Numerical modelling of vertical suspended solids concentrations and irradiance in a turbid shallow system (Vaccares,Se France)

In shallow ecosystems, the short temporal variability of available underwater irradiance is considered a major process controlling submerged macrophytes development. Mechanistic models that estimate photosynthetically available radiation (PAR) in shallow ecosystems at very short time scales are needed for use in predicting submerged macrophyte growth and persistence. We coupled a 2D horizontal circulation model with, first, a 1D vertical numerical model of suspended solid (SS) re-suspension, diffusion and settling, and next, with a model of vertical extinction of irradiance, previously validated at the same site. The study site was the Vaccarès lagoon (France) where a large data set of high frequency bottom irradiance and SS concentration were available. SS and irradiance measurements were conducted at a vertical study station, monitored over a 6 month period (from December 1995 to May 1996) characterized by wide-ranging wind velocities (1.5–18 ms⁻¹). In addition, grain-size analyses conducted over the whole lagoon, allowed adaptation of the 1D numerical model to the silt-sized (7 μm) and clay-sized (0.3 μm) fractions that prevail in the local sediment. First, model results showed that about 60% of the variance in bottom irradiance time series can be explained by our deterministic formulations, thus representing the same level of efficiency than those already obtained by a stochastic model previously developed with the same data set. Second, model results showed that the fit of the model to the field data (SS concentrations and bottom irradiance) depended mainly on storm occurrence and season (winter or spring). Finally, model results suggested that the underwater irradiance regime was controlled by seasonal succession of the horizontal circulation of turbid water in the lagoon, with increased solids concentrations in winter, followed by submerged canopy development and decreased solids concentrations in spring.