Re: Using a colorimeter in lakes/ponds?

Different components in a water body, including water molecules 
themselves, have differing spectra of absorption. For example, in the 
visible, water molecules absorb progressively more towards longer 
wavelengths (red) as well as towards UV; water has a minimal absorption 
in the blue wavelengths, which partially explains why ocean is blue. 
Microphytoplankton, containing chlorophyll, absorb both blue and red 
light, which also explains why leaves are green. Therefore the presence 
of phytoplankton (if in abundance) will shift the color toward green. 
Dissolved organic matter (mostly from decomposed dead tissues) absorb 
light in a pattern that increases exponentially towards shorter 
wavelengths (blue and UV). Therefore if the color of water is brownish, 
then there is a good indication of high concentration of dissolved 
organic matter.