| Abstract: | In a series of 188 experiments on the, light-saturation curve for natural assemblages of phytoplankton at 3 stations in Nova Scotia coastal waters, it was found that both the initial slope (α) of the curve and the assimilation number (PmB) varied about 5-fold throughout the year. No differences could be detected between stations, but both α and PmB decreased with depth. The mean value of α for all the experiments was 0.21 mg C[mg Chl a]?1· h?1· W?1· m2 with a range from 0.03 to 0.63. An explanation is offered for the nonconstancy of a in terms of the effect of cell-size and shape on self-shading. An estimate is made from first principles of the physiological maximum-attainable value of α. This estimate corresponds, within the limits of experimental error, to the highest values of α observed in the study. It is deduced that on the average the phytoplankton were photosynthesizing at only 44% maximum capacity. The mean value of PmB for all experiments was 4.9 mg C[mg Chl a]?1· h?1, with a range from 0.73 to 24.8. In the matrix of partial correlation coefficients, α and PmB were positively correlated with each other; α was correlated with mean solar radiation averaged over the 3 days prior to the experiment, but uncorrelated with temperature; PmB was correlated strongly with temperature but uncorrelated with recent solar radiation. The results show that PmB could be estimated from α and temperature using an empirical multiple regression equation, independent of depth. It is suggested that α and PmB are both correlated with some other factor not measured in the study, perhaps the mean cell-size of the populations, or the nutrient status of the cells. The predictability of primary production is discussed in the light of this evidence. |
