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A pulse amplitude modulated fluorometer (PAM) was used to investigate photosynthetic activity of microphytobenthos on an intertidal mudflat. Spectral irradiance measurements indicate that 75% of the signal detectable by the PAM originates in the upper 150 mu m of the sediment. From the photosynthetic electron transport rate (ETR) measurements, it was concluded that the PAM could be used to observe changes in photosynthetic parameters during the day or the season. Photoacclimation to lower irradiance was indicated by changes in the maximum ETR and the saturating photon irradiance parameter I-k. When cores were exposed to a high photon irradiance for several hours, vertical migration could be followed using reflectance spectra. The data also showed that the benthic algae did not seem to experience photoinhibition or CO2 limitation. To explain this, it is hypothesised that there is a continuous vertical migration in the top layer of the sediment, where algae can avoid photoinhibition due to prolonged periods of high irradiance and lack of CO2 by migrating downwards while others migrate upwards. [KEYWORDS: microphytobenthos; chlorophyll fluorescence photosynthesis; vertical migration; C-limitation Estuary sw netherlands; photosystem-ii; marine-phytoplankton; light; photoinhibition; sediments; stabilization; respiration;communities; limitations]

Determination of microphytobenthos PSII quantum efficiency and photosynthetic activity by means of variable chlorophyll fluorescence