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A simulation rnodel based on the short-term variability of in situ irradiance and productive biomass, defined as the fraction of total biomass contributlng to measurable production, was developed to descnbe the pnmary productivity of intertidal microphytobenthos in the Tagus estuary, Portugal. Hourly prirnary production was rnodelled based on the assumption that short-term vanations in the community-Ievel photosynthetic light response were caused by changes in productive biomass associated with the vertical rnigratory rhythms of motile microalgae The hourly variability in productive biomass was quantified by non-destructively measunng the dark-level chlorophyll a fluorescence, F,. ernitted from undisturbed microphytobenthos samples. F,, was found to be a good predictor of community-level photosynthcsis versus irradiance curve Parameters a (initial slope) and P, (maximum photosynthesis) under th.e range of conditions found in situ, which allowed for the estimation of hourly production rates from hourly time series of in situ observations of F, and irradiance. By modelling the hourly and fortnightly variability in F„ the rnodel was used to calculate hourly production throughout the annual cyclc, which resulted in an annual Pattern characterised by fortn.ightly oscillations in daily production coriiparable in amplitude with those occurnng at the seasonal time scale. A compari.son of the variability in production on hourly (intraday). fortnightly (within spring-neap tidal cycles) and seasonal (month-to-monthJ time scales confirmed that microphytobenthic production is dominated by variability on sub-seasonal (hourly and fortnightly) time scales. Significant variability in hourly primary production rates was detected on hourly and fortnightly time scales but not among seasons. A sensitivity analysis made on the rnodel showed that the occurrence of migratory rhythms rnay be expected to increase the variability in primary production both on hourly and on fortnightly time scales, and also to contribute to the high levels of primary productivity of intertidal microphytobenthos. The mean annual areal primary production was found to reach 156 g C rn-' yr-' By considenng the variation of production with tidal height. the rnodel was used to calculate the annual production of the entire intertidal area of the Tagus estuary, which resulted in a total of 4265 t C yr"

Modelling the primary productivity of intertidal microphytobenthos:time scales of variability and effects of migratory rhythms