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7 páginas, 5 figuras, 2 tablas., Lake Baciver, located in Pla de Beret (Val d’Aran), was formerly covered by aquatic plants (95% of its surface area). In autumn 1991 the lake was damned, flooding an area equivalent to its original surface and raising the water level by 3.5 meters. This event was catastrophic for the aquatic plants and only 17% of the formerly covered area, survived. This paper discusses the progress of the residual population and the aquatic plant colonization in the newly flooded areas over the past two decades. The study provides data of interest on the Restauration Ecology of soft-water oligotrophic lakes, which is otherwise very difficult to obtain. We found that the quillwort Isoetes lacustris L. can live up to 25 years under extremely low light conditions and sediment instability, but that the residual population does not recruit and is steadily regressing. We have also observed I. lacustris growing a new population from spores in an inoculated formerly terrestrial soil, although space occupation has been very slow, of less than 1 m2 per year on average. Sparganium angustifolium and accompanying species can colonize the new sedi- ments much faster, following exponential progression, and currently occupy 95% of the potential niche for the community. The new status of the lake has changed not only in terms of the landscape, but also in the biomass renewal. Fast growing annuals are taking most of the space against the ecological engineer Isoetes lacustris, which is a slow growing perennial species that plays a key role in controlling sediment biogeo- chemistry in oligotrophic lake sediments.

Potamogeton lucens L. i P. schweinfurthii A. Benn. són dues espècies aquàtiques morfològicament similars que fins fa ben poc han estat confoses a Europa. L'objectiu del treball és aclarir i actualitzar la informació sobre la situació actual i històrica d'aquests dos tàxons a Catalunya. El treball es basa en la revisió de material dipositat en diversos herbaris de Catalunya i en les dades de camp recents obtingudes pels autors. Addicionalment s'han estudiat les característiques dels hàbitats on hem localitzat poblacions d'aquestes espècies. S'han documentat 5 localitats segures de P. lucens, però la seva presència recent s'ha constatat en una sola d'aquestes localitats, un estany de muntanya dels Pirineus. Les altres quatre, actualment desaparegudes, es situaven en zones de baixa altitud i poc apartades del litoral a les conques dels rius Tordera i Llobregat. Aquesta regressió històrica està relacionada amb els canvis d'usos i d'ocupació del territori durant el segle XX, que han provocat la dessecació de gran part de les masses d'aigua estancada naturals i una forta eutrofització de les que s'han mantingut. En canvi, P. schweinfurthii no havia estat trobat a Catalunya fins a l'any 2010 i actualment se'n coneixen 3 localitats situades en zones de muntanya mitjana del centre i el nord del territori, en dues de les quals es constata que deu ser un colonitzador recent. La revisió del material dipositat als herbaris catalans no ha permès localitzar cap altre plec d'aquesta espècie. Sobre la base d'informacions actualitzades s'assigna a P. lucens la categoria IUCN En Perill Crític (CR) i a P. schweinfurthii la categoria Vulnerable (VU). L'únic híbrid amb participació d'alguna d'aquestes dues espècies conegut a Catalunya és P. x salicifolius -híbrid entre P. lucens i P. perfoliatus- documentat a la fi del segle XIX a l'estany de Banyoles i no retrobat. Potamogeton lucens L. and P. schweinfurthii A. Benn. are two aquatic species of similar morphology that have been confused in Europe until recent times. This study aims at elucidating and updating information about the current and historical distribution of both taxa in Catalonia. Data sources were herbaria specimen revisions and field observations. Habitat characteristics of the current populations were also studied. Despite verifying five P. lucens populations, only one has persisted in a high-altitude lake in the Pyrenees. The four disappeared populations were located in lowland areas in the basins of the rivers Tordera and Llobregat. This historical decline relates to changes in land uses and urban sprawl during the 20th century that have led to numerous desiccations and a large degree of eutrophication in the persisting lowland stagnant water bodies. In contrast, P. schweinfurthii was first reported from Catalonia in 2010 and three populations are currently known. They are located in mid-altitude areas at the center and north of the study area and two may be of recent colonization. Vouchers of this species were not found in the surveyed herbaria. Based on the current knowledge, the IUCN category assigned to P. lucens is Critically Endangered (CR) and to P. schweinfurthii is Vulnerable (VU). Regarding hybrids, only P. x salicifolius -hybrid between P. lucens and P. perfoliatus- was reported from Catalonia at the end of the 19th century at Banyoles lake and has not been found again.