DIURNAL MEASUREMENTS OF F-V F-M USED TO IMPROVE PRODUCTIVITY ESTIMATES IN MACROALGAE/

The ratio of variable fluorescence to maximal fluorescence (F-v/F-m) w as measured during a night and day cycle in five different macroalgae growing in the littoral zone at the Swedish west coast; the green alga e Ulva lactuca, Cladophora sp. and Enteromorpha flexousa, the red alga Ceramium nodulosum and the brown alga Fucus vesiculosus. All the gree n algae and C. nodulosum showed both diurnal fluctuations in F-v/F-m d uring days of high light intensities and a significant negative correl ation between F-v/F-m and photon irradiance (PI). An attempt has been made to improve calculations of macroalgal net growth based on carbon fixation (alpha(net)) considering this diurnal change in F-v/F-m. By a ssuming that the negative linear correlation between F-v/F-m and the m aximum quantum yield for photosynthesis (Phi) is proportional to that between F-v/F-m and alpha(net) it was possible to include the daily va riation of alpha(net) due to photoinhibition. To compensate for the va riation of F-v/F-m, a representative value for each day was obtained b y weighting all values of F-v/F-m over the diurnal period in relation to total PI. For all algal species there was a fairly good agreement b etween this representative value and the F-v/F-m value measured around noon. As the daily representative F-v/F-m value showed a negative lin ear correlation with the daily mean PI, it was possible to correct alp ha(net) for differences of daily mean PI.