Microphytobenthos plays an important role as primary producer in shallow embayments subject to a tidal regime. The present paper aims to characterize the spatial patterns of sedimentary chlorophyll a (chl a) biomass in 2 seasons in the wide intertidal mudflats of the Marennes-Oleron Bay (France); furthermore, it provides estimates of total microphytobenthic biomass. Accordingly, a systematic sampling scheme was performed in June 1995 and in January 1996. On average, biomass was higher in June (ca 120 mg chl a m-2) than in January (ca 85 mg chl a m-2). Moreover, the geostatistical analysis revealed that spatial similarities were propagated over larger distances in June (6 km) than in January (2.2 km). In order to account for this result , we suggest that seasonal dynamics of benthic microalgae biomass obeys a 'constant-density model' that describes the alternating spreading /shrinkage phenomenon around the few persistent patches of maximum biomass (ca 300 mg chl a m-2). Total biomass estimates were ca 13 t chl a (SE = 0.26) in June and 10 t chl a (SE = 0.28) in January. Calculating a raw approximation of the growth rate (0.1 d-l) of that biomass and considering that the total produced biomass is resuspended at flood tide, we found that microphytobenthos supplies about 2 µg chl a 1-', which corresponds to 30-90% of the chl a suspended in the water column, depending on the season. This suggests that microphytobenthos could be a major food source for secondary producers in both pelagic and benthic ecosystems of the Marennes-Oleron Bay.