In vivo absorption spectra, pigment concentrations from algal cultures, and field samples (Po Delta, Northern Italy, are analyzed to differentiate the contribution by each optically active constituent to the total absorption by the water. The results yeld systematic trends in the blue-red and blue-NIR absorption ratio for living phytoplankton cells and detrital material, which are then used to retrieve the absorption coefficient of dissolved organic material (yellow substance) at 410 nm from the total in vivo absorption coefficients of natural waters at 410 nm, 670 nm, and 730 nm.JRC.(SAI)-Space Application Institut