The spectral absorption coefficients of phytoplankton in oceanic waters were previously shown to vary with chlorophyll a concentration according to nonlinear relationships with a great deal of noise. We analyzed this biological noise on a data set of 596 simultaneous absorption and high-pressure liquid chromatography (HPLC) pigment measurements acquired within the surface layer (first optical depth) from various regions of the world's oceans. We observed systematic deviations from the average relationships for some oceanic areas and also seasonally within given areas. Using the detailed HPLC measurements, the influences of pigment composition and package effect (the two main sources of variability in algal absorption for a given chlorophyll a concentration) were explicitly separated for each sample. It was found that while the pigment composition experiences large variations, even within a restricted chlorophyll range, it is often not (at least within the first optical depth) the dominant source of the biological noise. Instead, these deviations mostly result from variability in the pigment packaging effect (for a given chlorophyll a concentration) due to variations in algal community size structure. This conclusion is fully confirmed by an independent approach, which consists of estimating a size index of algal populations from the relative concentrations of taxonomic pigments.