In situ light measurements were used to obtain information on inherent and apparent optical properties. The average vertical attenuation coefficient Kd(ave) varied from 1.1 to 4.6 In units m–1 During three periods the variation in Kd(ave) correlated with changes in chlorophyll a concentration and specific attenuation coefficients Ks, of 0.013, 0.014 and 0.022 m2 mg Chl a–1 were calculated. Chlorophyll-specific diffuse absorption coefficients (A,) for these periods were 0.012. 0.013 and 0.017 m2 mg Chl a–1 and only varied significantly from estimates of Ks in the period when scattering was intense. Absorption coefficients a(zmid) and scattering coefficients b(zmid) calculated for the mid-point of the euphotic zone ranged between 0.45 and 2.9 m and 3.5–52.0 m respectively. Chlorophyll-specific absorption coefficients Ka, of 0.005, 0.006 and 0.007 m2 mg Chl a–1 and scattering coefficients Kb of 0.05. 0.09 and 0.191 m2 mg Chl a–1 were measured during the three periods. The high Kb value occurred when gas-vacuolate cyanobactena were dominant. Algal photosynthesis and light absorption were related through the maximum quantum yield m which varied between 0.019 and 0.11 mol C Einstein–1 while average quantum yields a, varied between 0.006 and 0.024 with a mean of 0.013 mol C Einstein–1 A comparison of changes in the mean irradiance of the mixed zone and chlorophyll concentration indicated that growth was light limited below 0.04–0.05 Einsteins absorbed mg Chl a–1 day–1.