= Solution
The <photic zone> is the illuminated layer supporting <photosynthesis>; its <euphotic depth> is conventionally associated with about 1% of surface <photosynthetically active radiation>, integrated over approximately $400$–$700\ \mathrm{nm}$. This is a useful convention rather than a universal organism-specific compensation depth. Spectral filtering can be represented locally by
$$
E_d(z,\lambda)=E_d(0,\lambda)\exp\!\left[-\int_0^zK_d(\zeta,\lambda)\,d\zeta\right],
$$
where $K_d$ is the wavelength-dependent <diffuse attenuation coefficient>. It includes effects of absorption and scattering and is not simply equal to the absorption coefficient $a$.
In clear, low-chlorophyll water, pure <water> removes red light strongly, while weak blue pigment and CDOM absorption permits blue light, typically around $450$–$490\ \mathrm{nm}$, to penetrate furthest. The deepest usable light is consequently dim and blue-rich, and the <euphotic zone> can extend relatively deep. At high pigment concentrations, <phytoplankton pigments> preferentially remove blue and red light; associated detritus or CDOM can remove still more blue. The remaining transmission window commonly shifts toward blue-green or green wavelengths, roughly $500$–$570\ \mathrm{nm}$, and the total light falls off faster, so the <euphotic depth> is shallower. Exact window and depth depend on the full particle/dissolved mixture, not chlorophyll alone. The diagram normalizes the two deep spectra separately to show their different shapes, not equal absolute light at a common depth.
This confines photosynthetic growth to a thinner illuminated interval in high-chlorophyll water and allows a deeper potential habitat in clear low-chlorophyll water, subject also to nutrient supply, mixing and the organism's compensation requirement. Cells near the base benefit from low-light acclimation and pigment systems that capture the \b[remaining] wavelengths. In blue-rich deep ocean water, blue-absorbing pigments and some low-light-adapted picoplankton can be favored; where the surviving light is greener, groups with effective green-absorbing <accessory pigments>, such as certain <phycobilins>, gain an advantage. A pigment appears coloured by light it fails to absorb, so green-absorbing accessory pigments need not themselves look green. There is no universal deep species determined by chlorophyll concentration alone. A deep chlorophyll maximum can partly reflect increased pigment per cell rather than a proportional maximum in carbon <biomass>.<Phytopia's light and color by NASA>[Photic-zone discussion: the 1% convention, deep penetration in clear blue waters, and shallower illuminated habitat in productive turbid waters; pigment-based community selection here follows the surviving wavelengths and is conditional.]{ref}
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