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Effects of Cu and Zn through food as well as through the water on the growth and emergence of the young larvae of Epeoms latgolium (Ephemeroptera), were investigated using an indoor model stream. The critical lowest concentrations of Cu which have significant effects on the growth of the larvae were between 10 and 15 pg 1-' Cu. Growth and emergence of the larvae fed on the algae (diatoms) which accumulated more than 1000 pg CU g-(dry wt.) were significantly impaired. Growth of the mayfly larvae exposed to 100 or 300 pg 1-' Zn ceased after the second week, and all died before emergence. At 30 pg 1-' Zn, the growth rate decreased gradually and many larvae died before emergence. The molt interval also nearly doubled that of the control at these concentrations. Growth and emergence of the mayfly fed the algae which accumulated more than 2000 pg g -' of Zn were significantly affected.

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