MEPS 166:53-62 (1998)  -  doi:10.3354/meps166053

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

Peter Hartig*, Kirsten Wolfstein**, Sebastian Lippemeier, Franciscus Colijn

Research and Technology Centre Westcoast, University of Kiel, D-25761 Büsum, Germany
**Present address: Sustainable Forest Management Network, G-208 Biological Science Building, University of Alberta, Edmonton, Alberta T6G 2E9, Canada

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 (Pmax 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 Pmax measured with the fluorescence-based method. For other photosynthetic parameters [α (maximum light utilization coefficient), Ek (light saturation index), Emax (light intensity at which Pmax is reached)], differences between the 2 methods were much larger. Highest carbon quantum yields (φass) (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.

Photosynthetic activity · Fluorescence · PAM · Primary production · Microphytobenthos · German Wadden Sea

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