PHYTOPLANKTON PHOTOADAPTATION RELATED TO SOME FRONTAL PHYSICAL PROCESSES

Using phytoplankton pigments as biomarkers, we studied light adaptation of phytoplankton communities in a geostrophic front and the surrounding 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 shift (from high to low irradiances and vice versa), clearly demonstrated that frontal phytoplankton populations, especially diatoms (fucoxanthin) are less affected by light shifts than the populations of the adjacent oligotrophic zones. Kinetic experiments also showed that the ratio 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 estimated around 0.5 h-1, with significantly higher values for the frontal communities compared to the adjacent area. The rate of variation of this ratio, deduced from in situ measurements, is however one order of magnitude lower than the rate obtained from kinetic experiments. These 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 compared with potential causal physical motions (diffusion or advection).