Field Studies of DCMU-Enhanced Fluorescence as an Index of in situ Phytoplankton Photosynthetic Activity

Abstract

Comparison between the increase in fluorescence caused by DCMU (FD/FN: ratio of DCMU-enhanced to normal fluorescence) and the specific photosynthetic rate (P/B: photosynthetic activity per unit chlorophyll a) in the St. Lawrence Estuary showed that no overall correlation could be established between P/B and FD/FN, although there were significant correlations between these two variables on short transects. This reflected perhaps environmental and species composition changes. FD/FN vertical profiles in the Gulf of St. Lawrence and the Gulf of Guinea showed a characteristic increase with depth, as seen in laboratory cultures with a decrease in light intensity. This could be related to photosynthetic efficiency which is known to increase at lower light intensities. Technical problems involving the use of FD/FN as an estimator of P/B are also discussed. Further investigations are required before this fluorescence technique can be used regularly in field work to estimate photosynthetic activity.Key words: DCMU, fluorescence, photosynthesis, phytoplankton, St. Lawrence Estuary, Gulf of Guinea