Comparison Between Laboratory and “in Situ” Bioassays Using Chironomus xanthus in the Assessment of Sediment Toxicity in the Monjolinho River (São Carlos, SP)
DOI:
https://doi.org/10.5132/jbse.2006.02.014Keywords:
“in situ” bioassay, sediment, Chironomus xanthus, Monjolinho riverAbstract
“In situ” bioassays (96 h), using Chironomus xanthus, were conducted in October/03 and January/04, in 4 sampling stations at Monjolinho river. Laboratory bioassays were conducted using 60g of sediment and 240 ml of ASTM medium, for 96 h. Six organisms were added to each test-chamber and the toxicity evaluation was given by the mean survival. The “in situ” bioassay test chambers (PVC) were covered by a 300 μm mesh. The sediment of each sampling station was added to the chambers together with 10 organisms at IV instars. The laboratory bioassays showed survivals ranging between 0% (Federal, Oct/03) and 100% (Federal, Jan/04). The results “in situ” bioassays showed high mortality (between 84% and 97% in Oct/03 and between 60% and 70% in Jan/04). However, the results did not show, necessarily, that the sediments were toxic or not, because of the high mortality that occurred in the river ́s spring (over 60%), which may be associated to other factors, like screen saturation with suspended matter and different sediment textures. The presence of indigenous organisms may have influenced the results, due the competition and predation in the chamber. Thus, we can infer that the protocol for “in situ” bioassays using C. xanthus must be improved in order to minimize the influence of confounding factors on the tests results.
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