The proposed research aims at elucidating several issues related to the physico-chemical processes governing trace metals (TM) behaviour in pristine, but also in potentially contaminated transitional and coastal aquatic ecosystems. Whether the TMs behave as micronutrients or are highly toxic to organisms, their mobility, reactivity as well as bioavailability, strongly depends on chemical speciation. Our focus will be directed towards characterisation of interactions of TM with other macro and micro constituents of natural waters, particularly with natural organic matter, by employing the most recent approaches utilizing electrochemical and hyphenated spectrometric techniques. Particular effort will be invested in the development and improvement of speciation methods, measurement protocols and sensing electrodes. Methodologies developed under controlled laboratory conditions will be verified using natural water samples and applied in on-site and in-situ field-studies. The research will lead to the establishment of sets of specific measures and criteria which will allow reliable estimation of the actual state, but also the prediction of future behaviour of TMs (speciation, mobility, bioavailability/toxicity) in case of local environmental accidents (e.g. contamination) or global threats (e.g. ”ocean acidification”). Aquatic environments characterized by a variety of chemical compositions will be studied. In such conditions, variations in physical-chemical forms of the studied TMs (As, Cd, Co, Cr, Cu, Fe, Hg, Ni, Pb, Zn) are expected to occur. Study sites will be estuarine, oxic/anoxic, eutrophic and clean/contaminated areas, in which processes occurring at the phase boundaries, as well as in the sediment and water column will be explored. The specific subjects of the research are selected in a way to enable the outcome of the project be valuable not only to the broad scientific community working on TM speciation, but also to the authorities managing coastal zones.