In this paper, sensitivity analysis is applied to a mechanistic model developed to simulate microalgae growth. The Morris method of Elementary Effects (EEs) is applied to evaluate the sensitivity of model outputs with respect to a subset of key input parameters. For an easier interpretation, results were plotted as distributions of elementary effects means and standard deviations for each input parameter. The model outputs were very sensitive with respect to the maximum specific growth rate of microalgae (μalg). Results of the sensitivity analysis indicate that the transfer of ammonia (Ka,NH3) and carbon dioxide (Ka,CO2) have a non-linear relation with nitrogen uptake and carbonate concentrations, respectively. This analysis helped identify the parameters with the greatest impact on simulation outputs. The results indicated that maximum specific growth rate of microalgae (μalg) was the most critical parameter to calibrate properly.