In parallel with the growing use of nanoparticle-containing products, their release into the environment over the coming years is expected to increase significantly. Much of the research on the behaviour and fate of engineered nanoparticles (ENPs) in the environment has focused on terrestrial aquatic systems (lakes, rivers and groundwater) due to the expectation that ENPs would eventually be degraded in these systems without significant transport to other environmental compartments. However, with many large population centres located in near-coastal areas, the potential environmental load of nanoparticles in estuarine and coastal waters over the coming years is enormous. As there is increasing evidence that various nanoparticles may be toxic to a range of organisms, biota in estuarine and coastal waters may thus be particularly vulnerable. However, available data on how nanoparticles behave in high strength electrolytes, in particular the effect of their coating on colloidal stability, speciation or ion release kinetics are scarce. Further, there is little data on how such aged nanoparticles interact with the wide range of diverse organisms in brackish and coastal waters.This project will investigate the physico-chemical behaviour of silver, copper and polymer/plastic nanoparticles with a range of different sizes and surface coatings, as materials of concern, in brackish and marine waters based on a range of abiotic factors including salinity, pH, oxygen concentration and dissolved organic matter. Standardised biochemical tests, adapted to take into account the specific properties and interference effects of ENPs, will be used to measure a range of toxicity endpoints in selected organisms ranging from algae and macroalgae to urchins, mussels and crabs. The results from this project will provide for the first time a systematic overview of the behaviour of silver, copper and plastic nanoparticles in brackish water as a complex transition zone from fresh to salt water.