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The success of invasive macrophytes is due to the ability of introduced fragments to survive, regenerate and colonise freshwaters. In this study, carried out in an experimental garden, we evaluated: (i) the ability of Myriophyllum aquaticum fragments with or without an apex, to survive and colonise and (ii) the effects of a macrophyte bed on the colonisation success of two non-native species ( Ludwigia grandiflora, M. aquaticum). Fragments of M. aquaticum with apical stems had the highest survival rate (85 vs. 62%) and colonisation abilities (i.e. 9% of rate of anchorage with a macrophyte bed). The anchorage rate of L. grandiflora was stimulated by the presence of the non-native Elodea canadensis and the relative growth rate of M. aquaticum fragments was higher in the presence of a L. grandiflora bed. Facilitation may enhance the colonisation of some non-native species during the first stage of invasion (colonisation and regeneration). However, the presence of a macrophyte bed limits the chances of L. grandiflora rooting in the sediment. This result suggests that spatial competition may occur at the beginning of growth in shallow waters. Competitive or facilitative interactions between neighbouring species depend both on the introduced species and on macrophyte bed composition. [ABSTRACT FROM AUTHOR]

Aquatic ecosystems are particularly sensitive to the introduction of species due to human activities. Increasing salinity in freshwaters due to sea intrusion or to human activities (road salt, industry, etc.) is known to have a negative impact on aquatic organisms and could play a role in the dynamics of invaders. This study compares salt tolerance levels of two introduced aquatic plants Myriophyllum aquaticum (Velloso) Verdcourt and Ludwigia grandiflora (Michaux) Greuter and Burdet. Plants were collected in spring, summer and autumn 2010, and were exposed to a range of salt concentrations (1, 3 and 6 g/L). Plant growth was determined by measuring seven morphological traits and their photosynthetic activity. Increased salt levels induced a decline in growth and photosynthetic activity in L. grandiflora, while photosynthetic activity in M. aquaticum remained constant at all salt concentrations. The response of both species to salt varied according to the season. M. aquaticum allocated its energy to growth in autumn, whereas the growth of L. grandiflora remained constant whatever the season. Our results suggested that M. aquaticum stimulated root and leaf production at the end of summer, which conferred resistance to salt stress and allowed this species to become invasive by overcompensating for this stress. Conversely, L. grandiflora induced premature senescence and lost its leaves. In the context of worldwide salinisation of freshwaters, M. aquaticum could colonize brackish water and other water bodies, whereas L. grandiflora invasion could be limited. [ABSTRACT FROM AUTHOR]