The three-dimensional circulation of an eddy/front system is quantified using hydrographic data collected in the Channel of Ibiza, Western Mediterranean, during November 1990. The ageostrophic motion is derived from the quasi-geostrophic omega equation. Maximum upward velocities (approximate to 4 m/day) are diagnosed on the cold side of the front in the vicinity of a cyclonic eddy and coincident with a sub-surface convergence (below 50 m). This upward motion can be explained in terms of advection over isopycnals that uplift in the along-front direction. We also show the relevance of the three-dimensional circulation for the enhanced fertilization observed in the surface layer. Our results establish the importance of the ageostrophic dynamics induced by frontal mesoscale events for the regional ecology of the Balearic Sea.