Changes in specific photosynthetic rate of oceanic phytoplankton from the northeast Pacific Ocean

The study is based on data (n=244) from light-saturation experiments utilizing artificial incubation under fluorescent light. Values of maximum photosynthetic rate,P _max, and the light intensity at which it takes place,I _max, are estimated by non-linear regression using stepwise Gauss-Newton iterations. Estimated values ofP _max ranged from 0.85 to 5.48 mg C (mg Chla·h)^?1;I _max varied from 2.35 to 5.52 cal (cm²·h)^?1. The effects of time (months) and depth (illumination levels) and their interaction are evaluated by analysis of covariance using a linear model. A significant time-depth interaction is noted: The maximum specific primary productivity occurred in the surface layers during March, at the 50% light level during April, and at 1% level during May. Estimates ofP _max from simulated in situ primary productivity experiments for the same period are lower than those from light-saturation experiments. A comparison of data from light-saturation and simulated in situ experiments indicated that effects of duration of experiments and the quality of available light may affect primary productivity data considerably.