Relationships between spectral optical properties and optically active substances in a clear oligotrophic lake

The absorption and scattering coefficients in the euphotic zone of oligotrophic Lake Taupo, New Zealand, were measured at 19 stations across the 620 km(2) lake in late fall during the period of mixed layer deepening and development of the annual phytoplankton maximum. These coefficients were subsequently related to the water content of colored dissolved organic matter ( CDOM), phytoplankton, and nonalgal particles via measurements of the absorption spectra of these optically active substances and of chlorophyll a and suspended particle concentrations. Measurements of the spectral diffuse attenuation coefficient for downwelling irradiance (K-d) and of reflectance (L-u/E-d) revealed that the clear blue waters of Lake Taupo had a minimum K-d of 0.09 m(-1) at 500 nm and maximum reflectance at 490 nm. The measured K-d and L-u/E-d were well described by modeled spectra that were computed using a radiative transfer model (Hydrolight) assuming relatively low values for the backscattering ratio (0.008-0.014). The relationships established here between the optical properties and optically active substances were consistent with previous observations in case 2 marine waters, and they will provide a basis for prediction of eutrophication, climate, and other environmental effects on the blueness and transparency of large oligotrophic lakes.