The response of orthotropic rhizome elongation and primary production of Posidonia oceanica to anthropogenic perturbationsand potential confounding effects of shoot age were assessed using a Linear Multilevel Model (LMM). This model examined theconfounding effect of age by comparing the estimates of impact and variance components obtained by excluding and including Ageas an explanatory variable. Age had a negative effect on rhizome elongation and primary production with an annual decrease of0.6 mm y−1 and 7 mg dw y−1 respectively. According to the LMM when age effect was omitted, the differences between disturbedand control locations in rhizome elongation and primary production were 2.62 mm y−1 and 0.044 g dw y−1 respectively. Theseeffects were statistically not significant. On the contrary, when age effect was included in the statistical model, impacts becameevident for both variables, with significant differences between disturbed and control locations of 5.85 mm y−1 and 0.081 g dw y−1for rhizome elongation and primary production, respectively. Thus, particular attention should be paid to the potential confoundingeffect of shoots age in analyses of impacts on growth performance of P. oceanica.