Ciclo hidrológico e dinâmico de dois rios intermitentes da região semi-árida do Brasil, com ênfase em macrófitas aquáticas.

Abstract

The semi-arid region of Brazil presents rivers and intermittent streams with flood occurrence and of drought in different frequencies, intensity and duration. These events act as agents of hydrologic disturbance and they are decisive in the characteristics of the aquatic macophytes, in the concentrations of nutrients and in the fluvial geomorphology. This study was accomplished during the hydrologic cycles of 1999 and 2000 in two intermittent rivers of the semi-arid area of Brazil: a permanent pond in the stream Avelós (ephemeral) and two temporary ponds in the river Taperoá (temporary). The objectives were: (1) to determine the occurrence and the variation of the biomass of the species of aquatic macrophytes, (2) to determine the dynamics of nitrogen, ortophosphate, organic matter and particulate matter in the superficial water and hiporheic and in the fluvial sediment and parafluvial, (3) to verify the effects of the hydrologic variation in the level of the water and in the thickness of the sediment. Twenty-one visits were accomplished in the field during the weting phases, drying and dry. The sampling of the macrophytes for the study of the biomass was made in agreement with the method of the squares, the nitrate and the ortophosphate by the methodology pattern, and the variations geomorphology they were calculated through measures of the level of the water and thickness of the sediment. They happened floods in different frequency and magnitude during the study. The ephemeral stream presented phase of larger drought than the temporary river. The flood was a decisive factor in the occurrence and in the composition of the structure of aquatic macrophytes, in the concentrations of nutrients and in the geomorphology fluvial of the ponds. The macrophytes communities subject to floods they present smaller wealth when compared to those of atmospheres without the disturbance. The largest productivity values, biomass and the smallest time for the beginning of the recolonization happened in the flood of intermediate level. The concentrations of nutrients had significant increases in the flood phase, followed by falls of the values in the drying phase. The temporary ponds acted as areas of concentration of particulate matter in the drying phases and dry. The sediment was transported in the form of pulses. The largest significant variations in the depth and in the thickness of the sediment they happened in the phase drying and after events with intermediate magnitude.