You may have heard of "toxic 'Red Tides'" but not all Harmful Algal Blooms (HABs) are red... and not all "Red Tides" are harmful! What causes distinct coloration of water associated with HABs? It is natural to assume that the colors come from the phytoplankton themselves (e.g., that the algae are red in color) but that's not really the case. There are several possible reasons for the discolorations including the combined optical properties of all components in the water (not just the algae) and the depth distribution of the bloom.
Particles of light (i.e., photons) may be either scattered (path of light energy changes direction) or absorbed (light energy is removed). These processes determine the fate of light entering the ocean and thus ocean color. Different constituents (e.g., particles) found in the ocean absorb and scatter light differently; it is the sum of scattering and absorption by all these constituents that impart ocean color. (For more on optics, see the Smithsonian Environmental Research Center's "What is hydrologic optics?" website.)
To the eye, ocean color is the ratio of backscattering to absorption. Water itself both backscatters and absorbs light in a distinctive way; likewise, all the "stuff" in water has its own characteristic backscattering and absorption. Roll your cursor over the buttons below to see how backscattering, absorption, and ocean color vary in several conditions: clear water, water with very small phytoplankton, water with large/varied size phytoplankton, and blooms.
In addition to backscattering and absorption, other factors can also influence ocean color, especially in bloom conditions. By rolling your cursor over "Bloom conditions" (above) you'll see that blooms of small cells have low red backscatter; from this you'd expect that so-called "Red Tides" cannot be produced by small cells. However, red reflection CAN be produced when small cells concentrate near the surface to form an optically shallow ocean layer. Under these conditions, the shallow layer of phytoplankton prevents light from penetrating deep enough for water to absorb all the red photons. Roll your cursor over "Clear water" (above): note that the strong red absorption applies to the "deep blue sea." (Divers know that blood appears black at depth and red near the surface.)
It has been wrongly assumed by many (including scientists) that the colors associated with algal blooms result from the pigments of algae. However, algae do not contain unique enough pigment types to result in the wide variety of colors associated with HABs. In some cases it may not be the algae that are responsible for the color: rather, the water may be discolored by non-pigmented organic material produced during the bloom. Many scientists are developing "Inverse models" hoping to quantify the ties among ocean color, absorption and scattering, algae pigmentation and cell size, and depth of bloom layers.