Eichhornia azurea (Schwartz) Kunth is one of the most abundant species of aquatic macrophyte in the Upper Paraná River floodplain, and ephemeropteran nymphs frequently inhabit this plant. The objective of this study was to analyze the distribution of ephemeropteran nymphs associated with three sections (apical, intermediate, and basal) of E. azurea in two floodplain lakes of the Upper Paraná River as well as to establish the relation of this distribution to biotic and abiotic factors. The samplings of apical, intermediate and basal sections of E. azurea were carried out monthly from October 1997 to October 1998. Higher concentration of dissolved oxygen may contribute to greater density of nymphs on the apical plant segments near the water surface. Callibaetes willineri was the most abundant taxon, occurring on the apical and intermediate plant sections whereas Campsu- rus spp. were recorded on the basal and intermediate sections of the plant, near the sediment and, mainly during high water. Lower dissolved oxygen concentrations near the lake bottom during the high water phase might suggest that during this phase the nymphs of Campsurus violaceus also migrate to E. azurea roots. It was found that changes in water level influenced the variation of the abundance of nymphs. Other genera, such as Tricorythodes and Leptohyphes, were recorded during the high water phase, when river water enters the lakes and their shores are flooded.
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