Short time scale changes in underwater irradiance in a wind-exposed lagoon (Vaccares lagoon, France): efficiency of infrequent field measurements of water turbidity or weather data to predict irradiance in the water column

High frequency water sampling in the wind-exposed Vaccares lagoon revealed frequent and rapid changes in suspended solid (SS) concentrations in the water column. SS concentrations, sometimes higher than 800 mg l(-1), were significantly correlated with antecedent wind conditions. Mean wind velocity during the 5-33 h before water sampling or maximal wind velocity during the previous 8.5-22 h were good predictors of SS concentrations in the water column. Underwater irradiance at canopy level was modeled (r(2) = 0.66, n = 7584) using the SS calculated from the relationship between SS and antecedent mean wind velocity and the surface irradiance data measured at the weather station close to the study site. On the other hand, we have shown that in this wind-exposed lagoon, mean underwater irradiance can not be effectively estimated using infrequent measurements of the optical properties of water.