Multiscale environmental characterization of Aldrovanda vesiculosa in the Danube Delta, Romania

Abstract Aldrovanda vesiculosa is a critically endangered aquatic plant in the Danube Delta, where it is vulnerable to water level fluctuations. In this context, this study established conservation measures for the species A. vesiculosa following investigations on the analysis of habitats and characteristics of the species in the polders of the Perișor abandoned facility in the Danube Delta. The study was conducted in the Perișor dunes (Tulcea County), between June and August 2024. To identify habitat and phytocoenological preferences, relevés were carried out, with a sample area of 1m². A ProDSS multiparametric digital water quality meter was used to analyze the physicochemical factors of the water, and molecular absorption spectrometry methods were used to analyze the physicochemical factors of the sediment. Topographic and bathymetric measurements were performed using the SonTek Acoustic Doppler Current Profiler (ADCP) RiverSurveyor M9 for the hydrological analysis. Aldrovanda vesiculosa was reported in four EUNIS habitat types. The most favorable habitat was C1.32 free-floating vegetation, where the highest densities and individual lengths were reported. In the habitat Q53 - tall sedge beds, the lowest values for density and length of A. vesiculosa individuals were recorded. Cluster analysis revealed six associations, such as Typhetum angustifoliae, Spirodelo-Aldrovandetum, Caricetum ripariae, Schoenoplectetum lacustris, Nymphaeetum albae, and Scirpo-Phragmitetum. Plant length ranged from 4 to 21 cm, and the density of individuals ranged from 3 to 300 per 1 m². Physicochemical analysis of the water showed that A. vesiculosa preferred waters with moderate temperatures, from 22 °C to 27 °C, a more alkaline pH, from 8.5 to 10.5, low ammonium and nitrate concentrations and depths of up to 23 cm. Correlation analysis indicated that with increasing specific conductivity, vegetation cover, water depth and transparency, there is a decrease in the average length of A. vesiculosa individuals. Also, increasing vegetation cover caused a decrease in the minimum length of A. vesiculosa individuals. Moreover, the number of A. vesiculosa individuals increases with atmospheric pressure. In addition, a slight increase in individuals was recorded in waters with high nitrate concentrations. PCA analysis showed a strong correlation between the number of individuals per 1 m² and atmospheric pressure. Hydrological measurements in the central basin feeding the polders showed that A. vesiculosa growth indicated increased water levels and flow in 2024. In conclusion, urgent interventions are required to conserve A. vesiculosa in the Danube Delta through transplantation. Protecting this plant requires constant monitoring and integrating measures into local environmental policies.