Growth and diel vertical migration patterns of the toxic dinoflagellate Protoceratium reticulatum in a water column with salinity stratification: the role of bioconvection and light

Abstract

Novel data on growth and migratory behaviour of a southern Norwegian strain of Protoceratium reticulatum (Dinophyceae) were obtained from long-lasting (14-22 d) experiments in a water column with salinity gradient (ΔS = 4 [30-34 psu] and 14 [20-34 psu]), light intensity of 100 µmol photons m-2 s-1 and light:dark cycle of 14:10 h. Based on specific growth experiments (maximum growth rate: 0.3 divisions d-1), these environmental conditions were considered near optimal. Upward migration at an average speed of 0.6 m h-1 (maximum 4.0 m h-1) was controlled by positive phototaxis. Ascending populations managed to pass the halocline after an osmotic adjustment time of 4 and 8 d in the case of ΔS = 4 and 14, respectively, and once this was achieved the vertical migration cycle was restricted to the upper layer. Descent from the surface started once the dark period was initiated. Very slow sinking speeds (0.03-0.08 m h-1) during darkness resulted in a random distribution of algal cells within the upper layer, indicating a balance between negative and positive geotaxis. After about 1 wk, bioconvection (i.e. pattern-forming motions caused by hydrodynamic instabilities in suspensions of swimming microalgae) became an important part of the migration pattern during the day. Dense surface patches of cells were formed after 1-2 h in the light, followed by fast-sinking (5 m h-1) plumes/droplets extending down to the halocline. This motion pattern lasted for the rest of the light period, being repeated for several light cycles. Despite nitrate limitation in the upper layer, P. reticulatum did not migrate down below the halocline where nutrients were sufficient.