Mercury levels in concentrations similar to those in natural waters contaminated with mercury (0.1 and 1.0 μg/ℓ) caused significant reduction in algal numbers, standing stock, and diversity. Reductions in diversity resulted from a decrease in the evenness of distribution of numbers among species and a decline in the number of species. Although the algal community was affected by mercury ions, the magnitude of change was small at the mercury levels tested and the number of species affected were few. The decline in algal standing crop could indirectly affect other food chain members that possess physiological resistance to the mercury. There was no evidence of direct or indirect impact on the herbivorous or carnivorous midges. The impact on primary producers was not sufficient to be transferred to consumer trophic levels.

Oxygen consumption (µl% mg dry wt . • hr' corrected to STP) in Chaoborus punctipennis was measured in a Gilson differential respirometer at two-hour intervals during 24-hour periods . Animals were held at controlled conditions similar to those at the time of collection . Respiration was measured under controlled temperatures and natural photoperiod . Measurements were begun about 36 hours after collection .Winter-collected animals showed a lower and more stable respiration than summer and fall-collected animals which did not differ from one another . Respiration increased with temperature becoming quite variable at 3o°C where mortality was highest . Animals collected at different seasons and subjected to a variety of temperatures did not show a predictable pattern of respiration during the daily cycle under any condition tested .Since the 36-hour delay in respiration measurements could have dampened a diet cycle, oxygen consumption was determined within an hour of collection and followed for 24 hours . For this experiment animals were collected at six-hour intervals over a 24-hour period . Respiration did not show a predictable pattern of variation during the day-night cycle .

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