The dilemma of underestimating freshwater biodiversity: morphological and molecular approaches

The Lower Rhine ecosystem has been extensively shaped by human activities, destroying, modifying, and even creating novel water bodies as habitats. Freshwater systems are hotspots of biodiversity and highly complex ecosystems. However, knowledge and description of its biodiversity across all trophic levels is still incomplete and the complexity of interactions remains unresolved, especially below the micro scale. This is even true for important international inland waterways such as the River Rhine. We here document the biodiversity of the Lower Rhine and associated water bodies, spanning from the level of protists up to the level of larger invertebrate predators and herbivores organized in faunal size classes (nano-, micro, meio- and macrofauna). This study is part of a long-term ecological research project (LTER-D REES). Our study includes two riprap sections of the river's main channel, two oxbows and two gravel-pit lakes in the river's flood plain. Diversity was assessed through morphotype richness and metabarcoding, as well as accounting for sediment composition.