PHOTOINHIBITION: ALGAL RESPONSES TO BRIGHT LIGHT DURING DIEL STRATIFICATION AND MIXING IN A TROPICAL ALPINE LAKE1

Abstract

ABSTRACTNear surface thermoclines form each day in the limnetic waters of Lake Titicaca (Peru‐Bolivia) and thereby retain phytoplankton under extreme irradiances. This bright light exposure results in strongly depressed chlorophyll fluorescence and photosynthesis which both decay (bright light) and recover (dim light) by first order rate kinetics. During each afternoon the phytoplankton are redistributed by wind‐induced mixing, and full recovery is accomplished soon after nightfall. In vivo fluorescence was measured over this diet cycle both with (Fb) and without (Fa 3‐(3,4‐dichlorophenyl)‐1,1‐dimethyl urea. Strongest bright light effects were on the parameter (Fa– Fa), a crude measure of operational photosystem II reaction centers (RC IIs). On dates of strong thermocline development, surface (FFa– Fa) was reduced to 5% or less of that for the mixed layer maximum. Fluorescence depression was greater in the lake than in Pyrex bottles incubated at fixed depths for 4 h. Ultraviolet light intensified the photoinhibitory response, but strong (Fb– Fa) depression could be induced by photosynthetically available radiation alone. In Lake Titicaca, photoinhibition apparently operates by reversible in‐activation of RC IIs. It occurs in the natural water column and is not simply an artifact affixed bottle incubations.