Three-Dimensional Variation in Light Quality in the Upper Water Column Revealed With a Single Parameter

For the first time the vertical variation in light quality in the global ocean is quantified with a single parameter—the hue angle (
$\alpha _{E}$
, in degree) in chromaticity of downwelling irradiance. For oceanic waters,
$\alpha _{E}$
is ~140° at surface, but it becomes ~230° at the bottom of the euphotic zone;
$\alpha _{E}$
changes rapidly near the surface, and we term this layer of rapid change in light quality as chromocline, analogous to the thermocline or pycnocline in oceanography. The 3-D variations in light quality are further highlighted with data from satellite ocean color measurements, where global distributions of
$\alpha _{E}$
for depths of 99%, 37%, and 1% of surface photosynthetically available radiation (PAR) are presented. As an example to demonstrate the importance to consider the change in light quality, the relationship between the light quality and the ratio of phytoplankton absorbed light to PAR is presented where this ratio may vary by a factor of 3 or more under different chlorophyll-a concentrations; otherwise, the ratio would be constant vertically. We advocate quantitative measurement and report the light quality in the upper ocean with such a single and objective parameter to accompany the routine measurement and report the light intensity, which will greatly improve our understanding of light-related processes and further bridge ocean optics and oceanography.