The decline and recovery of populations of Potamogeton coloratus in Hungary

Fen pondweed (Potamogeton coloratus) is a characteristic species of oligotrophic waters, therefore, one of the rarest and most endangered aquatic plants in Europe because of habitat loss and population decline all over the continent. In Hungary, all of its populations disappeared during the 1950s and 1960s and then dramatically recovered in recent years during which many populations that had disappeared recovered and new populations established. In this study, we investigated the role of environmental variables in the recovery of this species and in determining its phenology-environment dependency, as well as the role of dispersal in the establishment of new populations. Our study is the first to assess the dispersal ability of P. coloratus by means of endozoochory, evaluated by force-feeding mallards with its seeds. We found that variation in annual precipitation does not affect the presence of P. coloratus; however, the variation in the karst water level coincides with the number of populations in the region studied. This indicates that the disappearance-reappearance of P. coloratus has been driven by fluctuations in regional karst water levels, which was greatly affected by the development of socialist heavy industry and especially the operation of bauxite and coal mines. Moreover, we show that water temperature determines the phenology of this species. Importantly, we also demonstrate that a relatively high proportion of the seeds of P. coloratus force fed to mallards passed through their digestive tract and many of them contained viable embryos. Thus, birds could serve as possible dispersal vectors of P. coloratus seeds, hence they might be responsible for the establishment of new populations. Overall, P. coloratus appears to be recovering, but regular monitoring is needed to ensure its persistence.