Urban aquatic ecosystems: Habitat loss and depletion of native macrophyte diversity during the 20th century in four Swiss cities

Urban areas are among the most affected by human activities. In Europe, urbanization has essentially occurred since the end of the 19th century. However, the influence of this dramatic process on aquatic ecosystems has rarely been quantified and analyzed using historical data. In this study, we investigate the evolution of the hydrological system and native aquatic macrophytes in urban areas between the end of the 19th century and the beginning of the 21st century. Four urban areas in Switzerland were chosen for the analyses, Zürich, Basel, Lausanne and Fribourg, and we analyzed the changes in aquatic plant diversity based on the historical and recent floristic data available for the same areas and the same time periods. Our results show that a significant proportion (∼30 %) of aquatic habitats has disappeared from the investigated locales during the last 130 years and that the extinction rate of the aquatic plant species is notably higher in the studied cities (28 %) than in Switzerland as a whole (2 %). Thus, between the end of the 19th and the beginning of the 21st centuries, urban development in Swiss cities has prompted a degradation of aquatic habitats that resulted in a significant reduction of the aquatic biodiversity. However, our study shows that urban areas still have the capacity to shelter a large diversity of aquatic organisms, including some of the most threatened species. Thus, it is important to integrate urban areas in the conservation strategies for these species.