Tidal and diel periodicities of size-fractionated phytoplankton pigment signatures at an offshore station in the southeastern English Channel

The influence of the tidal cycle on the vertical distribution of phytoplankton was investigated in neap-tide conditions during a 36 h sampling period in the southeastern English Channel. Phytoplankton distribution was investigated through algal pigment signatures analyzed by HPLC, in three size fractions: 0.7–5 lm, 5–20 lm and 20 lm. In spite of the physical homogeneity of the water column, vertical and temporal variabilities were observed in the size-class distribution and phytoplankton-community composition (revealed by biomarker pigments). Microplankton biomass (chl.a, 20 lm) dominated the community, accounting for up to 60% of the total chl.a. Tidal current cycles were responsible for periodical horizontal advection of biomass in the surface layer with every low tide moving from the coastal waters to the offshore. Tidal currents were responsible for cyclical vertical advection of phytoplankton through the water column, such as for the resuspension of benthic and/or tychoplanktonic diatoms from the bottom. Time-dependent biological variability was also related to the diel scale, with vertical migration of phytoflagellate cells (especially Cryptophytes). Over the daily cycle, it appeared that physiological changes (through measurements of in vivo chl.a fluorescence) occurred in the sub-surface layer, revealing photoinhibition under high light.