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Hussner A (2012). Alien aquatic plant species in European countries. Weed Research52, 297–306. Summary Alien aquatic plant species cause serious ecological and economic impacts to European freshwater ecosystems. This study presents a comprehensive overview of all alien aquatic plants in Europe, their places of origin and their distribution within the 46 European countries. In total, 96 aquatic species from 30 families have been reported as aliens from at least one European country. Most alien aquatic plants are native to Northern America, followed by Asia and Southern America. Elodea canadensis is the most widespread alien aquatic plant in Europe, reported from 41 European countries. Azolla filiculoides ranks second (25), followed by Vallisneria spiralis (22) and Elodea nuttallii (20). The highest number of alien aquatic plant species has been found in Italy and France (34 species), followed by Germany (27), Belgium and Hungary (both 26) and the Netherlands (24). Even though the number of alien aquatic plants seems relatively small, the European and Mediterranean Plant Protection Organization (EPPO, http://www.eppo.org) has listed 18 of these species as invasive or potentially invasive within the EPPO region. As ornamental trade has been regarded as the major pathway for the introduction of alien aquatic plants, trading bans seem to be the most effective option to reduce the risk of further unintended entry of alien aquatic plants into Europe.

Crassula helmsii, Hydrocotyle ranunculoides, Ludwigia grandiflora and Myriophyllum aquaticum are four well known invasive aquatic plants in European waters. In this study, plant growth at different nutrient availabilities, regeneration capacity and photosynthesis were investigated. Results show high relative growth rates (RGR) of the species of up to 0.132 ± 0.008 g g )1 dry weight (dw) day )1 (H. ranunculoides) and a significant increase in RGR with increasing nutrient availability. All species show a high regeneration capacity and the ability to form new shoots from single nodes, even though it differs between the species. Ludwigia grandiflora and M. aquaticum also show regeneration from single leaves. Species differed in maximal amounts, and in temperature and light optima of net assimilation rates: H. ranunculoides leaves reach maximum photosynthetic rates of up to 3500 lmol CO 2 · h )1 g )1 dw, L. grandiflora (leaves) up to 2200 lmol CO 2 · h )1 g )1 dw, M. aquaticum (shoots) 400 lmol CO 2 · h )1 g )1 dw and C. helmsii (shoots) up to 200 lmol CO 2 · h )1 g )1 dw. Hydrocotyle ranunculoides, L. grandiflora and M. aquaticum preferred high light intensity and high temperatures, whilst C. helmsii was negatively affected by intense sunlight. Summarising, it can be assumed that at least H. ranunculoides, L. grandiflora and M. aquaticum can grow well under current and likely future central European climate conditions.

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SUMMARY1. Alien plant species are rapidly spreading in aquatic ecosystems around the world, causing major ecological effects. They are typically introduced by humans, after which natural vectors facilitate their further spread. Migratory waterbirds have long been recognised as important dispersal vectors for native and aquatic plants, yet little is known about their role in the spread of alien species. 2. We determined experimentally the potential for long-distance dispersal of native and alien wetland plants in Europe by two abundant waterfowl: mallards Anas platyrhynchos and greylag geese Anser anser. We fed seeds from two plants alien to Europe and two native plants to 10 individuals of each bird species, testing for the effects of bird and plant species on the potential for dispersal. 3. Intact seeds were retrieved from faeces for up to 4 days after ingestion. The proportion of seeds retrieved intact varied significantly between plant, but not bird, species. Retrieval was highest for the invasive water primrose Ludwigia grandiflora (>35% of ingested seeds), lowest for the invasive cordgrass Spartina densiflora (<3%) and intermediate for the native glasswort Arthrocnemum macrostachyum and seablite Suaeda vera (5-10%). 4. Seed retrieval patterns over time varied between both plant and bird species. Contrary to expectations, seeds were retained in the gut for longer in the smaller mallards. No Spartina seeds germinated after retention for over 8 h, whereas some seeds of the other species germinated even after retention for 72 h. Germinability was reduced by gut passage for Ludwigia and Arthrocnemum seeds. Ludwigia seeds recovered from geese were more likely to germinate than those recovered from mallards. Time to germination was reduced by gut passage for Spartina and Ludwigia, but increased with retention time. 5. Ducks and geese evidently have the potential for long-distance transport of alien and native plant seeds, with maximal dispersal distances of well over 1000 km. The much greater potential of Ludwigia than Spartina for dispersal by waterfowl is consistent with its faster expansion across Europe. Maximum retention times of wetland seeds have been underestimated in previous experimental studies that lasted only 1-2 days. Contrary to previous studies, wetland plants with large seeds, such as Ludwigia, can still show high potential for long-distance dispersal. More attention should be paid to the role of waterbirds as vectors of alien plants and to the role of migratory geese as vectors of plants in general.