Species composition and occurrence of aquatic macrophytes in the Amazon Basin are associated with the hydrology and geomorphology of the catchment area. Nonetheless, the distribution of the Neotropical genus Montrichardia in the Amazon floodplains remains unclear. Three taxa of the genus were described: Montrichardia linifera (Arr.) Schott (ML), associated with nutrient-rich rivers; Montrichardia arborescens (L.) Schott (MA) and M. arborescens var. aculeata (G. Mey.) Engl., growing in nutrient-poor rivers. We hypothesize that different taxa can be discriminated according to variation in density, morphology, physiology, and biomass along environmental gradients of water characteristics and soil fertility. Samples of 55 populations of Montrichardia were taken in rivers with contrasting water types in the Amazon Basin along a gradient of declining soil fertility from Western to Eastern Amazonia spanning about 2,500 km. Local phenotypic variations and discriminant analysis separated ML and MA with MA as subset of MA. Water chemistry influenced the distribution of taxa. The nutritional soil gradient at the basin level clearly explained the distribution of ML, but not that of MA. The distribution pattern of ML was similar to that of tree species in the Amazon Basin, with higher density in West Amazon. © 2015, Springer International Publishing Switzerland.
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