Cyanobacterial blooms can affect a wide range of aquatic organisms due to the presence of toxic compounds. However, no study so far has shown the effects of natural blooms samples on the physiological parameters related to the ecology of Daphnia. In this study we used a natural bloom sample obtained from a reservoir in Colombia to evaluate its effects on five parameters related to Daphnia's feeding behavior, swimming movements and physiology: second antennae movement (swimming), mandible movement (feeding), thoracic appendages (feeding), postabdomen movement (rejection of food particles) and heart rate (physiology). The results revealed significant changes in all parameters evaluated at two different concentrations of aqueous extracts of the bloom: second antennae movements increased significantly and there were significant reductions in mandibular movements, thoracic movements and heart rate. Although postabdominal movements showed high variability with no distinctive pattern between control and treatments, the reduction in the other parameters (such as heart rate over time) could possibly reflect an intoxication by microcystins or a behavioral response (e.g., feeding inhibition).