AbstractSix laboratory model streams derived directly from natural stream communities were used to study the effects of atrazine and a hexachlorobiphenyl isomer on community structure and function. Two streams were treated with 2,4,5,2′,4′,5′‐hexachlorobiphenyl (0.10 μ/L) dissolved in acetone, two streams received atrazine (25 μg/L) dissolved in dimethylsulfoxide and each of the two remaining streams received one of the carrier solvents at the same solvent concentration as the solute‐treated streams (0.69 μl/L). The chemicals were administered to the streams for four 30‐d periods, each of which was followed by a 60‐d recovery period.The results indicate that at the concentrations used, neither chemical had any significant or lasting effects on the structure of macroinvertebrate populations, periphyton standing biomass or rates of primary production and community respiration. The negative nature of the results for hexachlorobiphenyl are of special interest because in these experiments all components of the biota accumulated substantial quantities of this chemical.Macroinvertebrate drift in the streams increased abruptly upon the injection of both solutes and their carrier solvents. This response appeared to be a reaction primarily to the solvents rather than the solutes. This effect should be investigated in more detail as it indicates that carrier solvents may alter the outcome and interpretation
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