Diel variation in microphytobenthic productivity in areas of different tidal amplitude

In situ diel patterns of productivity, total daily productivity (C-14-uptak e) and microphytobenthic biomass were measured at 3 sites from different ti dal regimes: a shallow subtidal site with negligible tidal fluctuations (Sw eden), a shallow subtidal site exposed to distinct salinity and turbidity v ariations (Italy) and an intertidal site in a macrotidal estuary (Portugal) . Measurements were taken at regular intervals (5 to 7) throughout the peri od that the sediment was Light-exposed and replicated over 3 d. The pattern of diel productivity varied between the sites. The subtidal Swedish site s howed the dearest pattern, closely following daily irradiance, while produc tivity at the Italian subtidal site was influenced by varying freshwater in put affecting the turbidity. Daily productivity at the intertidal site was controlled mainly by the tidal exposure patterns, but changes in rates of p roductivity also occurred across the emersion period. This variation could not be explained purely by irradiance, suggesting the influence of further factors, e.g. nutrient Limitation. Despite different sediment type, tidal a mplitude and species composition, the biomass and total daily primary produ ctivity at the sites was of the same order of magnitude, suggesting a simil ar carrying capacity. The main difference between the sites was in the degr ee of intra-site variation, being greatest for the intertidal site. The rel ationship between in situ productivity (measured across the diel cycle) and irradiance varied between sites. No relationship could be determined for t he highly motile intertidal microphytobenthic community, while a statistica lly significant relationship between in situ microphytobenthic productivity and irradiance was found at the 2 subtidal sites.