Photosynthetic Active Radiation (PAR) was measured using single planar and two-bulb spherical light sensors. The attenuation coefficient (Kd) was found to vary significantly during the year. The highest Kd values were obtained in the station with higher influence of currents and run-off. Our data suggested a reflection of 50% of light that reaches the bottom, which is associated with a decrease in the Kd value obtained with the spherical sensor of 0.15 m-1. This means that flat sensors may underestimate PAR and that spherical sensor may underestimate Kd. This is a critical issue given that knowledge on light attenuation is essential for modeling approaches and quality assessments.