The light absorption properties of a dinoflagellate (Noctiluca scintillans Macartney) bloom in Manila Bay were analysed during the onset of a whole-bay-scale bloom in March 2004. The chlorophyll a concentrations varied over a very wide range from 1.4 μg l−1 to extremely high values of 521 μg l−1. The chlorophyll specific absorption coefficients of phytoplankton (a ph()) varied significantly in shape and magnitude. The spectrally averaged values of a ph() varied by two orders of magnitude