As a result of the increasing emission of complexformers like EDTA (ethylenediaminetetra-acetic acid) or NTA (nitrilotri-acetec acid), the speciation characteristics of metals will change. In this study the effect of EDTA (in a synthetic medium) on the speciation and biological activity of copper has been studied. The experimental set-up was three-fold: -the determination of the copper speciation electrochemically (Differential Pulse Anodic Stripping Voltammetry) in combination with UV-destruction and ion-exchange (Chelex), -the determination of the biological activity of the algae Selenastrum capricornutum (photosynthesis) and with the Microtox-test (light inhibition), -modelling of the speciation pattern. The calculated "not-EDTA" bound copper gives a better description of the photosynthetic activity than total copper concentrations, but it can not be used quantitatively. The experimentally determined "chelex- bound" copper fraction seems to be a good descriptor of the observed changes in the photosynthetic activity of S. capricornutum. More research to the validity of this relation for other water types is desired to enable the possibility to generalize and to improve the extrapolation methods.