<p>The species–area relationship (SAR) and ecosystem buffering concepts suggest that habitat size can regulate both macrophyte diversity and invasion outcomes in human-impacted floodplains. The aim of this study was to evaluate how lentic-system size, associated environmental heterogeneity, and anthropogenic pressures relate to aquatic macrophyte assemblages including invasive species in tropical floodplains. To achieve this, macrophyte community composition, diversity metrics and importance value index (IVI), along with key water and sediment physicochemical variables, were assessed across ten lentic systems of varying sizes in the Barak River basin of southern Assam, India. Overall, 38 macrophyte species belonging to 34 genera and 23 families were recorded, exhibiting diverse assemblage and distribution patterns. Species richness in wetlands was nearly twice that of ponds. Across systems, total richness increased with habitat area, driven mainly by non-invasive richness, whereas invasive richness showed no significant area dependence. Statistical analyses indicated that environmental variables such as transparency, electrical conductivity, dissolved oxygen, total alkalinity, hardness, sediment moisture content, sediment pH, and sediment organic matter significantly differed across aquatic systems, contributing to environmental heterogeneity. Canonical correspondence analysis revealed distinct associations between habitat types and species along these environmental gradients. Larger systems exhibited greater environmental heterogeneity and higher total (especially non-invasive) richness. In contrast, smaller systems showed higher community-level importance of dominant invasive species, consistent with disproportionate invasion impacts in small floodplain waterbodies. The findings suggest that management and policy efforts for invasive species control should also prioritize smaller lentic systems, which are prevalent in tropical floodplain landscapes.</p>