MODELING SPECTRAL IRRADIANCE IN FRESH-WATER 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 a nd C-IV of lake Banyoles during a two-year period. Epilimnetic light a ttenuation coefficients at 350, 450, 550, 650 and 750 nm were estimate d from a physical radiative model and related to chlorophyll concentra tions. Two light attenuation models have been tested in order to obtai n the best agreement between extrapolated and measured values. The res olution of the exponential model was found to be better than the arcta ngent model at low light intensities. Chlorophyll a concentrations up to 6 mu g . 1(-1) as well as diversity of phytoplankton found in the c hecked waterbodies allow to screen variability of optical characterist ics for oligo-mesotrophic waters. Light absorption by phytoplankton ha s been found to modify spectral composition significantly, shading spe ctral absorption ranges of photosynthetic sulphur bacteria. A linear r elationship between light attenuation coefficients and chlorophyll con centrations has been found, with the best correlations at the central region of light spectrum. Dispersion at the extreme wavelengths was at tributed 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 tho se 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.