DCMU-enhanced fluorescence as an index of photosynthetic activity in phytoplankton

Tests have been carried out, both on phytoplankton cultures and in the field, on the relation between the ratio of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU)-enhanced to normal fluorescence (F _D /F _N ) and the specific photosynthetic rate (P/B). In all cases observed, significant linear correlations were found between these two values. Differences in this relation were observed according to species and physiological conditions. Nutrient stress that occurs in batch cultures decreases both F _D /F _N and P/B, while lowered light intensity has a different effect on both, increasing F _D /F _N and decreasing P/B. This is interpreted as indicating that F _D /F _N is an index of photosynthetic efficiency at different light levels and an indicator of the specific photosynthetic rate (P/B) when physiological conditions vary at a given light intensity. The practical use of DCMU-enhanced fluorometry to estimate instantaneous P/B values in the field is discussed, stressing the frequent calibrations needed, as in all in situ fluorometry studies.