Relationship between the maximum quantum yield of carbon fixation and the minimum quantum yield of chlorophyll a in vivo fluorescence in the Gulf of St. Lawrence

Abstract

Intensive monitoring of the maximum quantum yield of carbon fixation (&,,,) and of the minimum quantum yield of chlorophyll a in vivo fluorescence ($& was followed along the path of drifting buoys in the estuary and the Gulf of St. Lawrence, Canada. Samples were collected every third hour during two 24-h periods. Over the 170 measurements, &,,X varied by a factor of 20 [from 0.004 to 0.08 mol C (mol quanta))‘]. As predicted by a model of primary processes in photosystem 2, a significant and positive linear relationship was observed between $,,, and q5I;,. In the estuary, the covariability of &,,,, and $t;, seems to be driven by light, because it was observed only during daytime and was characterized by a parallel increase of &,,, and $r;, with depth. Examination of the spectral values of $FO suggests a contribution of photoprotectant carotenoids. In the gulf, because a parallel, although weaker, increase of &,, and rbr;, with depth also persisted during nighttime, nutrient deficiency is probably an additional cause of covariation between (6,,, and cbr;,. In both regions, an overall diel cycle was observed for $,,X with a maximum at around noon; $I;, did not show a clear diel periodicity. Despite the vertical covariability of &,,X and 4F0, the diel cycle of &,,X was uncoupled to $FO and thus to photosystem 2 activity. These results suggest a significant diel variation in the photosynthetic quotient of natural phytoplankton.