Aquatic plant communities and predictors of diversity in a sub-tropical river floodplain: The upper Rio Paraná, Brazil

Analysis of field survey data collected during 1999, from waterbodies (river channel, secondary channels and backwaters, temporary lagoons, permanent lagoons) within the riverine floodplain wetlands of the Upper Rio Paraná in southern Brazil showed: (i) strong dominance by five aquatic macrophyte species (most important being Eichhornia azurea), out of 28 species recorded at the survey stations; (ii) based on TWINSPAN clustering, the existence of three major aquatic vegetation types. These comprised an E. azurea-Limnobium laevigatum sub-community (at 20% of stations sampled); an E. azurea-Salvinia sub-community (commonest, at 67% of stations); plus a Nymphaea amazonum-Myriophyllum aquaticum community (rarest, at 13%) which occurred in habitats showing significant between-type differences in environmental conditions for water (depth, underwater light regime, conductivity, pH, alkalinity, phosphate) and sediments (redox potential, calcium, total Kjeldahl nitrogen, phosphate, iron); (iii) predictable patterns of variation in plant size and shape, among populations of the dominant plant species present: E. azurea, Cyperus diffusus, Polygonum ferrugineum, Salvinia spp., Eichhornia crassipes, N. amazonum, along gradients of water and sediment physicochemistry; (iv) significant variation in both macrophyte community abundance and species diversity between the three vegetation types, with the highest species richness occurring in the E. azurea-L. laevigatum sub-community, but highest community biomass in the E. azurea-Salvinia sub-community. Minimal linear models are presented which use a combination of environmental and vegetation attribute data as predictor variables to predict species richness and biomass of aquatic vegetation for the floodplain waterbodies. Such models can be used to assess vegetation response to likely scenarios of future change in river conditions in sub-tropical areas, for example modification of flow regime by dam construction.