As tidal stream energy sites contribute to a significant part of water transport in coastal shelf seas, turbines’ implementation may lead to system-wide changes of the hydrodynamic circulation with potential effects on marine water quality. These aspects were investigated in north-western coastal waters of Brittany (France, western Europe) by simulating the decay of a tracer concentration within a control domain surrounding the tidal stream energy site of the Fromveur Strait. Simplified simulations were adopted to analyse the sensitivity of tracer distribution to stream energy extraction. While this marine area showed important renewal capacity with a 50% loss of tracer concentration in less than three days,residual values were still present after two months. This result exhibited that the concentration decay could not be calibrated by simple exponential functions. The renewal of marine waters appeared furthermore dependent on tracer release time during a tidal cycle. A full energy-extraction scenario was finally simulated suggesting weak effects of turbines on the renewal capacity with changes in residence times below 5%. The spatio-temporal Eulerian evolution of the concentration revealed, however, local trapping areas in close correlation with the Lagrangian residual circulation that vanished after two months.