Confirmation of Species Identification and New Locations of Potamogeton nodosus Poir. in Biebrza National Park Region

In 2013, plants tentatively identified as Potamogeton nodosus Poir. were discovered in the Biebrza River (NE Poland). In this study, the authors confirm the presence of P. nodosus by collecting new specimens at the original location and analyzing their microscopic characteristics, an essential step due to significant overlap in macromorphological traits with the closely related P. × fluitans complex. Additionally, new occurrences of the species within Biebrza National Park are reported, and the possibility that its spread is linked to rising river water temperatures is discussed. The authors provide evidence of an increasing average water temperature in the Biebrza River and of a northbound expansion of P. nodosus in Europe. Given similar trends observed elsewhere in Northern Europe, it is likely that P. nodosus will continue to expand its range northward in response to ongoing climate change.