Ecology of <i>Elodea canadensis</i> Michx. and <i>Elodea nuttallii</i> (Planch.) H. St. John—Insights from National Water Monitoring in Croatia

<i>Elodea canadensis</i> Michx. (common waterweed) and <i>Elodea nuttallii</i> (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. <i>Elodea canadensis</i> was found at 30 sampling points, mostly in rivers, and <i>E. nuttallii</i> at 15 sampling points, mostly in artificial canals. Nearly three-quarters (72.5%) of all elodea sampling points were in the Pannonian Ecoregion. <i>Elodea canadensis</i> 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 <i>E. canadensis</i> and <i>E. nuttallii</i> was confirmed with the ANOSIM test. Furthermore, Indicator Species Analysis revealed eight species characteristic of <i>E. canadensis</i> sites and eleven species characteristic of <i>E. nuttallii</i> sites. Fitting multivariate models (CCA and NPMR) to species abundance revealed the ecological reaction of <i>E. canadensis</i> and <i>E. nuttallii</i> to environmental descriptors. The most strongly contributing environmental descriptors that influence the distribution of both <i>Elodea</i> species are biochemical oxygen demand, electrical conductivity and total phosphorus. In Croatia, the replacement of <i>E. canadensis</i> with <i>E. nuttallii</i> was observed in several water bodies with high nutrient loads.