Importance of mixing, thermal stratification and light adaptation for phytoplankton productivity in Lake Tahoe (California-Nevada) [Physiological response of algal populations]

Lake Tahoe is highly transparent (Secchi readings 20-42 metres) and never freezes over. Deep mixing throughout its 500-metre water column occurred in March 1974 and 1975. Between June and October 1974, a distinct epilimnion of 15- to 30-metre thickness existed. Phytoplankton, which is dominated by diatoms, showed no significant vertical shifts in species composition. Chlorophyl @a concentrations were highest at the bottom of the trophogenic zone (75-100m) during thermal stratification. Maximum photosynthetic rates were observed at greater depths during winter mixing of the lake than during thermal stratification. In situ experiments, in which incubations flasks containing phytoplankton from 3 discrete depths were distributed over the trophogenic zone, were conducted to study light adaptation. Results indicated that seasonal variation in the vertical distribution of photosynthesis was controlled to a greater extent by algal adaptation to prevailing subaquatic light conditions than by variations in solar irradiance. During deep mixing of the lake, algae adapted to average prevailing low light intensities. Thermal stratification, however, caused populations to remain at certain depths sufficiently long to adapt to ambient light intensities. Thus, phytoplankton at shallow depths was light adapted, whereas deep populations remained dark adapted. During thermal stratification, light inhibition was minimized and dim light in deep water was utilized efficiently. Thus, at a defined light-saturated photosynthetic rate, integral photosynthesis in general was enhanced during thermal stratification relative to levels attained during lake mixing.