Human and natural drivers of changing macrophyte community dynamics over 12 years in a Neotropical riverine floodplain system

The Rio Paraná is the world's tenth largest river (discharge in the upper river, studied here, during major flood events exceeds 12 700 m3 s-1). Together with its associated floodplain water bodies, this Neotropical river system supports a freshwater biodiversity of worldwide conservation significance, but one increasingly affected by human pressures, particularly the effects of river impoundment. Multivariate analysis of data from repeated (2000–2002 and 2011) macrophyte–environment surveys of a set of stations within a 230 km stretch of functioning riverine floodplain, downstream of the most recently completed dam on the Upper Paraná, the Porto Primavera Dam, together with published data (from 1999 and 2005–2007), was used to test the hypothesis that human influences might outweigh the importance of natural factors in driving the aquatic vegetation dynamics (macrophyte community composition, alpha-diversity, and abundance) of this system during the period 1999–2011. The macrophyte communities present showed differences of species composition and abundance that could be strongly related to the scale and duration of the immediately preceding annual flood pulse, as well as local variations in physico-chemical conditions. For rare Neotropical endemic species, which are a good marker of the conservation status of the system, there was a reasonable degree of stability between the 2000–2002 and 2011 survey periods, with eight of 16 such species being present in both surveys, three present in 2000–2002 only, but an additional five such species found only in 2011. The conclusion is that vegetation changes in the system can be referred both to direct human impacts (river regulation), and to indirect human or natural impacts (floods, alien species invasion by Hydrilla verticillata, and local environmental factors). Overall there was evidence that major floods associated with El Niño - Southern Oscillation (ENSO) climatic events had a much greater impact than other drivers, whether natural or anthropogenic