The spectral attenuation of solar irradiation was measured during summer in two types of coastal waters in southern Chile, a north Patagonian fjord (Seno Reloncavi) and open coast (Valdivia). In order to relate the light availability with the light requirements of upper subtidal seaweeds, the saturating irradiance for photosynthesis (E-k) from P-I curves was measured. In addition the UV risk was assessed. Based on the z(1%) of PAR, the lower limit of the euphotic zone in the studied systems averaged 21 m (K-d 0.24 m(-1)) in Seno Reloncavi and 18 m (K-d 0.27 m(-1)) in the coast of Valdivia. Photosynthesis of the studied seaweeds was saturated at markedly lower irradiances than found in their natural depths at the time of the study. Solar radiation penetrating into these depths at both locations largely supports the light requirements for the photosynthesis of subtidal species: 50-160 mu mol m(-2) s(-1) for seaweeds from Seno Reloncavi (7 m tidal range) and 20-115 mu mol m(-2) s(-1) for Valdivia assemblages (2 m tidal range). Optimal light conditions to saturate photosynthesis (E-k) were present at 10-16 m water depth. The attenuation of solar irradiation did not vary significantly between the fjord and coastal sites of this study. However, the underwater light climates to which seaweeds are exposed in these sites vary significantly because of the stronger influence of tidal range affecting the fjord system as compared with the open coastal site. The patterns of UV-B penetration in these water bodies suggest that seaweeds living in upper littoral zones such as the intertidal and shallow subtidal (< 3 m) may be at risk. (C) 2010 Elsevier Ltd. All rights reserved.