Which wavelength to use to determine algae using a colorimeter?
From Biotek I learned the following:


  Spectral scans of algae cultures demonstrate significant absorbance below 400 nm as well as two distinct absorbance peaks at 440 nm and 675 nm (Figure 1). Cell number determination is most consistent when light scatter is used rather than absorbance by cellular constituents. In order to avoid influence from absorbing material 600 nm [is best] to monitor growth.



Figure 1. Absorbance spectral curve of Microcystis aeruginosa suspension cultures.Source: Biotek