Environmental predictors of the occurrence of exotic <i>Hydrilla verticillata</i> (L.f.) Royle and native <i>Egeria najas</i> Planch. in a sub-tropical river floodplain: the Upper River Paraná , Brazil

The Upper River Paraná Floodplain System comprises the rivers Ivinheima, Baía and Paraná, which with their associated waterbodies form three subsystems, each showing individual characteristics. <i>Hydrilla verticillata</i> recently invaded the Upper Paraná Floodplain, while <i>Egeria najas</i> is the native most abundant submersed macrophyte. A large flood-pulse, during January-March 2007, abruptly reduced macrophyte stands in many areas to near-zero and dispersed propagules over the entire floodplain. From April 2007 to April 2008, we conducted three surveys sampling for the presence-absence of <i>H. verticillata</i> and <i>E. najas</i> and environmental variables aiming to answer: (1) How rapid is the colonization-regeneration process for both species? (2) Which habitats seem to be more susceptible to their colonization? (3) Which environmental factors can best predict their occurrence? Neither <i>H. verticillata</i> nor <i>E. najas</i> colonized the Ivinheima subsystem. In the Baía subsystem, <i>E. najas</i> had only two occurrences while <i>H. verticillata</i> was not present. In the Paraná subsystem, <i>E. najas</i> predominantly occurred in river channels, but it was also common in floodplain lakes. In April 2007, it was found in 13% of the sites in the Paraná subsystem, increasing to 30% in November 2007 and reaching 34% in April 2008. <i>H. verticillata</i> did not successfully colonize floodplain lakes of the Paraná subsystem. In channels, it had 34% occurrence in April 2007, increasing to 62% in November 2007 and remaining at 62% in April 2008. The role of environmental variables in predicting species occurrence changed depending upon the scale of the analysis. Considering the whole Upper Paraná floodplain, water transparency followed by electrical conductivity were the strongest predictors for both species. Colonization by submersed plants seems improbable in the Ivinheima subsystem owing to its low water transparency besides frequent localized floods; in the Baía subsystem, it seems inhibited by transparency and low alkalinity. Considering just the Paraná subsystem, the proportion of organic matter in sediment, ten times higher in floodplain lakes than in channels, was the best predictor for <i>H. verticillata</i> occurrence (also related to water pH and transparency), while <i>E. najas</i> was only significantly explained by transparency