This study was sponsored by Empresas Públicas deMedellin (EPM, Colombia), grant 29990832845; NH thanksto EPM for a fellowship, besides to Drs. Wayne Carmichael,Linda Lawton and Sandra Azevedo for their contributionsduring this research.oftoxic compounds. However, no study so far has shown the effects of natural blooms sam-ples on the physiological parameters related to the ecology of Daphnia. In this study weused a natural bloom sample obtained from a reservoir in Colombia to evaluate its effectson five parameters related to Daphnia’s feeding behavior, swimming movements and phys-iology: second antennae movement (swimming), mandible movement (feeding), thoracicappendages (feeding), postabdomen movement (rejection of food particles) and heart rate(physiology). The results revealed significant changes in all parameters evaluated at twodifferent concentrations of aqueous extracts of the bloom: second antennae movementsincreased significantly and there were significant reductions in mandibular movements,thoracic movements and heart rate. Although postabdominal movements showed highvariability with no distinctive pattern between control and treatments, the reduction inthe other parameters (such as heart rate over time) could possibly reflect an intoxicationby microcystins or a behavioral response (e.g., feeding inhibition).