The ocean presents itself in a spectrum of colors, ranging from deep indigo in the open sea to bright turquoise or murky green near coastlines. The color observed is not an inherent property of the water molecules themselves, but rather a direct result of how sunlight interacts with the water and the materials suspended or dissolved within it. These differences in ocean color provide scientists with important data on the biological and physical composition of various marine environments.
The Baseline: Why the Ocean is Usually Blue
For the vast expanse of the open ocean, the characteristic blue color is a matter of physics involving how light interacts with water molecules. Sunlight contains the full spectrum of visible colors, each corresponding to a different wavelength. When this light penetrates the water’s surface, the water molecules selectively absorb the longer wavelengths of light, such as red, orange, and yellow, very quickly.
This absorption process leaves the shorter wavelengths, specifically blue and violet light, to travel much deeper into the water column. The blue light is then scattered by the water molecules, a phenomenon known as Rayleigh scattering, which redirects it in all directions. It is this scattered blue light that returns to the viewer’s eye, making deep, clear ocean water appear a rich blue hue.
The sheer volume and depth of the ocean amplify this light scattering effect, making the water appear blue. Any deviation from this deep blue signals the presence of other materials influencing the light transmission. The clarity of the water is a direct measure of how little dissolved or particulate matter is present to interfere with this natural blue scattering process.
The Primary Reason for Green: Phytoplankton
The shift from blue to green in ocean water is most often a biological signal, primarily caused by the presence of microscopic marine organisms called phytoplankton. These single-celled plants form the foundation of the marine food web and contain the green pigment chlorophyll. Chlorophyll captures light energy to perform photosynthesis, much like the leaves of terrestrial plants.
Crucially, chlorophyll molecules absorb light most efficiently in the blue and red parts of the spectrum to power their energy production. However, they reflect light in the green wavelengths. When phytoplankton populations are dense, the cumulative effect of billions of cells absorbing the blue light and reflecting the green light changes the water’s optical properties.
The concentration of chlorophyll effectively acts as a color filter, shifting the ocean’s appearance from deep blue toward a vibrant blue-green or even a murky green. Scientists use satellite instruments to measure the ratio of blue light to green light reflected from the ocean surface to estimate chlorophyll concentration. This technique allows for large-scale mapping of phytoplankton blooms and overall ocean productivity.
Other Factors Influencing Green Coloration
While phytoplankton are the main biological driver of green color, non-biological components also play a significant role, particularly in coastal regions. One such factor is the influx of terrigenous sediment, which includes fine particles of silt and clay carried by land runoff and rivers. These suspended particles increase the scattering of all wavelengths of light, leading to a turbid, less transparent appearance.
When a large river plume meets the ocean, the water can take on a brownish-green or opaque color because the sediment scatters the light indiscriminately. This high amount of scattering overwhelms the water’s natural blue scattering and the effects of biological pigments. This is a common occurrence near river mouths or after large storms that churn up the seabed in shallow areas.
Another factor is Colored Dissolved Organic Matter (CDOM), also known as yellow substance. CDOM is a complex mixture of organic molecules derived from the decay of plant and animal matter, both on land and in the sea. This dissolved material strongly absorbs blue and ultraviolet light, much like a dark tea stain.
Because CDOM removes the blue wavelengths from the water, the remaining reflected light appears yellow or yellow-brown. When this yellow hue mixes with the blue light scattered by the water, the resulting color is a shade of green. CDOM concentrations are much higher in coastal areas and estuaries due to the inflow of river water, explaining why nearshore waters are often more yellow-green than the open ocean.