A primary production simulation model was used to assess the effect of a brown tide (Chrysophytes) on benthic microalgal photosynthesis. This model is based on the assumption that photosynthesis of microphytobenthos is primarily determined by irradiance at the sediment-water interface and by the photophysiological response of microalgae to changes of this irradiance. So, irradiance recordings at the sediment-water interface were used as the forcing variable. The simulation indicates that before the introduction of the brown tide, primary production of microphytobenthos was physically controlled. Light levels, hence primary production rates, were very variable (<1-132 mg C m(-2) d(-1), CV = 80%) because wind-induced resuspension generates turbidity within the water column. During the chrysophyte bloom, the mean production rate of microphytobenthos dramatically decreased by two orders of magnitude (0.25-1.31 mg C m(-2) d(-1), CV = 46%) due to shading by the planktonic compartment. Simulations further indicate that the indirect effects of this light reduction (decrease of P-I parameters and biomass) had a higher impact on microphytobenthic production rates than its direct effect (reduction of light energy at the surface of the sediment). As a result, the collapse of microphytobenthic productivity could partially explain the observed decrease of macrofaunal abundance in Baffin Bay, since microphytobenthos is an important food source for benthic invertebrates.