Photoadaptation and primary production study in tidally mixed coastal waters using a Lagrangian model

In theoretical and field pnrnary productlon studies, much interest is currently focused on the influence of apenodic vertical mixing generated at the surface by wlnd speed and/or heat flux.In the present work, a Lagrangian random walk model was used to study the interactions between penodic vertical tidal mixing and both photoadaptation and primary productlon of phytoplankton, in typical shallow coastal waters, such as the eastern English Channel.The model considers a depthdependent d~ffusion coefficient fluctuahng 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 tn light-shade adzpt, accordizg to the vertical mixing tune scdles, by aitenng their photosynthetic properties in response to vanations 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 tlde, and at the scale of neap-spring tidal cycles in shallow coastal systems.Secondly, it appears that the decreasing vertical mixing intensity between spnng and neap tide condihons 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 b e 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 intenslty of a water column in a turbulent regime is valid only from a populational point of view.From the model used, ~t appears also that our present knowledge on photosynthetic dynamic modeling is unsuited to generating pronounced vertical grad~ents of photosynthetic parameters In all water columns.