Introduction

What is the first color that comes to mind when you think of the ocean? Blue, right? Pictures of the planet from space show that what we call Earth is mostly a blue ocean (if we knew this earlier, we may have named it Ocean rather than Earth!). But if we dig a little deeper, we find that the sea can have a range of colors, varying from blue to green or even yellowish-brownish. You can see some of these differences yourself the next time you take a plane trip. Near the coast, the water is often a different color from what it is out at sea. Have you ever wondered what those different colors mean? In this article, we will look at why the color of the sea changes, how this can be captured by satellites in space, and what it can tell scientists about the oceans.

First, What Is Color?

Objects that are black absorb (extinguish) all light, so you could say they have no color, because what our eyes actually see as color is the light that is reflected back from an object. White light (like sunlight) consists of a mixture of light of all colors. Milk is white because it contains lots of very small fat particles that reflect all colors equally well. Similarly, a sheet of white paper reflects all colors. Some objects appear a certain color to our eyes because they absorb specific colors, meaning that they actually remove certain colors of light. Light is absorbed by pigments, which are chemical compounds that can be found in living organisms (such as green plants) and inanimate objects (such as paints). The colors that are not absorbed by an object are reflected by the object, and that is what we see or measure with scientific instruments. So, what we see is, in fact, the light that is reflected by the object we are looking at.

What Controls the Color of the Sea?

Some of the sunlight that shines on the sea surface is absorbed by the water and by pigments that can be found in it, and the rest is reflected, creating the blue, green, and yellowish-brownish colors we see when looking at the ocean (Figure 1). Which colors are reflected depends on what is in the water and the properties of water [1]. Water naturally absorbs most red light and reflects a lot of blue light (Figures 1B, 2), which is the one that penetrates deepest into the water and is the most reflected (Figure 2). This can be observed, for instance, in the clearest blue-water tropical oceans (and mountain lakes). Other colors of water result from three things: dissolved compounds, plants, and very fine grains of sand.

Dissolved Compounds: The Ocean as a Cup of Tea

Make yourself a cup of tea and you will see the water in your cup become yellow or brown, as the pigments in the tea leaves dissolve into the water. The pigments from the dried tea leaves absorb blue light, and the light reflected ends up being yellow or brown. This example helps to explain why seawater can be yellow or brown near places where rivers flow out into the sea. The same way water dissolves compounds in tea leaves, falling rain dissolves soil, which drains into rivers and finally makes its way to the ocean (Figure 1C).

Plants: Just Drifting Around

Just like plants on land, plants in the ocean have a green color. Plants have pigments that they use to capture sunlight as a source of energy, through photosynthesis. Most plants in the ocean are part of a group of organisms called phytoplankton, which are very tiny—the width of human hair or even smaller (see this Young Minds article for some examples). Because of their size, phytoplankton cells mostly drift around with the ocean currents. When there are lots of phytoplankton cells in the water the ocean (called a bloom), the ocean becomes green (Figures 1A, 2), which can often be seen in spring and summer. Some groups of phytoplankton have certain red pigments, making the ocean look red when those phytoplankton bloom. This phenomenon is called a red tide.

Sediments: The Oceans’ “Milky Way”

As rocks and soils experience the effects of wind, rain, and erosion over the years, they are broken into smaller pieces, ranging from coarse sand grains to very fine silt that forms clay. These products from eroded rocks and soils are called sediments. The very fine grains sink so slowly that they remain suspended in the water for a long time. Near the coasts, these grains are either supplied by rivers or whirled up from the seafloor by storms. Although sediments can be pigmented (colored), they mainly change the color of the water by reflecting (scattering) light in all directions (Figure 2). This is similar to what the fat particles in milk do—making the water become what we often call “milky” (Figure 1D). Some phytoplankton called coccolithophores have white plates made of chalk, which also scatter the light. When these phytoplankton bloom, the ocean water can become light blue or even white (Figure 1B).

