ABSTRACT: PAM (pulse-amplitude-modulated) fluorescence measurements of motile microphytobenthic algae were carried out in June 1996 at Sylt, Germany. Comparisons between ¹⁴C-based and fluorescence-based production rates were made. A very high correlation between ¹⁴C- and fluorescence-based production rates was found for maximal production rates (P_max values). ¹⁴C-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 [a (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 (f_{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 (Fm' - F)/Fm' (F and Fm': 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.}

KEY WORDS: Photosynthetic activity · Fluorescence · PAM · Primary production · Microphytobenthos · German Wadden Sea

Published in MEPS Vol. 166 (1998) on May 28
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1998