PHYTOPLANKTON PHOTOADAPTATION RELATED TO SOME FRONTAL PHYSICAL PROCESSES

Using phytoplankton pigments as biomarkers, we studied light adaptatio n of phytoplankton communities in a geostrophic front and the surround ing area by (i) onboard kinetic experiments and (ii) detailed analyses of the vertical profiles of a selected photoadaptative index. Kinetic experiments, in which populations were subjected to a sudden light sh ift (from high to low irradiances and vice versa), clearly demonstrate d that frontal phytoplankton populations, especially diatoms (fucoxant hin) are less affected by light shifts than the populations of the adj acent oligotrophic zones. Kinetic experiments also showed that the rat io diadinoxanthin/Chl. a, positively correlated with light intensity, is a useful index to characterize and identify certain time scales of photoadaptation in microalgae. The rate of change of this index was es timated around 0.5 h-1, with significantly higher values for the front al communities compared to the adjacent area. The rate of variation of this ratio, deduced from in situ measurements, is however one order o f magnitude lower than the rate obtained from kinetic experiments. The se two approaches allowed us (i) to compare different populations and (ii) to define an upper limit of the adaptation rates. Typical in situ profiles of the diadinoxanthin/Chl. a ratio showed obvious day-night variations. Relative anomalies in these profiles were interpretated in terms of ''light histories'' of phytoplankton communities and compare d with potential causal physical motions (diffusion or advection).