The aim of this study was to examine the influence of the net cage system of fish culture on the composition, abundance and diversity of the zooplankton community. Comparisons were made between three collection points located near a group of 30 net cages, with individual volumes of approximately 4 m3, containing newly hatched, young and adult Nile tilapia, positioned in an arm of the Furnas hydroelectric reservoir; point 1 was in the limnetic region and points 2 and 3 were upstream and downstream of the cages, respectively, near the bank. The sampling was carried out at all three points from July to December, 2005. The composition of the zooplankton community differed among the points analysed. The lowest diversity was recorded at point 1. Point 2, upstream of the cages, was characterized by greater species richness and higher density of organisms pertaining to the Rotifera group. The results of the study suggest that variations in the composition and abundance of Rotifera and Cladocera are a consequence of the impact of net cage fish culture, although the magnitude of the variations is small.

Water samples were collected quarterly for 1 year from the newly created Osamu Utsumi uranium mine pit lake, Brazil, which is affected by acid mine drainage (AMD). The water presented mean pH values of 3.8, high mean electrical conductivity values (2391 lS/ cm), manganese (74 mg/L), sulfate (1413 mg/L) and uranium (3 mg/L). The density of rotifera was significantly higher than that of cladoceran. Rotifera Keratella americana, K. cochlearis and the Cladocera Bosminopsis deitersi, Bosmina sp., are being reported for the first time in samples from a uranium pit lake with AMD. Of the species registered, the order Bdelloidea was the most important in terms of density (17,500-77,778 ind/m 3 ), since it occurred throughout the whole sampling period. The combined effect of moderately acidic pH and other potential stress factors, such as high concentrations of stable and radioactive contaminants, probably influenced the zooplankton species composition in the pit lake.

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