Aquatic macrophytes produce large amounts of organic matter and have an essential structuring role in floodplains. This process highlights the importance of this community to aquatic biodiversity maintenance. We investigated the role of a flood disturbance on the response of macrophyte assemblages in regional and local structuring in the Upper Paraná River floodplain. Plant species were recorded before (November 2006) and after (March 2007) an uncommon increase in water level caused by the El Nin˜o South Oscillation, which is considered a disturbance. Samples were taken in lakes and backwaters located in the floodplain and connected to three distinct rivers (that differentiate three sub-systems). Species richness and the assemblage structure of macrophyte patches underwent significant changes after the flood disturbance, depending on the specific sub-system (rivers) to which the lakes were connected. In addition, flood disturbance had a strong impact on community organization at the local scale.However, regionalization with respect to sub-systems remained significant after the flood disturbance. Our results emphasize the importance of connection to the river on macrophyte community composition and richness, and suggest that flood events in the Upper Paraná River floodplain disrupt community organization only at fine (local) scales.

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In the last two decades, the relationship between diversity and stability/ecosystem functioning has been widely discussed and has become a central issue in ecology. Here, we assessed the relationship between wetland plant diversity and community resilience after a disturbance. Our study area was located in the Upper Paraná River floodplain (Brazil). An experiment was carried out in situ (18 1 m ¥ 1 m plots with richness varying from 1 to 18 species). In each plot, we recorded the number of species, total per cent vegetation cover and per cent age cover of each species. The above-ground biomass of wetland plants was removed, simulating a disturbance by animal trampling or an extreme flood. The recovery of vegetation was monitored over 3 months. According to a linear regression, the recovery of wetland plants was positively correlated with diversity. Comparisons with plots containing monocultures of one of the dominant species (Polygonum stelligerum) suggested that this species did not overyield in mixed cultures. Thus, our experiments indicate that the higher resilience in richer plots after a disturbance is mainly due to the fact that species have different resource use requirements (complementarity effect) and not due to the presence of a single, more productive species. Our experiment carried out in a more real condition (in situ) showed that biodiversity is important to wetland functioning and stability, paralleling the results obtained in laboratory and mesocosms experiments.These results also suggest that the loss of plant diversity in our study area could compromise community recovery following strong disturbances.

-Reservoirs cause hydrological changes (i.e., water level stabilization) that favor the colonization of aquatic macrophytes. Knowing the ecological factors that determine the occurrence of these plants is critical for water management (e.g., plant control) and biodiversity conservation. In this sense, the present study investigated colonization patterns of Ludwigia sedoides in Lajeado reservoir (Tocantins River, Amazon Basin), in order to identify variables that influence colonization at habitat scale. We investigated the relationship between colonization (coverage area and occurrence) and morphometric (fetch, slope, depth and distance from shoreline) and biotic variables (local diversity of macrophytes and co-occurrence patterns). Stepwise regression selected fetch, depth and slope as the best variables to explain the variation in L. sedoides coverage, which together explained 46% of data variability. Fetch and slope were negatively correlated with coverage, whereas depth showed a positive correlation. No biotic variable was included in the model (P > 0.05). However, the investigation of the geometric shape of bivariate correlations (null models) showed positive relationships with local species richness and richness of life forms (i.e., submerged, emergent, floating and epiphytic). In addition, an analysis of species co-occurrence (C-score) revealed that L. sedoides is negatively associated with some macrophyte species. We believe, however, that these results may be associated with species preferences for particular environmental conditions. In conclusion, the present study indicated that morphometric variables are potential predictors of the colonization of L. sedoides in Lajeado reservoir. Sheltered sites with low slope and moderate depths represent favorable environment for colonization and growth.

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