give the water a green tint in certain regions. SeaWiFSA person looking at the ocean from the coast of Maine experiences a completely different color palette compared to someone gazing at the sea from a sun-soaked beach on a Greek island. So what makes the ocean blue and why does it appear in so many distinct shades? Let’s explore!
Myth or Truth: Is the Ocean Really Blue?
To begin with, as NASA oceanographer Gene Carl Feldman explains, "The ocean's water isn't blue, it's clear. The color we see on the ocean's surface mainly depends on its depth, its contents, and what lies beneath." Thus, while the ocean may seem blue, in reality, it's clear.
Explaining Why the Ocean Looks Blue
A glass of water will obviously look clear since visible light passes through it with minimal obstruction. However, when the water is deep enough that light doesn’t reflect off the bottom, it appears blue. This can be explained by basic physics: Sunlight consists of various wavelengths. Longer wavelengths appear as red and orange light, while shorter wavelengths are seen as blue and green. When sunlight hits the ocean, it interacts with water molecules and is either absorbed or scattered. If the water contains only water molecules, shorter wavelengths are more likely to scatter, giving the ocean its blue appearance. On the other hand, the longer, red wavelengths are absorbed near the surface.
The Impact of Ocean Depth
The ocean’s depth and its floor also affect whether the surface appears dark navy blue, like in parts of the Atlantic, or shows a bright blue or lighter shade as in tropical waters. According to Feldman, "In Greece, the water is this stunning turquoise because the bottom is either white sand or white rocks." What happens is that light reaches the seafloor and bounces back, reflecting the light and creating the beautiful light blue hue visible in the water. However, most of the ocean appears dark navy or completely dark, as light rarely penetrates deeper than 656 feet.
The Color of the Ocean Also Indicates Its Health
Additionally, the ocean is rarely crystal clear; it's often filled with tiny organisms or suspended particles, pollutants, and sediments. Oceanographers observe the ocean’s color much like doctors monitor a patient's vital signs. The color visible on the ocean's surface is an indicator of the conditions in the deep ocean.
Feldman, a researcher at the NASA Goddard Space Flight Center in Maryland, analyzes images captured by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite, launched in 1997. From its position over 400 miles (644 kilometers) above Earth, the satellite captures swirling patterns of the ocean’s colors. These patterns are not only beautiful but also reveal areas where sediment and runoff may cause the water to appear muddy brown, as well as where phytoplankton gather in nutrient-rich waters, giving the water a light green hue.
Phytoplankton contain chlorophyll, a green pigment that absorbs sunlight to transform water and carbon dioxide into organic compounds. Through photosynthesis, phytoplankton produce about half of the oxygen we breathe. While most phytoplankton turn ocean water green, some can give it a yellow, reddish, or brown hue, according to Feldman.
Oceans with high levels of phytoplankton can appear blue-green to green, depending on their concentration. Although greenish water might not sound appealing, as Feldman points out, "Without phytoplankton, we wouldn’t be here." Phytoplankton are the foundation of the food chain, providing nourishment for zooplankton, which are eaten by fish. These fish are then consumed by larger animals such as whales and sharks.
Oceans become polluted with runoff, leading to an overgrowth of phytoplankton. These organisms feed on the pollutants, grow rapidly, and eventually die, sinking to the ocean floor where they decompose, consuming oxygen in the process and depleting the water's oxygen levels.
The Impact of Climate Change
In the past 50 years, the size of ocean zones with depleted oxygen has increased more than fourfold, now covering an area roughly equivalent to the size of the European Union, or 1,728,099 square miles (4,475,755 square kilometers), as revealed in a study published in the January 2018 issue of Science. A contributing factor may be the rising ocean temperatures due to climate change, which reduces oxygen levels in the water. In coastal areas, phytoplankton blooms are suspected to be a significant cause. Phytoplankton are vital to the ocean's food chain, but as Feldman points out, "Too much of a good thing is not a good thing."
In Feldman's office, a map shows a location where human activity has minimal impact, and the ocean water is possibly the clearest on Earth. Off the coast of Easter Island in the southeast Pacific Ocean, the water is deep and exceptionally clear due to its position within a vast oceanic gyre, or circular current. This central location prevents mixing of ocean layers, and nutrients from the deep ocean aren't brought up. The combination of clarity and depth causes the water here to appear an intense indigo, unlike anywhere else.
"The light just keeps going down, down, down; there's nothing that bounces it back," says Feldman, "This is the deepest blue you will ever see."
A type of bacteria known as Synechococcus cyanobacteria can change its color to match various wavelengths of light found in oceans worldwide. These bacteria capture light to absorb carbon dioxide and generate energy. According to research published on Feb. 12, 2018, in the Proceedings of the National Academy of Sciences, these bacteria possess genes that enable them to adapt their color, much like a chameleon, allowing them to thrive in waters of any color and optimize their ability to absorb surrounding light.
