The clarity and color of ocean water are determined by marine optics—specifically, how light interacts with the molecules and materials within the water column. Clarity, or transparency, is a measure of how far light can penetrate before it is absorbed or scattered away. This transparency is highly variable globally and reflects the amount of suspended particulate matter and dissolved substances present.
How Suspended Sediments Block Light
Clarity is significantly reduced in coastal areas due to inorganic particulate matter, scientifically termed turbidity. Turbidity is primarily caused by terrestrial runoff, where rivers carry vast amounts of fine silt, clay, and sand from the land into the ocean.
Since clay and silt particles are extremely fine, they remain suspended in the water column, clouding the water and scattering light. In shallow coastal regions, wave action and currents constantly stir up the seabed, leading to the resuspension of fine sediment and decreasing clarity. Storm events cause sharp, short-lived increases in turbidity. In colder regions, meltwater from glaciers carries glacial flour—pulverized rock particles—which scatter blue and green light, often giving the water a milky or striking turquoise appearance.
The Influence of Biological Life and Dissolved Matter
Biological productivity is a major factor that influences both the color and clarity of ocean water, often shifting it from deep blue toward green. Microscopic phytoplankton contain chlorophyll, a pigment that strongly absorbs red and blue light but reflects green light. High concentrations of phytoplankton, such as in productive upwelling zones, make the water appear greener and less transparent.
Colored Dissolved Organic Matter (CDOM), often called “yellow substance” or gelbstoff, consists of the decayed byproducts of phytoplankton and other organisms. CDOM strongly absorbs light in the ultraviolet and blue regions of the electromagnetic spectrum. This selective absorption of blue light makes the water appear brown or yellow, reducing clarity and making it less blue than open ocean water.
The availability of nutrients dictates biological activity, leading to large differences in clarity across ocean basins. Oceanic deserts, such as the vast subtropical gyres, are regions with very low nutrient concentrations, which limits the growth of plankton. This low biological productivity results in exceptionally clear water with minimal suspended organic matter, allowing the water’s inherent blue color to dominate. However, even in these clear gyres, tiny picophytoplankton can produce CDOM, which still affects the water’s optical properties by absorbing blue light.
The Physics of Light Absorption and Water Depth
The fundamental reason clear ocean water appears blue is due to the inherent optical properties of the water molecules themselves. Water molecules preferentially absorb the longer wavelengths of the visible light spectrum, such as red, orange, and yellow. Conversely, the shorter wavelengths, primarily blue light, are scattered in all directions.
The scattering of blue light directs the color back to the viewer, making the open ocean appear blue. Deeper water increases the distance light must travel, ensuring longer wavelengths are completely absorbed, leaving only blue light to be scattered. In very shallow, clear waters, clarity is enhanced because light reflects off a bright, sandy or coral seabed. This reflected light creates the vibrant, light blue or turquoise colors seen in tropical shallows.