 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 bywhere $K_d$ 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 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.^[ref]