1. Wetland ecosystems may, besides having considerable economical value, increase landscape biodiversity and function as traps for nutrients from land to freshwater- and marine systems. As a result of these features, wetlands are nowadays often protected and restored, and many countries have even initiated wetland construction programmes. 2. In the present study, we aim at increasing the knowledge on how to improve the design of a wetland with respect to both biodiversity and nutrient retention, by analysing physical, chemical and biological features of a large set of constructed wetlands. 3. Our results show that a combination of the wetland features, namely shallow depth, large surface area and high shoreline complexity are likely to provide a high biodiversity of birds, benthic invertebrates and macrophytes and to have high nitrogen retention, whereas a small, deep wetland is likely to be more efficient in phosphorus retention, but less valuable in terms of biodiversity. 4. Hence, among the features used to design new wetlands, area, depth and shoreline complexity have fundamental, and sometimes conflicting, effects on nutrient retention and biodiversity. This means that there are, within limits, possibilities to direct the ecosystem function of a specific wetland in desired directions.