The present study was carried out at laguna de Rocha (Rocha, Uruguay) from March 1996 to May 1999. This polymictic lagoon exhibits large hydrological dynamics and presents from limnic conditions at the tributaries influence zone to brackish ones at the area influenced by the marine intrusion. As a consequence of the periodic communciation between the lagoon and the Atlanctic Ocean, well-defined horizontal gradients can be observed on the optic and chemical features. Also, the extreme shallowness and the wind pattern favour the resuspensión of bottom sediments, which introduce a large variability on a short-term basis (day).

In this study the following hypothesis were tested: a) the hydrological regime determines the attenuation of the UV-A (320-400 nm) and UV-B (290-320 nm) solar radiation; b) the UV radiation exerts a negative effect on the photosynthetic capacity of the phytoplankton; c) the most severe effects are associated to marine intrusion events, which exhibits low concentration of dissolved organic substances and inoculates a microalgal biota not adapted to high UV radiation fluences.

The penetration of the UV-B radiation into the water column was mainly controlled by the dissolved organic carbon concentration (DOC) and the organic suspended material, while the UV-A penetration depended on the DOC and the chlorophyll a content. The attenuation was higher at the freshwater end of the system than at the brackish area, as a consequence of the mixing of continental and marine waters with different content of humic subtances. The penetration was maximum at the Southern zone during marine intrusion events, when the depth where 10% of the surface UV radition accounted for 66 and 100% of the water column for the UV-B and UV-A range, respectively.

The mean inhibition of the photosynthetic capacity of the phytoplankton was ca. 30% (maximum ~55%). The inhibition levels exherted by the UV-A and UV-radiation were close to the relation 2:1. The negative effect of the UV radiation on the phytoplankton photosynthetic rate ocurred under maximum penetration of the radiation into the water column, which was observed in summer during marine intrusion events. The inhibition was 2-fold lower at the limnic area as a consequence of a higher attenuation of the UV radiation. No intracellular substances absorbing in the UV range were detected in the phytoplankton, suggesting that other mechanisms as the vertical mixing are relevant to mitigate the effects of the UV radiation. These results suggest that the ocurrence of drought periods will reduce the input of substances that attenuate the UV radiation, thus incrementing the negative effects on the microalgal communities of these coastal systems.