Hans Brix is specialised in freshwater ecology, wetland ecology and plant ecophysiology. Focus is on the ecophysiology of invasive plants in wetlands and aquatic ecosystems, and biogeochemical cycling. The research involves studies on ecosystem level, e.g. the emission of greenhouse gases from wetlands, as well as on the individual plant level, e.g. release of oxygen from plant roots, internal gas-transport in aquatic plants, and physiological characteristics of aquatic plants in relation to growth conditions. Applied aspects of the research involve the use of natural and constructed wetlands for the treatment of various kinds of polluted water.