|MadSci Network: General Biology|
Water absorbs light, but it absorbs different wavelengths of light to different degrees. Oceans and deep lakes look blue because blue light is absorbed the least so it penetrates water the furthest. Red light (and infrared energy or heat) is absorbed strongly. So a red object more than a few meters deep in water is going to look black because it absorbs the other wavelengths, but there is no red light to reflect.
So you seem to have the right idea about this. Adding a pigment that absorbs a particular color will change the spectrum of light penetrating water. Green dye will absorb other colors but let green light pass will rob an aquatic plant of the blue light its chlorophyll could absorb. If the aquatic plant was deep enough it would look black for the same reason the red object above looks black, the chlorophyll will absorb blue wavelengths but no green light is around to be reflected or transmitted. Altering the wavelengths of light will definitely affect photosynthesis to the degree that the different wavelengths absorbed by chlorophyll are present.
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