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Myriophyllum aquaticum is a semi-submerged exotic macrophyte that was introduced to China for many years. This species may be found in an emergent form in aquatic environments or in an amphibious form under drained conditions. Nuisance growth of this species has often been attributed to excessive amounts of nutrients. Therefore, we tested the following hypotheses: (1) high nutrient availability facilitates the establishment of M. aquaticum and (2) fragment type interacts with nutrient availability to determine the colonization and regeneration capacities of M. aquaticum. Two types of fragments were grown in water solutions with two levels of phosphorous. After 3 weeks, the survival rates showed no significant difference between the phosphorous treatments. However, emergent fragments showed higher RGR in the low and high phosphorous treatments than amphibious fragments. In addition, emergent fragments also showed higher regeneration capacities, indicating higher invasiveness in emergent fragments compared to amphibious fragments. Moreover, the high phosphorous concentration caused emergent fragments to produce more branches, indicating that nutrient availability may increase M. aquaticum propagule pressure. Our study highlights that nutrient supply increased emergent fragment establishment and shaped the invasion dynamics of macrophytes, which could help predict the spread and potential impact of exotic macrophytes in natural aquatic ecosystems. [ABSTRACT FROM AUTHOR]

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]

Clonal plants benefit from the ability to translocate resources among interconnected ramets through clonal integration to colonise ubiquitous heterogeneous habitats, which may contribute to the invasiveness of exotic clonal plants. To test this hypothesis, a greenhouse experiment was conducted to investigate the effects of clonal integration on a non-native clonal plant, Myriophyllum aquaticum, subjected to spatial and temporal heterogeneity of water supply. The daughter ramets were grown with stolon connections either severed from or connected to the mother plant and subjected to different amounts or frequencies of water supply. Clonal integration significantly improved growth and photosynthetic performance of daughter ramets that were exposed to spatial and temporal heterogeneity of water supply. Biomass allocation to roots of offspring ramets changed with water supply to enhance the capacity for water uptake. The decrease of the maximum quantum yield of photosystem II ( F/ F) as a function of reduced water supply was greatly alleviated by stolon connection. Moreover, clonal integration facilitated stabilisation of foliar N concentration and C/N ratio to support healthy growth of the ramets. These results suggest that clonal integration may facilitate invasion of M. aquaticum when subjected to heterogeneity in resource supply under ever-changing environments. [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]

Nuisance growth of Myriophyllum aquaticum has often been attributed to high amounts of nutrients. The uptake of nitrogen and phosphorus from sediments and their allocation have been documented in both natural and laboratory populations. However, nutrient loading to surface water is increasingly becoming an important issue for water quality standards. Aquatic macrophytes that develop adventitious roots may be able to survive through the uptake of water column nutrients. Our objectives for this study were to assess M. aquaticum growth when combinations of nitrogen and phosphorus were added to the water column. Mesocosm experiments were conducted where nitrogen (1.8, 0.8, and 0.4 mg l; high, medium, and low) and phosphorus (0.09, 0.03, 0.01 mg l; high, medium, and low) concentrations were paired and added to the water column. After 12 weeks, the combination of 1.80:0.01 N:P resulted in greater ( P < 0.01) total biomass and greater biomass for all plant tissues. Total biomass at the 1.80:0.01 N:P combination was 53% greater than biomass at all other combinations. The yield response of M. aquaticum was a quadratic function of tissue nutrient content. Yield was positively ( r = 0.82) related to increasing nitrogen content, whereas a negative ( r = 0.89) relationship was determined for increasing phosphorus content. We propose the negative relationship is due to increased nutrient competition and shading by algae resulting in reduced M. aquaticum growth. Tissue nutrient content indicated that critical concentrations (1.8% nitrogen and 0.2% phosphorus) for growth were not attained except for nitrogen in plants grown in the 1.80:0.01 N:P combination. These data provide further evidence that M. aquaticum requires high levels of nitrogen to achieve nuisance growth. Survival through uptake of water column nutrients may be a mechanism for survival during adverse conditions, a means of long distance dispersal of fragments, or may offer a competitive advantage over species that rely on sediment nutrients. [ABSTRACT FROM AUTHOR]