Use of new microbiotests with Daphnia magna and Selenastrum capricornutum immobilized forms.

A preliminary assessment of toxicity to aquatic life of water from rice fields treated with pesticides, and particularly with the herbicides molinate and quinclorac, was performed from June to August 1996 in a rice paddy area integrated in a Natural Reserve. New rapid-screening microbiotests with Daphnia magna and Selenastrum capricornutum were used. Water samples were collected from: a water source for irrigating the rice fields, two rice plots, a drainage channel and the river in which the water is discharged. The results showed that while the water for irrigation was not toxic to D. magna, the samples collected from the river were very toxic causing 100% immobility in D. magna and substantial inhibition of Selenastrum growth. This, however, is due to the brackish nature of the river water. Water collected in the drainage channel of the rice fields were also found to be quite toxic. A greater effect was detected in samples from the rice plot treated with molinate than from that treated with quinclorac, suggesting that molinate treatment was more toxic to both species. The results indicate that the rapidity, simplicity and relatively low cost of the new microbiotests make them very practical for initial acute toxicity screening and offer an alternative to provide evidence for changes needed in agriculture practices for a better protection of the aquatic environment.