Modelling the dynamics of the microalgal biomass in semi-enclosed shallow-water ecosystems

The main objective was to formulate a deterministic model, which simulates the dynamics of the microalgal biomass in semi-enclosed littoral ecosystems, in order to test the hypothesis that resuspension of the microphytobenthos can occur without any concomitant sediment erosion. Benthic diatoms form a dense biofilm at the mud surface during daytime emersion periods. Cells in the first centimetre of the mud migrate to the surface to get a sufficient amount of light energy for photosynthesis and production. Cell renewal in the biofilm was taken into account, with an average residence time equal to 1h. When the “migration period” (i.e. the total amount of time spent by microalgae at the surface of the mud) exceeds the duration of the emersion period, the biofilm is resuspended at the beginning of the flooding tide. A critical biomass above which the biofilm is resuspended was calculated. The dynamics can reach equilibrium in the vicinity of this critical threshold, as long as the balance between production and losses is in favour of production. Even with a low initial biomass, below the maximum concentration in the biofilm, the convergence to equilibrium remained possible. Despite of the deterministic nature of the model, steady-state solutions are sensitive to initial conditions and occurrence of resuspension can only be predicted in terms of probability. This biofilm resuspension without any concomitant sediment erosion was observed in the Marennes-Oléron Bay (French Atlantic Coast), in conditions defined by the model. A good agreement between simulations and observations is a first significant step in the quantification of the overall microalgal productivity and in the estimate of the export of the biomass from the littoral zone. This study will permit to precise the role of the microphytobenthos in the global carbon cycle in semi-enclosed littoral ecosystems.