Diel variations in the photosynthetic parameters of coastal marine phytoplankton

The diel variations in the assimilation number, PmB, of natural assemblages of coastal marine phytoplankton were examined and a working model proposed to characterize the apparent diel rhythm. No rhythm could be distinguished for the initial slope of the light saturation curve, α, although α and PmB were highly correlated and the daily and hourly variations in α were as great as those in PmB. Diel variations in the derived parameter Ik(≡PmB/α) were described by a model similar to that proposed for PmB. The rhythm PmB was found to have a significant effect on the estimation of daily productivity. Correlations with environmental covariates support the hypothesis that the photosynthetic rhythms observed in the field result both from intrinsic oscillations within the phytoplankton cells and from varying environmental factors.