Past and present distribution of the rare aquatic plant Luronium natans (Alismataceae) in Belgium shows marked decline and bad conservation status

Background and aims - Luronium natans is a rare endemic plant of West- and Central-Europe and protected by the Habitats Directive. The present study gives a comprehensive and up-to-date overview of its past and present distribution in Belgium, which lies within the core area of Luronium natans. Methods - We assessed the distribution by consulting herbaria, literature and databases, and by additional field visits, recording the number of ramets, flowering, general site characteristics and accompanying plant species for extant populations. Key results - The core area of Luronium natans is located in the Campine phytogeographical district, a region with sandy soils in northern Belgium. Overall, it was recorded at c. 250 sites in 155 16 km2-squares, with 93 populations occurring since 1980. Field observations show thirty extant populations, all but one in the northern part of the country. Less than half of them are large flowering populations with a good conservation status as far as the species population characteristics are concerned; 30% are located in nature reserves and 70% in Special Areas of Conservation. The number of records decreased since 1985, especially outside the core area. This decline related well to eutrophication, but not to anthropogenic acidification. In general, populations are short-lived and population size fluctuates strongly. Population size correlates with flowering incidence. For one isolated population it appears that the species was introduced with fish. Conclusions - The number of Luronium natans records shows that the Belgian territory accounts for a significant part of its global distribution. There was however a marked decline of records since 1985, especially outside its core area. Moreover, most of the remaining populations are small and have a bad conservation status. They show little or no flowering, which is worrying, considering the alleged importance of sexual reproduction for the survival of the species.