Environmental DNA metabarcoding (eDNA metaB) is fundamental for monitoring marine biodiversity and its spread in coastal ecosystems. We applied eDNA metaB to seawater samples to investigate the spatiotemporal variability of plankton and small pelagic fish, comparing sites with different environmental conditions across a coast-to-offshore gradient at river mouths along the Campania coast (Italy) over 2 years (2020–2021). We found a marked seasonality in the planktonic community at the regional scale, likely owing to the hydrodynamic connection among sampling sites, which was derived from numerical simulations. Nonetheless, spatial variability among plankton communities was detected during summer. Overall, slight changes in plankton and fish composition resulted in the potential reorganization of the pelagic food web at the local scale. This work supports the utility of eDNA metaB in combination with hydrodynamic modelling to study marine biodiversity in the water column of coastal systems. This article is part of the theme issue ‘Connected interactions: enriching food web research by spatial and social interactions’.

Plastic debris in river ecosystems provides persistent surfaces for microbial colonization, forming complex plastic-associated biofilm communities known as the riverine plastisphere. Despite the growing number of studies on plastic-associated microorganisms, current knowledge remains strongly biased toward marine environments, whereas riverine plastisphere communities are still comparatively understudied. This mini-review critically examines the current state of knowledge on the riverine plastisphere, focusing on microbial community composition, ecological drivers, biofilm development, ecological significance, and microbiological aspects related to pathogens and antibiotic resistance genes (ARGs). Current evidence indicates that riverine plastisphere communities are influenced by a complex interplay of environmental conditions, anthropogenic pressures, and substrate-related factors. The available literature suggests that these plastic-associated biofilms may support distinct microbial assemblages and microbial traits linked to nutrient cycling, pollutant transformation, pathogen occurrence, and ARG persistence. However, current knowledge remains largely focused on community composition and occurrence patterns, whereas evidence linking plastisphere communities to ecosystem-level processes remains limited. Recent studies have also highlighted the importance of considering the plastisphere as a biofilm system developing at the interface between plastic substrates and the surrounding environment, where biological, chemical, and physical processes interact. Information on ecological relevance, microbial viability, virulence, and gene mobility remains comparatively scarce. The mini-review also highlights the challenges associated with methodological heterogeneity, limited comparability among studies, and the absence of harmonized indicators capable of supporting robust ecological and public health risk assessments. Overall, the riverine plastisphere represents a rapidly developing research field that offers new opportunities to investigate the interactions among plastic pollution, microbial ecology, and lotic ecosystem processes. Future progress will depend on methodological harmonization and on the development of approaches capable of linking microbial occurrence to ecosystem processes and translating occurrence data into ecologically meaningful information and comparable risk assessments.