Distribution and habitat characteristics of Vallisneria spiralis L. in Croatia

Abstract Eight new localities of the rare, strictly protected macrophyte Vallisneria spiralis have been recorded during a comprehensive survey of water bodies in Croatia. One record is located in the Mediterranean Region, in Butoniga Reservoir, while the remaining records are from the Continental Region, the majority of them situated along the Kupa River. Vallisneria spiralis occurred in slightly basic and alkaline environments and in a wide range of nutrient availability. In Continental localities, the species was present within Myriophyllo-Nupharetum luteae of the alliance Nymphaeion-albae, while in Butoniga, it occurred within the vegetation of the alliance Potamogetonion. Prior to our research, the species was recorded in Croatia only twice, with the older record dating back to the 19th century and the second from recent years, but neither one was confirmed in later surveys. The low number of records of V. spiralis, in spite of a wide-ranging search through hundreds of localities, confirmed the rare status of this species, although the reasons for such limited distribution remain unclear, especially bearing in mind that suitable aquatic habitats are quite widespread in the country.