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 (ϕmax) and of the minimum quantum yield of chlorophyll a in vivo fluorescence (ϕFo) 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, ϕmax varied by a factor of 20 [from 0.004 to 0.08 mol C (mol quanta)− 1]. As predicted by a model of primary processes in photosystem 2, a significant and positive linear relationship was observed between ϕmax and ϕFo. In the estuary, the covariability of ϕmax and ϕFo seems to be driven by light, because it was observed only during daytime and was characterized by a parallel increase of ϕmax and ϕFo with depth. Examination of the spectral values of ϕFo suggests a contribution of photoprotectant carotenoids. In the gulf, because a parallel, although weaker, increase of ϕmax and ϕFo with depth also persisted during nighttime, nutrient deficiency is probably an additional cause of covariation between ϕmax and ϕFo. In both regions, an overall diel cycle was observed for ϕmax with a maximum at around noon; ϕFo did not show a clear diel periodicity. Despite the vertical covariability of ϕmax and ϕFo, the diel cycle of ϕmax was uncoupled to ϕFo and thus to photosystem 2 activity. These results suggest a significant diel variation in the photosynthetic quotient of natural phytoplankton.