Modelling spectral irradiance in freshwater in relation to phytoplankton and solar radiation

Spectral irradiance and phytoplankton chlorophyll concentrations were measured both at the surface and at different depths of basins C-III and C-IV of lake Banyoles during a two-year period. Epilimnetic light attenuation coefficients at 350, 450, 550, 650 and 750 nm were estimated from a physical radiative model and related to chlorophyll concentrations. Two light attenuation models have been tested in order to obtain the best agreement between extrapolated and measured values. The resolution of the exponential model was found to be better than the arctangent model at low light intensities. phyll a concentrations up to 6 μg·l−1 as well as diversity of phytoplankton found in the checked waterbodies allow to screen variability of optical characteristics for oligo-mesotrophic waters. Light absorption by phytoplankton has been found to modify spectral composition significantly, shading spectral absorption ranges of photosynthetic sulphur bacteria. ar relationship between light attenuation coefficients and chlorophyll concentrations has been found, with the best correlations at the central region of light spectrum. Dispersion at the extreme wavelengths was attributed to the absorption by elements other than phytoplankton. Since light climate (intensity and spectral distribution of irradiance) is a main factor affecting photosynthetic populations, and especially those thriving at the higher depths as photosynthetic sulphur bacteria do, this model has a special interest for microbial ecologists in order to determine the effect of light spectral absorption by phytoplankton on available light for photosynthetic sulphur bacteria.