Nowadays high metal loads are still found in watersheds and overall in mining areas. At present there are not many specific biomarkers to detect metal pollution at acute or chronic exposure. Antioxidant enzyme activities (AEA) like SOD, CAT, APX, GR and GST, can be used as early warning systemsbecause they have a functional response and it is expected that functional changes will be detected before than the structural ones used in former investigations. Moreover, these enzyme activities might also be biomarkers of adaptation, since their activation is expected to contribute to metal detoxification under chronic exposure. The main goal of this presentation is to assess the use of AEA as biomarkers of metal pollution under real exposure scenarios: Riou Mort and Osor stream by means of translocation experiments. These translocation studies were carried out in two streams with heavy metal inputs due to natural and anthropogenic sources. The Riou Mort (SW France), and the Osor stream (NE Spain). The experimental design of the Riou Mort (from 0 to 24h) allowed us to assess in situthe effects of short-term exposure (by comparing translocated biofilms) and chronic exposure (by comparing the non-translocated ones) on the AEA responses. And in the Osor stream, the translocation lasted for 5 weeks allowing as to assess in situthe effects of a gradient of metal concentration after short-term (from 0 to 24h) and chronic exposure (up to 5 weeks) on the AEA responses. Metal exposure affected the fluvial biofilm causing transitory physiological responses (short exposure) and structural and functional alterations (chronic exposure). According to this, AEA showed a different response depending on the time of exposure and also depending on water metal concentrations.