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 chi a m(-2)) than in January (ca 85 mg chi 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 or 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 chi a m-2). Total biomass estimates were ca 13 t chl a (SE = 0.26) in June and 10 t chi a (SE = 0.28) in January. Calculating a raw approximation of the growth rate (0.1 d(-1)) of that biomass and considering that the total produced biomass is resuspended at flood tide, we found that microphytobenthos supplies about 2 mu g chi a l(-1), which corresponds to 30-90 % of the chi 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.