Ecological conditions and conservation strategies for 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. The study was conducted in the Perișor fish farm (Tulcea County) between June and August 2024, within the polders of the fish farm. To identify habitat and phytocoenological preferences, relevés were carried out using a 1 m² plot size. A ProDSS multiparameter digital water quality meter was used to analyze the physicochemical properties of the water, and spectrometric methods were employed to analyze the physicochemical properties of the sediment. 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 favourable habitat was C1.32 free-floating vegetation, where the highest densities and individual lengths were reported. In habitat Q53 (Tall sedge beds), the lowest values for density and length of individuals of A. vesiculosa were recorded. Cluster analysis revealed six associations, such as Typhetum angustifoliae, Spirodelo-Aldrovandetum, Caricetum ripariae, Schoenoplectetum lacustris, Nymphaeetum albae, and Phragmitetum australis. Length of individuals ranged from 4 to 21 cm, and number of individuals per m² ranged from 3 to 300 individuals per m². Physicochemical analysis of the water showed that A. vesiculosa occurred under moderate temperatures (22–27 °C) and alkaline pH values (7.8–11.6), with variable, sometimes elevated ammonium and nitrate concentrations and shallow waters (7–30 cm). Correlation analysis indicated that the length of A. vesiculosa individuals was more sensitive to environmental gradients than the number of individuals per m². Thus, the minimum and medium length of individuals showed significant negative relationships with water conductivity, while the medium length of individuals also decreased with increasing water depth and transparency. In contrast, the number of individuals per m² was weakly correlated with most environmental variables. In conclusion, the results underline the importance of habitat quality and hydrological stability for the conservation of A. vesiculosa in the Danube Delta, supporting the need for continuous monitoring and targeted conservation measures. Comparisons between the main Perișor Channel and the polders indicate that maintaining controlled connectivity and water-level management at the main-channel connections may be crucial for hydrological stability and habitat quality in A. vesiculosa sites.