Patterns of spatial and temporal variation in estuarine meiofaunal communities assessed through DNA metabarcoding: a case study in the Lima estuary (NW Portugal) Maria Fais 1, 2, 3* , Sofia Duarte 1, 2 , Pedro E. Vieira 1, 2 , Ronaldo G. Sousa 1, 2 , Mehrdad Hajibabaei 4 , Carlos A. Canchaya 3 and Filipe O. Costa 1, 2 • 1 Centre of Molecular and Environmental Biology, Department of Biology, University of Minho, Portugal • 2 Institute of Science and Innovation for Bio-Sustainability, University of Minho, Portugal • 3 Department of Biochemistry, Genetics and Immunology, University of Vigo, Spain • 4 Centre for Biodiversity Genomics, Biodiversity Institute of Ontario, University of Guelph, Canada Meiobenthic organisms are key components of estuarine environments, providing invaluable functions and services. In particular, meiofaunal organisms participate actively in nutrient cycles and energy flux, supporting higher trophic levels, as well as the stabilization of sediments. Due to their small size (between 30 µm and 1mm) and high dispersal potential, meiofauna was considered to have a ubiquitous distribution. However, with the recent progress of DNA-based technologies, in particular, DNA metabarcoding, the cosmopolitan meiofauna paradox is being questioned, suggesting that meiobenthic communities can comprise also endemic and narrow-range species. Several studies on the spatial variability of meiofauna at small-scale (within meters) and at meso-scale (meters to km) have been carried out in an attempt to elucidate community structure, both using traditional approaches (such as morphological identification) and DNA-based tools. All these studies focused on the importance of environmental features (i.e., salinity, sediments’ size, percentage of organic matter) in the distribution of particular taxonomic groups, as well as of their bio-ecological characteristics, which may overall explain the spatial structure of the meiobenthic communities. However, little attention has been given to the temporal variation of meiofauna communities in estuaries, and to the best of our knowledge, no research on the spatial-temporal dynamics of Iberian estuarine meiofauna has been yet performed using DNA-