The photosynthetic performance was analyzed in common macroalgae on site and under seminatural conditions in Patagonia, Argentina, using a portable PAM instrument. Solar radiation was monitored in parallel with a filter dosimeter, ELDONET (Real Time Computer, Möhrendorf, Germany), in three wavelength ranges, UV-A, UV-B and PAR. Fluence rate-response curves indicated that the species is a typical shade plant which developed profound non-photochemical quenching at intermediate and higher irradiances. Fast induction and relaxation kinetics indicated a fast adaptation of the photosynthetic apparatus to the changing light conditions by fast adjustment of the photochemical and non-photochemical quenching. Upon exposure to solar radiation the algae responded with photoinhibition but recovered in the subsequent shade conditions within several hours. When the thalli were allowed to free float in the water photoinhibition was less profound and recovery was faster than when the algae were confined in a fixed position. While most of the photoinhibition was due to visible radiation, the UV wavelength range, and especially UV-B, considerably enhanced the decline in photosynthetic quantum yield.
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