Elodea canadensis Michx. (common waterweed) and Elodea nuttallii (Planch.) H. St. John (Nuttall’s waterweed), two invasive aquatic plants from North America, have coexisted in European water bodies since the early 20th century. New localities for both species in Croatia continued to be discovered during a study that ran from 2016 to 2023 as a part of the annual implementation of Water Framework Directive monitoring that covered the entire territory of Croatia (786 sampling points in total). Based on these data, the distribution and ecology of both species were analysed. Elodea canadensis was found at 30 sampling points, mostly in rivers, and E. nuttallii at 15 sampling points, mostly in artificial canals. Nearly three-quarters (72.5%) of all elodea sampling points were in the Pannonian Ecoregion. Elodea canadensis was discovered for the first time in the Continental—Dinaric and Mediterranean—Dinaric Subecoregions. To study the ecology of the species, for each sampling point, vegetation relevés were performed and monthly measurements of physico-chemical parameters were collected. The most common accompanying species for both elodeas are presented, and the difference in species assemblages between the sites with E. canadensis and E. nuttallii was confirmed with the ANOSIM test. Furthermore, Indicator Species Analysis revealed eight species characteristic of E. canadensis sites and eleven species characteristic of E. nuttallii sites. Fitting multivariate models (CCA and NPMR) to species abundance revealed the ecological reaction of E. canadensis and E. nuttallii to environmental descriptors. The most strongly contributing environmental descriptors that influence the distribution of both Elodea species are biochemical oxygen demand, electrical conductivity and total phosphorus. In Croatia, the replacement of E. canadensis with E. nuttallii was observed in several water bodies with high nutrient loads.

Pondweeds, an important component of macrophyte vegetation, are influenced by various ecological factors of the aquatic ecosystem. In turn, pondweeds affect the nutrient and sediment dynamics and provide food and shelter for other organisms. As different species have specific environmental preferences and tolerances, they can serve as indicators of the ecological status of water bodies. Here, the ecological preference of the seven most frequent pondweeds in Croatia (Potamogeton berchtoldii, P. crispus, P. lucens, P. natans, P. nodosus, P. perfoliatus and Stuckenia pectinata) for chemical and physical water parameters was studied using 218 vegetation relevés and the accompanying water parameters. CCA revealed the main environmental gradients described by six parameters (chemical oxygen demand, total nitrogen, total phosphorus, electrical conductivity, dissolved oxygen and pH), while ecological responses of the species were further explored by GAMs. Potamogeton berchtoldii, P. lucens, P. natans and P. perfoliatus prefer clean, oxygenated, oligo- to mesotrophic water, and P. crispus and S. pectinata thrived in eutrophic water with low oxygen levels, while P. nodosus is a widespread generalist. The results of this study explain the distribution patterns of Potamogeton and Stuckenia species in Croatia, and add to the general knowledge on their role as bioindicators.

Galanthus reginae-olgae Orph. occurs in mainland Greece, the Ionian Islands, Corfu, Albania, Italy, Sicily, Montenegro, Bosnia and Herzegovina, and Croatia. This paper aims to extend the knowledge of the distribution and ecology of G. reginae-olgae in Croatia and the northwestern Balkans. A field study in 2023 revealed as many as 35 populations in southern Croatia, some of them counting up to 100.000 individuals. Most of the populations exhibit semi-sciophilic behaviour within transitional habitats (ecotones) from insolated grassland to shaded eu- and sub-Mediterranean thickets and forests, and are found in the altitudinal range from sea level to 664 m a.s.l., at an average distance of 5.44 km from the coastline. Analysis of the bioclimatic variables reveals that the populations studied find an optimum in Mediterranean conditions with a mean annual air temperature of 14.61 °C and mild winters. The annual precipitation is high, exceeding 1600 mm, although it is limited to the winter period. The analyses of the temperature data provided insight into the environmental conditions that might serve as triggers for the initiation and duration of anthesis. In view of the the observed leaf length and flowering time, the populations are designated G. reginae-olgae subsp. vernalis Kamari. Since the Croatian populations are sparsely scattered along the northern limit of the species’ distribution on the Balkan Peninsula, future research should investigate the adaptability of this species to habitat and climate changes in the eastern Mediterranean region.