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The effect of high (ambient) and low (shaded to 50% of ambient) light on microphytobenthic biofilm primary production, vertical migration and allocation of photoassimilated carbon into extracellular carbohydrates was investigated over the low tide emersion period at Sarilhos Pequenos on the Tagus estuary, Portugal, in July 2000. Carbon uptake ( 14 C measurements), electron transport (pulse-amplitude modulated [PAM]-fluorescence) and carbon allocation into extracellular carbohydrate fractions were measured. There was a decrease in the maximum rate of primary production (P max ) ( 14 C) over the emersion period (16 decreasing to 5 mg C m -2 h -1 at midday and the end of emersion, respectively) that was independent of the incident light and the light history to which the biofilms had been exposed. Primary production ( 14 C) did not correlate with measurements of in situ relative electron transport rate (rel.ETR), due in part to the light-induced migration of the cells away from the sediment surface during periods of high irradiance (photosynthetic photon flux density [PPFD] > 1200 mol m -2 s -1 ). Downward migration coincided with the light level at which P max ( 14 C) was reached, 750 mol m -2 s -1 , and with a point of inflexion on the rel.ETR versus PPFD curve. Above this light level rel.ETR was over-estimated due to migration of the cells into a lower light field in the sediment. Shading significantly increased the extracellular polymeric substance (EPS) production rate (from 0.4 to 1.0 mg C m -2 h -1 in ambient and shaded light, respectively) and percentage allocation of photoassimilates into EPS (from 4 1.0 to 18 3.5% of total production in ambient and shaded light, respectively). In contrast, percentage allocation of photoassimilates to low molecular weight extracellular carbohydrate was higher in the non-shaded treatments (35 4.5% compared with 13 3.8% in ambient and shaded treatments, respectively), probably due to photosynthetic 'overflow'. In non-shaded biofilms there was little or no increase in the concentration of all extracellular carbohydrate fractions until the last hour before tidal immersion. Results indicate that diel patterns in microphytobenthic primary production are not predictable solely from light data.