Modelling Upwelling Irradiance using Secchi disk depth in lake ecosystems

A simple model for upwelling irradiance has been developed. The model represents the relationship between Photosynthetically Active Radiation diffuse attenuation coefficients and Secchi disk depth described with a physical-mathematical expression. This physical mathematical expression allows the evaluation of the sub surface upwelling irradiance that was generated by the interaction between downwelling irradiance and the water column. The validation of the relation was performed using experimental data collected from five different aquatic ecosystems at different latitudes, solar elevations and irradiance levels. We found a good linear, positive correlation between the theoretical and measured up,,welling irradiance (R-2 = 0.96). The residues were well distributed, around the null value, according a Gaussian curve (R-2 = 0.92). The results confirm the importance and the versatility of the Secchi disk measurements for aquatic optics.