LAKE OPTICS AND DEPTH LIMITS FOR PHOTOGENESIS AND PHOTOSYNTHESIS IN CHAROPHYTE MEADOWS

Two series of lakes with increasing attenuation were examined for trends in spectral composition. They became the basis for an evaluation of the light environment at the lower boundary (LB) of Nitella meadows in three other series of lakes. Increased attenuation (K(d) PAR) was marked by progressive erosion of the 'blue window' and caused primarily by humic substances. An increase in K(d) PAR from 0.06 to 0.81 produced, at the floor of the euphotic zone, a shift in K(d)min from 440 to 580 nm. Regressions of boundary depths of Nitella meadows on water clarity produced similar slope coefficients for the three series of lakes. Several trends became evident: 1, PAR irradiance at the LB increases with depth of the LB; 2, red light (E(d) 660) declines from richness at shallow LB to near extinction in deep water LB in clear lakes; while 3, blue light (E(d) 450) increases to an asymptote. Blue light appears to substitute, although less effectively, for red light irradiance in the growth regulation of charophytes. These data support an hypothesis that spectral quality is involved in the determination of lower boundary depths for benthic macro-algae.