The development of a Rayleigh-Benard convection inside a formerly imposed stable density stratification was studied during potentiostatic copper electrolysis. By means of such a density barrier the turbulent, three-dimensional emission of buoyant fluid elements, plumes, can be constrained to a slow, quasi-one-dimensional process. In this regime the current transient displays paradoxical behaviour. Although the concentration gradients at the electrodes are seemingly higher, the current density stays for a long time below that without the density barrier. An interferometric analysis of the concentration profiles during the growth of the plume ensemble, in connection with the analytically given profile in the diffusion phase, is able to find the reason for the behaviour of the current. (c) 2007 Elsevier B.V. All rights reserved.