An alternative approach to absorption measurements of aquatic particles retained on filters

We made modifications to the procedure used for absorption measurements of aquatic particles retained on glass‐fiber filters that extend the procedure's application range to “case 2” waters with high suspended sediment content and allow the determination of phytoplankton pigment absorption in situations where the standard solvent extraction method is not effective. The first result was achieved by combining light‐transmission and light‐reflection measurements (the latter made possible by the use of a commercially available integrating‐sphere attachment for the dual‐beam spectrophotometer) so as to remove the spurious contribution to the measured absorption caused by sample backscattering. The second result was obtained by bleaching the sample with a NaClO solution‐a method that proved satisfactory even with water‐soluble pigments (e.g. the important class of the phycobilins) and solvent‐resistant Chlorophyceae. This process also allows for depigmentation of the particle suspension, and thus it was used to evaluate the empirical expression for converting the filter‐retained sample absorbance to the equivalent particle suspension value. Both features of the modified procedure have been positively tested through measurements carried out on solvent‐resistant phytoplankton species and on samples of inorganic suspended sediment.