LAUSR

Abstract The immense value of ecosystem services provided by freshwater bodies is incomputable. However, the productivity and biodiversity of freshwater bodies are undergoing dramatic degradation as a result of climate- and anthropogenic-induced changes worldwide. Despite a well-documented literature on freshwater biodiversity, we still lack a comprehensive and objective overview of biodiversity research conducted in different types of freshwater bodies. Here, we conducted a comparative study of freshwater biodiversity in different ecosystem types at the global scale to describe the current state of knowledge, map the scientific geographical distribution and international collaboration network and identify persistent and emerging themes. Thus, metadata of biodiversity studies carried out in six freshwater ecosystem types - river, lake, wetland, stream, reservoir and pond- were extracted from SCI-E and SSCI databases of the Web of Science and then visualized for science mapping using bibliometric analysis. Our findings showed that in the field of freshwater biodiversity (i) the number of publications in various freshwater habitats has risen considerably over the last two decades with the highest average annual growth rate in river- and lake-related studies, (ii) there was a clear bias in the geographical distribution of the research with fewer scientific production and international collaboration in less developed parts of the world. The USA (20%) and China (19%) are the main leading countries and Europe (30%), Asia (26%, mainly China and India) and North America (24%) are major continents contributing to this field, (iii) taxonomic groups have been investigated disproportionally among different freshwater habitats, (iv) habitat heterogeneity and fragmentation, land use change, climate change, eutrophication and invasive species are the most frequent driving factors, (v) the persistent and emerging themes differ among water ecosystems, e.g. disproportionate assessment of biodiversity measures. Furthermore, functional and phylogenetic facets in this area are poorly understood. Overall, clear obtained distinctions in the hotspots and thematic areas of different freshwater environments can contribute to identify the missing pieces in current research, and put an effort to investigate the non-explored or under-explored areas of research.