Fine scale vertical displacement of Phaeodactylum tricornutum (Bacillariophyceae) in stratified waters: influence of halocline and day length on buoyancy control.
Diatom blooms are frequently seen in low-turbulent stratified coastal waters. The present work aims at demonstrating the ability of marine diatoms to regulate their vertical position in a strongly stratified water column, with special emphasis on the fine scale displacements within and around the halocline. Here we give results from such experiments in a specially designed artificial water column. The experiments were conducted on the pennate diatom Phaeodactylum tricornutum. Surface irradiance was 100 µmol quanta m-2 s-1, halocline strength was 28-34 psu, and temperature around 20°C. Our results show that once introduced into a water layer P. tricornutum are able to remain within that water layer for 6-17 d, or even ascend/descend into new water layers by exerting active buoyancy control. We conclude that this was mainly due to cell density regulation and not to size. The presence of a strong halocline has a great impact on the vertical movement of P. tricornutum as revealed by a high degree of accumulation of cells within or just below it for descending and ascending cells. Cells needed about 4 d to pass through the halocline and into the upper layer. We have also seen that the physiological ability to exert buoyancy control varies among cells of the same species. The importance of day length for buoyancy control was demonstrated by the fact that the high gain of photosynthetic energy obtained during a 14 h light period cannot compensate for the low gain of respiratory energy during 10 h of darkness. Therefore cells being inoculated at the bottom only managed to ascend to the upper layer (above the halocline) and cells being inoculated in the upper layer to remain above the halocline during subsequent diel cycles when living in a 24:0 LD regime.