The aim of this work is to assess the impact of data assimilation on the dynamics of mesoscale eddies. We do this based on the use of a recent mathematical method from nonlinear dynamics field that is capable of detecting Lagrangian eddies from birth to death. We apply this method on two composites time series of two configurations covering the whole globe, with and without assimilation, and carry on various statistical analysis to evaluate the impact of data assimilation on several properties of these Lagrangian mesoscale eddies.