Pirjo Huovinen

This study reports 5 years of (1998–2003) data on continuous solar‐irradiation measurements from a scanning spectroradiometer (SUV‐100) in Valdivia, Chile (39° S), accompanied by evaluation of the impact of ultraviolet radiation (UVR) on marine macroalgae of this site. UVR conditions showed a strong seasonal variation, which was less pronounced toward longer wavelengths. Daily maximum dose rates (clear days) averaged in winter‐summer: UV‐B_{(290–315 nm}) 0.30–2.1, UV‐B_{(290–320 nm)} 0.703.7, UV‐A_{(315–400 nm)} 20.6–62.1, UV‐A_{(320–400 nm)} 20.2–60.5 W m^‐2, and photosynthetically active radiation (PAR) 969–2423 μmol m^‐2 s^‐1. The corresponding daily doses (all the days) ranged: UV‐B_{(290–315nm)} 2.6–40.7, UV‐B_{(290–320 nm)} 6.7–78.5, UV‐A_{(315–400 nm)} 228–1539, UV‐A_{(320–400nm)} 224–1501, and PAR 2008–13308 kJ m^‐2 d^‐1. Taking into consideration action spectra of a biological interest, the risk of UV exposure could be up to 37 times higher in summer than in winter. The photosynthetic activity (as maximum quantum yield of chlorophyll fluorescence, F_v/F_m) of the brown alga Lessonia nigrescens from the infralittoral zone was markedly more sensitive to UVR than of the green alga Enteromorpha intestinalis from the upper midlittoral, and the UV‐B wave band increased markedly photoinhibition. In L. nigrescens, maximal photoinhibition (40%) took place at weighted (the action spectrum for photoinhibition of photosynthesis) UVR doses of 800 kJ m^‐2, irrespective of the season (corresponding midsummer daily dose in Valdivia is 480 kJ m^‐2). In winter, when this alga was at its most sensitive, the weighted W dose causing 35–40% photoinhibition was around 200 kJ m^‐2. In E. intestinalis, weighted doses of 800 kJ m^‐2 resulted in low photoinhibition (<10%) and no clear seasonal patterns could be inferred. These results confirm that midday summer levels of UV‐B and their daily doses in southern Chile are high enough to produce stress to intertidal macroalgae.