MEPS 192:13-30 (2000)  -  doi:10.3354/meps192013

ABSTRACT: A simulation model based on the short-term variability of in situ irradiance and productive biomass, defined as the fraction of total biomass contributing to measurable production, was developed to describe the primary productivity of intertidal microphytobenthos in the Tagus estuary, Portugal. Hourly 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 rhythms of motile microalgae. The hourly variability in productive biomass was quantified by non-destructively measuring the dark-level chlorophyll a fluorescence, F_o, emitted from undisturbed microphytobenthos samples. F_o was found to be a good predictor of community-level photosynthesis versus irradiance curve parameters α (initial slope) and P_m (maximum photosynthesis) under the 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_o and irradiance. By modelling the hourly and fortnightly variability in F_o, the model was used to calculate hourly production throughout the annual cycle, which resulted in an annual pattern characterised by fortnightly oscillations in daily production comparable in amplitude with those occurring at the seasonal time scale. A comparison of the variability in production on hourly (intraday), fortnightly (within spring-neap tidal cycles) and seasonal (month-to-month) 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 model showed that the occurrence of migratory rhythms may 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 m^-2 yr^-1. By considering the variation of production with tidal height, the model 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^-1.

KEY WORDS: Microphytobenthos · Primary production · Modelling · Migratory rhythms · Chlorophyll a fluorescence