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Abstract
This paper presents the results of a field study on the short-term variability in photosynthetic activity of inter-tidal microphytobenthos using non-destructive techniques in situ. Photosynthetic rate (measured using oxygen microelectrodes), productive biomass and photosynthetic efficiency (dark-fluorescence and effective quantum efficiency of PSII, respectively, measured by PAM fluorometry) were measured in situ during a low-tide exposure period on an intertidal mudflat in the Tagus Estuary, Portugal. Community photosynthesis displayed large temporal variation. Maximum activity (24.3 mmol O2 m-2 h-1) was found at noon while minimum activity (8.7 mmol O2 m-2 h-1) was found at the end of the exposure period. Productive biomass followed a similar pattern. During the course of the exposure period, productive biomass peaked halfway through, and decreased by 80% towards the end of the exposure period. These changes in productive biomass were followed by significant changes in the species composition/cell size as well as changes in photosynthetic efficiency of the microphytobenthic community in the surface sediment. The obtained results therefore suggest that microphytobenthic community-level photosynthesis is controlled by migratory movements of microalgae and by variations in photosynthetic efficiency. The latter could be induced by changes in the species composition/cell size in the photic zone due to microalgal migrations. It is suggested that the migration pattern and, hence, photosynthesis is linked to changes in salinity, temperature and light, changes that are difficult to simulate accurately in the laboratory. Respiration rates obtained during the day were much higher than those obtained at night, and also displayed a marked hour-to-hour variation. Depth integrated respiration within the photosynthesis zone (0.33 mm) represented 80 to 90% of the respiration in the entire oxic zone, suggesting a tight coupling between photosynthesis and respiration.