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Long-term effects of full-spectrum solar radiation, solar radiation without UVB, and solar radiation without total UV (UVA + UVB) radiation were studied in intertidal Ulva rigida C. Agardh (Chlorophyta). The experiment was carried out under natural conditions and at prevailing winter levels of UV radiation. Changes in relative growth rate, photosynthesis and pigment content were studied after 7 and 20 d of cultivation under the three solar radiation conditions. Relative growth rate was enhanced (50 %) in the absence of UVB radiation after one week, but no difference was found after 20 d. Pigment content (chlorophyll a, chlorophyll b and carotenoids) was significantly higher in the presence of UVB, suggesting an efficient protective-pigment mechanism. When UVB was removed, photosynthetic performance measured as oxygen exchange and fluorescence parameters exhibited higher values than under the other …