The aim of this study was to use digestive enzyme activities in Gammarus fossarum as biomarkers during active biomonitoring. Standardised gammarids were transplanted for 7 days to five sites in the Riou Mort watershed contaminated by polymetallic pollution. This experiment was conducted on seven different dates from February 2009 to June 2010. Feeding rates were tracked, along with amylase, cellulase and trypsin activities. We found that feeding rate and digestive capacity were reduced in the most polluted site, "Joany," in comparison with the reference site "Up.Lot". The results suggested that trypsin was more sensitive than the other two carbohydrases. In the four other sites, seasonal differences were observed during the 2yr but no clear pattern can be established. This study highlights the ability of G. fossarum to demonstrate environmental disturbances and suggests the use of a caging process in certain seasons. Caging organisms and feeding ad libitum is advantageous, as it reduces inter-individual variability and removes dependence on the native food fluctuations. However, confounding factors other than temperature were present, and the interpretation of digestive enzyme activities is complex.