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

A simulation model based on the short-term variability of in situ irradianc e and productive biomass, defined as the fraction of total biomass contribu ting to measurable production, was developed to describe the primary produc tivity of intertidal microphytobenthos in the Tagus estuary, Portugal. Hour ly primary production was modelled based on the assumption that short-term variations in the community-level photosynthetic Light response were caused by changes in productive biomass associated with the vertical migratory rh ythms of motile microalgae. The hourly variability in productive biomass wa s quantified by non-destructively measuring the dark-level chlorophyll a fl uorescence, F-o, emitted from undisturbed microphytobenthos samples. F-o wa s found to be a good predictor of community-level photosynthesis versus irr adiance curve parameters cc (initial slope) and P-m (maximum photosynthesis ) under the range of conditions found in situ, which allowed for the estima tion of hourly production rates from hourly time series of in situ observat ions of F-o and irradiance. By modelling the hourly and fortnightly variabi lity in F-o, the model was used to calculate hourly production throughout t he annual cycle, which resulted in an annual pattern characterised by fortn ightly oscillations in daily production comparable in amplitude with those occurring at the seasonal time scale. A comparison of the variability in pr oduction on hourly (intraday), fortnightly (within spring-neap tidal cycles ) and seasonal (month-to-month) time scales confirmed that microphytobenthi c production is dominated by variability on sub-seasonal (hourly and fortni ghtly) time scales. Significant variability in hourly primary production ra tes was detected on hourly and fortnightly time scales but not among season s. A sensitivity analysis made on the model showed that the occurrence of m igratory rhythms may be expected to increase the variability in primary pro duction both on hourly and on fortnightly time scales, and also to contribu te to the high levels of primary productivity of intertidal microphytobenth os. The mean annual areal primary production was found to reach 156 g C m(- 2) yr(-1). By considering the variation of production with tidal height, th e model was used to calculate the annual production of the entire intertida l. area of the Tagus estuary, which resulted in a total of 4265 t C yr(-1).