Physiological differentiation of the invasive macrophyte Vallisneria australis along a thermal gradient in the Lake Hévíz outflow

Abstract Invasive species pose a significant threat to biodiversity, particularly in freshwater ecosystems. While temperate climates often act as filters, thermally stable environments such as thermal springs can serve as refugia for tropical non-native species by providing conditions similar to their native habitats. The large natural thermal lake of Lake Hévíz in Hungary and its outflow have facilitated the establishment of several allochthonous species from tropical and subtropical regions. This study investigated whether physiological or morphological differentiation exists between Vallisneria australis populations at upper (warmer) and lower (cooler) sites that were 7 km apart. Results showed that temperature difference between the upper and lower parts of the outflow of 4.6 °C significantly affected the photophysiology of V. australis . Lower photophysiological parameters related to primary production (ETR max and I k ) were recorded at the upper site compared to the lower site. Significant differences between sites were found for most photochemical parameters, while seasonal variations were observed. Correlation patterns between water temperature and photophysiological parameters also differed between the two sites, suggesting different response strategies, while the complementary laboratory transplant experiments at controlled temperatures (19 °C and 24 °C) mirrored the field differences in photochemical parameters, supporting the hypothesis of physiological differentiation. These results suggest that the variations are not just short-term plastic responses, but reflect a more established physiological divergence. The thermal gradient is likely to exert selective pressure, possibly leading to local acclimation or even adaptation, although genetic evidence for this differentiation is not presented. The potential escape into nearby Lake Balaton poses a significant ecological risk. The results support the application of the precautionary principle in managing the spread of V. australis .