GENETIC CHANGES IN PHYTOPLANKTON COMMUNITIES EXPOSED TO THE HERBICIDE SIMETRYN IN OUTDOOR EXPERIMENTAL PONDS

Possible changes in the species composition of phytoplankton communities and the susceptibility of individual strains to the triazine herbicide, simetryn, were investigated in outdoor experimental ponds exposed to the herbicide. Centric diatoms remained predominantly longer in the treated ponds than in the control ponds during the early experimental period, but there was little difference in the overall floristic composition. In one of the two treatments, the phytoplankton community whose abundance had been suppressed by the chemical, subsequently became more abundant in spite of the much higher concentration of simetryn than previously. All the strains isolated from the herbicide-treated ponds during the latter experimental period were tolerant to 100 mu g/L simetryn (relative yields: >70%), whereas all the strains isolated from the control ponds at the same time were susceptible (relative yields: <15%). Scenedesmus gutwinskii var. heterospina showed a marked difference in susceptibility between the strains isolated from the treated and the control ponds. Tolerance of the isolated strains remained for nearly two years in the absence of simetryn, which indicate that changes in genetic composition occurred in the phytoplankton communities when they were exposed to the herbicide.