Measuring the Color of Water

We can see a lot using only our eyes: the blue of the open ocean, the brown and milky river waters flowing to the sea, and the green phytoplankton blooms at the beach in summer. But with instruments called sensors, we can turn the colors we see into precise numbers. We can use those numbers to follow very small changes in water color, which are invisible to the naked eye. These sensors are called radiometers. Some radiometers only measure a specific color, whereas others can measure lots of colors at the same time—either individually or by combining the total signal across several colors (see this Young Minds article for some examples).

Satellites: Sentinels From Space

A satellite is any man-made or natural object that orbits another, larger object. Often, satellites with a range of sensors are intentionally placed into orbit around the Earth. These sensors can include radiometers used to monitor water color, among other things. This technique is called ocean color remote sensing (OCRS) and, although it has “ocean” in its name, it can be used to monitor the color of water in lakes, rivers, and other aquatic environments, too. The OCRS technique has been running since the 1970s, when the first ocean color sensor, the Coastal Zone Color Scanner, was integrated into the satellite Nimbus 7. A sensor on board a satellite acts like a camera that takes several pictures of the Earth’s surface during its orbit around the planet. That means these sensors capture the sunlight reflected by ocean water after that light is absorbed and/or scattered by dissolved compounds, phytoplankton, and sediments (Figure 2). The individual pictures are then combined into a mosaic, which represents a global map of how and why ocean color changes. This mosaic can be produced once a day, every day! The daily mosaics can be combined over several years to form a time series. Scientists use time series images to study how the color of the oceans varies regionally (for example, across different seas or in coastal vs. open sea areas) and over time (year to year, month to month), and to evaluate if ocean conditions are changing [2].

Why Is It Important to Study the Color of the Oceans?

The color of the oceans can provide scientists with lots of information. For instance, ocean color can tell us about the quantities and locations of phytoplankton, dissolved compounds, and sediments in different regions. What do those three components have in common? They all include the element carbon. That means the oceans are involved in the global carbon cycle. Carbon dioxide (CO₂) is a very important gas in the carbon cycle. CO₂ can influence global temperature and climate. Like other plants, phytoplankton use CO₂ during photosynthesis and therefore help to control the amount of CO₂ in the atmosphere (see more in this Young Minds article). Phytoplankton also represent the basis of the marine food chain, and changes in phytoplankton quantity and distribution may affect the overall life cycle in the oceans and can even impact humans, in terms of the availability of fish for us to eat (see more in this Young Minds article) [3]. Thus, the color of the oceans represents more than beautiful hues, it gives us information about fundamental elements that control life in the oceans and the climate of the entire Earth.

Do You Want to Help Ocean Scientists Collect Data?

Due to some impressive technological developments in the last decades, citizens (including you!) can help to advance this science through what are called citizen science projects. Using your smartphone, you can download applications that collect information about the color of waterbodies near you. This info is sent to scientists so they can, for instance, correct the data collected from the satellites. One example is called Project Citclops (Citizen’s Observatory for Coast and Ocean Optical Monitoring), which has developed a smartphone app called Eye on Water. This app is fun and quite easy to use, and a great way for you to contribute to research and help the scientific community. So, get your hands dirty (or should we say wet) and help ocean-color science!

Glossary

Pigments: ↑ Chemicals that add color to a material.

Phytoplankton: ↑ Single-celled organisms of water environments that make their own food from sunlight through photosynthesis.

Bloom: ↑ When phytoplankton density increases rapidly due to lots of nutrients and sunlight available in the water. It generally happens during Spring season.

Sediments: ↑ Fragments of material that have been broken down. Those materials can be organic (for example, soil) and inorganic (for example, rocks).

Radiometer: ↑ Instrument to detect and measure the intensity of light.

Ocean Color Remote Sensing (OCRS): ↑ A method to collect information on the color of the ocean’s surface and further transform it into information (data) which can be used by scientists to study the oceans.

Carbon Cycle: ↑ Process of how the element carbon travels from living to non-living objects.

Citizen Science: ↑ Science projects in which the public participates by collecting data. Some projects eventually also train the public to analyze data.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

RG-A has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant Agreement No. 839311.