Lake Cadagno is a meromictic lake of 21-m depth in the central Swiss Alps at 1,923 m asl. Its mixolimnion reaches from the surface down to 10-m depth. A dense population of phototrophic bacteria dominated by Chromatium okenii is located in the subsequent 2-m layer containing up to 200 mg bacteriochlorophyll (Bchl) m-7.Maximum cell concentration was found at 10.8-m depth during the summer season. The light intensity at the upper edge of the bacterial layer averages 4.7% of the subsurface radiation (at 0.05 m depth) and at the density peak of the layer it averages 0.4%. Variations over the summer season in the photosynthetic properties of the bacterial population at the depth of highest cell density have been observed in in situ 14C-CO, incorporation experiments. These changes are random, and no photoadaptation effect was found. Thus, the population of phototrophic bacteria was physiologically uniform during the investigation period and it had the following photosynthetic properties (medians for the 25th and 75th percentiles are in parentheses): P,,,, 0.154 mg C (mg Bchl)-I h-l (0.048, 0.174); Ik, 0.016 mol quanta m-2 h-l (0.012, 0.019); (Y, 7.7 mg C (mg Bchl) I h-l (mol quanta mP2 h-l)-' (4.8, 11.6). The specific dark production rate was 0.016 mg C (mg Bchl)-' h-l (0.014, 0.023). The average quantum yield for CO, assimilation at the depth of maximum cell density, (b,, was low at 0.012 (0.007, 0.020). The cells were sensitive to light intensities higher than the optimum found at -0.036 mol quanta mm2 h-I. Maximum inhibition by excessive light was determined to be 80% at light intensities >0.108 mol quanta m-2 h-l (=30 pmol quanta m-2 s-l) when cells were exposed for periods of 2.5 h or longer. The bacterial biomass increment assuming no losses, p*, was calculated to be 0.05 d-l (0.03, 0.08) and the loss rate, A, to be 0.03 d-l (-0.10, 0.06).
