Seasonal changes in the spectral composition of downwelling irradiance in Lake Kinneret (Israel)1,2

A submersible scanning radiometer was used to measure seasonal variations in the intensity and spectral characteristics of downwelling irradiance in Lake Kinneret, Israel. Extinction coefficients for photosynthetically active radiation (350–750 nm) varied from 0.2 to 3.3 ln units·m −1 , the euphotic zone (1% of light energy at surface) ranged from 1.7 to 11 m. Specific extinction coefficients of chlorophyll, k c (ln units mg −1 ·n −2 ), and water k w (ln units·m −1 ) were dependent on the wavelength at which they were measured (at 650 nm, k c = 0.011, k w = 0.693; for 350 to 750 nm, k c = 0.007, k w = 0.489). High concentrations of chlorophyll and other cell pigments during the annual Peridinium bloom strongly attenuated light penetrance and caused a spectral shift toward the red to give elevated quantum to joule (Q:W) ratios (2.96 × 1018)