MEPS 169:43-54 (1998)  -  doi:10.3354/meps169043

ABSTRACT: In theoretical and field primary production studies, much interest is currently focused on the influence of aperiodic vertical mixing generated at the surface by wind speed and/or heat flux. In the present work, a Lagrangian random walk model was used to study the interactions between periodic vertical tidal mixing and both photoadaptation and primary production of phytoplankton, in typical shallow coastal waters, such as the eastern English Channel. The model considers a depth-dependent diffusion coefficient fluctuating according to the high-low tidal cycles and neap-spring tidal cycles, water columns of different euphotic zone and mixed layer depths, and photoresponse time constants of natural phytoplankton populations collected in the eastern English Channel. Cells were allowed to light-shade adapt, according to the vertical mixing time scales, by altering their photosynthetic properties in response to variations in light. The simulation results indicate first that vertical tidal mixing could control photoadaptation processes at the scale of the high-low tidal cycles at spring tide, and at the scale of neap-spring tidal cycles in shallow coastal systems. Secondly, it appears that the decreasing vertical mixing intensity between spring and neap tide conditions is responsible for a significant increase in daily primary production rates, despite the occurrence of photoinhibition at neap tide. Therefore, primary production in coastal seas would be a function not only of light and nutrient concentrations, but also of photoadaptation processes in relation with vertical tidal mixing. In another way, the Lagrangian model suggests that the theory according to which cells are adapted to the mean light intensity of a water column in a turbulent regime is valid only from a populational point of view. From the model used, it appears also that our present knowledge on photosynthetic dynamic modeling is unsuited to generating pronounced vertical gradients of photosynthetic parameters in all water columns.

KEY WORDS: Models · Phytoplankton · Photosynthetic parameters · Photoadaptation kinetic · Primary production · Vertical tidal mixing · Coastal waters · Eastern English Channel