Climate-driven distribution shifts and phytocoenological patterns of invasive and range-expanding aquatic macrophytes in the Danube Delta Biosphere Reserve, Romania

Aquatic macrophytes with invasive or range-expanding behaviour are a growing ecological concern, especially in vulnerable wetland habitats under pressure from climate change. The present study analyses the phytocoenological preferences and climate-influenced expansion potential of three macrophyte species in the Danube Delta Biosphere Reserve: two alien invasive species, Azolla filiculoides and Elodea nuttallii , and one non-native range-expanding macrophyte, Vallisneria spiralis . The study was conducted during 2024–2025, in selected lakes from the Danube Delta Biosphere Reserve, Romania. For vegetation analysis, phytocoenological relevés were carried out, subsequently processed by cluster analysis and DCA ordering, in order to highlight floristic groupings and gradients of vegetation variation. Species presence data and environmental variables were used for spatial modelling. The three aquatic species studied differed clearly in their current distribution, the factors controlling their occurrence and their response to future climate scenarios. Azolla filiculoides showed the highest current suitability and a generally stable to expanding trend under SSP3-7.0, although a reduction was projected under SSP5-8.5 by 2071–2100. Elodea nuttallii showed a reduction under most scenarios, but a late-century increase in suitable area was projected under SSP5-8.5. Vallisneria spiralis was more stable overall, although the models indicated an initial increase in suitable area, followed by a decline towards the end of the century, more pronounced under SSP5-8.5. Cluster analysis showed that each species was mainly linked to its characteristic vegetation type, while DCA revealed moderate to relatively strong floristic gradients between relevés. In conclusion, climate change may increase the risk of invasion of some aquatic macrophytes in the Danube Delta, but the magnitude and direction of change differ depending on the species. Azolla filiculoides appears to benefit most from future conditions, E. nuttallii may become more restricted, and V. spiralis appears comparatively more stable. This species-specific response highlights the need for targeted monitoring and adaptive management in vulnerable wetland habitats. The qualitative hydromorphological assessment further indicates that differences in lake connectivity, water exchange and transparency may contribute to the ecological variation observed among the studied lakes and should be considered in future monitoring of submerged macrophyte development.