make life on Earth possible. The coincidence of the solar spectrum to the window of
water clarity allowed selective tuning of the light-absorbing pigments energizing
photosynthesis by phytoplankton to the blue-dominated spectrum of light available at
even moderate depths. The only light reaching depths below 100 m or so peaks very
narrowly in the blue, so visual pigments of deep-sea fish and invertebrates (shrimp,
squid) are adapted for generation of visual nerve impulses by absorption of those
specific wavelengths.
Fig. 1.5 (a) Light absorbance coefficients, k, of pure water (solid line) and seawater
(dotted line) as a function of wavelength, showing a window of clarity around the
visible band. (Data compiled from various sources.) (b) Detail of absorption (k) and
scattering (b) spectra of seawater in the visible band plus near UV and near IR (data
of Smith & Baker 1981). (c) Spectrum of radiative solar energy arriving at the
surfaces of the atmosphere and ocean, with the differences labeled according to the
principal absorbing gases in the atmosphere accounting for the difference.
(^) (Repeatedly published without attribution. Reproduced here from Falkowski & Raven 2007.)
The ocean layer that is sufficiently illuminated to support positive net
photosynthesis, meaning more organic-matter generation than phytoplankton will
respire themselves, is often considered to extend down to about the level receiving 1%
of mid-day irradiance, and is termed the euphotic zone. It is not fully dark below that
depth, and in clear tropical waters net photosynthesis may extend somewhat deeper,
reaching to 120 m or so. In waters of a natural ecosystem, much additional absorbance
comes from dispersed cells containing pigments, shoaling the euphotic zone depth.