Photosynthetic activity of natural microphytobenthos populations measured by fluorescence (PAM) and 14C-tracer methods:: a comparison

PAM (pulse-amplitude-modulated) fluorescence measurements of motile microphytobenthic algae were carried out in June 1996 at Sylt, Germany. Comparisons between C-14-based and fluorescence-based production rates were made. A very high correlation between C-14- and fluorescence-based production rates was found for maximal production rates (P-max values). C-14-based maximal production rates varied during the study period between 0.65 and 1.7 mg C mg chl a(-1) h(-1), comparable to variations of P-max measured with the fluorescence-based method. For other photosynthetic parameters [alpha (maximum light utilization coefficient), E-k (light Saturation index), E-max (Light intensity at which P-max is reached)], differences between the 2 methods were much larger. Highest carbon quantum yields (phi(ass)) (mol C mol quanta(-1) absorbed) were obtained at low irradiances. Considering the whole range of investigated carbon quantum yields, we found that initially these values decreased at low to moderate irradiances without a concomitant decline of the actual photochemical efficiency (F-m' - F)/F-m' (F and F-m': minimal and maximal fluorescence signals in the Light). Therefore, a high linearity between the actual photochemical efficiency and the carbon quantum yield could only be observed up to values of 0.018 mol C mol quanta(-1). This is different to higher plants, for which Linearity can be observed up to carbon quantum yields of 0.042 mol C mol quanta 1. It was shown that, for the calculation of the overall production rates based on the fluorescence method, it is necessary to carefully measure the mean specific absorption coefficient (a*) of the algae. Unless this is achieved, PAM measurements cannot be used to calculate absolute production rates.