Continuous measurement of the DCMU-induced fluorescence response of natural phytoplankton populations

The effect of 3x10^-6 M DCMU 3-(3,4-dichlorophenyl)-1, 1-dimethylurea] on in vivo chlorophyll a fluorescence was observed in nearshore waters of the Southern California Bight. We compared fluorescence readings in the presence and absence of this inhibitor using parallel flow-through fluorometers. The increase in fluorescence induced by DCMU is expressed as the FRI (fluorescence response index). Theory and laboratory studies on batch cultures of phytoplankton suggest that the FRI is correlated with photosynthetic efficiency and/or physiological state, but other studies have produced results in apparent conflict with this interpretation. Although sufficient information does not exist to justify the use of fluorescence response as a precise physiological indicator in the field, we suggest that very low FRI values are a manifestation of photosynthetic debility in a sample. Vertical profiles showed a wide range of the fluorescence response index. At a station close to shore, low FRI values were observed well below the 1% light level, but the fluorescence response of the phytoplankton throughout the euphotic zone was similar to that of growing cultures. Farther offshore, the FRI was depressed near the surface, but increased in the enhanced nutrient conditions of the lower euphotic zone. The patterns observed were strong, and consistent with hypotheses which relate low values of the FRI to diminished photosynthetic capacity.