Plankton are drifting organisms that form the foundation of the marine food web. Their name comes from the Greek word planktos, meaning “wanderer,” reflecting their inability to swim against ocean currents. The term “plankton” encompasses a vast array of life forms, ranging from microscopic bacteria and single-celled algae to the larval stages of fish, crustaceans, and even large organisms like jellyfish. Because the plankton community includes both plant-like and animal-like organisms, their appearance is incredibly diverse, leading to a complex answer regarding their color.
The Green and Gold Hues of Phytoplankton
The color most commonly associated with ocean productivity is green, which comes from phytoplankton, the plant-like members of the plankton community. These organisms contain chlorophyll, the same photosynthetic pigment found in terrestrial plants. Chlorophyll absorbs red and blue light from the sun for energy production, but it reflects green light, causing phytoplankton cells to appear green.
The concentration of chlorophyll dictates the overall green hue of surface seawater in biologically rich areas. However, phytoplankton often display colors beyond simple green due to accessory pigments. Carotenoids, for instance, are secondary pigments that help capture light energy and can impart yellow, orange, or golden-brown colors.
Diatoms, a major group of phytoplankton, frequently exhibit golden-brown shades due to high concentrations of pigments like fucoxanthin, a type of carotenoid. These accessory pigments allow the organisms to absorb a wider range of light wavelengths, which is beneficial where light quality changes with depth. The distinct color of a phytoplankton species is determined by the specific mix and concentration of these pigments within their cells.
Why Zooplankton Are Often Invisible
In contrast to their pigmented counterparts, zooplankton, the animal-like members of the plankton community, are often clear or nearly invisible. Zooplankton, which include organisms like copepods and krill larvae, do not photosynthesize and therefore do not require light-absorbing pigments.
The primary driver for this transparency is camouflage in the open-ocean environment. Since the upper ocean is a featureless habitat, being clear is an effective strategy to avoid visual predators, such as fish. Transparency allows light to pass through the organism’s body, making it difficult to distinguish from the surrounding water.
For an organism’s tissue to be transparent, it must minimize both the absorption and scattering of light. Zooplankton achieve this by having unpigmented cells and a highly ordered internal structure that reduces light scattering. While some surface zooplankton may develop light pigmentation, such as carotenoids, to protect against damaging ultraviolet radiation, most shallow-water zooplankton rely on their translucence for survival.
Plankton Blooms and Changing Ocean Colors
The colors of plankton become most visible during a bloom, which is a rapid, dense multiplication of a single species. Blooms are caused by a sudden influx of nutrients and favorable light conditions. These massive concentrations of organisms can change the color of entire ocean regions, moving the visual effect from the microscopic to the macroscopic scale.
A bloom of chlorophyll-rich phytoplankton, such as diatoms or cyanobacteria, will turn the water bright green or turquoise. Some species, like coccolithophores, produce plates of calcium carbonate called coccoliths, which are shed into the water during a bloom. These tiny, white plates scatter light, turning the ocean a milky white or pale blue, an effect often visible from space.
Other blooms are known as Harmful Algal Blooms (HABs), frequently called “Red Tides,” due to the dark colors they produce. These events are often caused by specific dinoflagellates, a group of phytoplankton that contain highly concentrated red or brown pigments. The sheer number of cells—sometimes millions per gallon—is enough to stain the water a deep red, burgundy, or brown color.
The color of a Red Tide is not always red, as the shade depends on the species, cell concentration, and the angle of the sun, sometimes appearing yellow or brown. Scientists use satellite imagery to track these ocean color changes. This tracking allows them to monitor the health and distribution of plankton populations globally, offering a broad view of marine ecosystem dynamics.