The distribution of photosynthetic pigment concentrations, as an index of microalgal biomass, was studied on shallow sandy bottoms in the Gulf of Fos near Marseille, France. Analyses of grid and transect samples of surface sediments gave evidence of several patchiness scales, which can be related to the size scales of sedimentary structures: (1) on a large scale (some km), stations located on exposed shores exhibit more heterogeneous distributions than those in sheltered areas; (2) on a medium scale, sand-waves (h = ca 10 m) induce accumulation of plant pigments in the depressions; (3) on a small scale, ripple marks (h = 3 to 10 cm) may leave a pigmentary record even after they fade out. The influence of vertical distribution within sediments and of seasonal effects on these horizontal features is analysed. Finally, a sampling strategy is discussed which is based on the maximum dispersal of samples within any previously defined strata. Def~nition of the strata depends upon the aim of the study.

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