Chlorophyll a, nutrients and salinity distributions were studied at two spatial scales (10 cm and 0.25 to 2.5 km) in the St. Lawrence Estuary (Quebec, Canada), in order to investigate the role of tidal currents and winds in the formation and maintenance of spatial structures. Data were collected according to a synoptic sampling pattern using three sampling platforms simultaneously, and they were analyzed using analysis of variance. The sampling pattern was repeated on four occasions during July 1980.Analyses of variance indicated significant spatial heterogeneities of about the same magnitude at the two scales studied for chlorophyll and nutrients, whereas salinity showed only large–scale variability. At the kilometer scale, the frequency distribution spectra of patch length for chlorophyll showed the existence of patches of various dimensions between 0.2 and 6.0 km with a dominance of small patches (≤0.5 km). Frequency maxima were usually observed at the smaller (≤0.5 km) and larger (≥2.0 km) scales for the nutrients and only at larger (≥2.0 km) scale for salinity. The distribution spectra of patch dimensions were characteristic for each sampling experiment, depending on tidal currents and prevailing wind conditions. Estimated patch dimensions were larger parallel to the current direction than perpendicular to current direction, implying that spatial structures are elongated in the sense of the current direction. Higher winds have, first, a tendency to increase the small–scale structure of the environment by breaking up larger patches into smaller patches, before structures are completely eliminated. The implication of these findings is that different results could be obtained depending on the sampling strategy used (sampling either at anchor stations or at random, independent of current direction), which could lead to different conclusions.