ABSTRACT:The naturally occurring, aerobic, heterotrophic bacterial populations of two impounded ecosystems were studied in a laboratory tank system. One reservoir received runoff from a rural drainage basin only, while the second received treated municipal sewage, industrial waste and heavy recreational use. Water from each reservoir was treated with 1.4 ppm final concentration of diuron and studied for total bacterial counts and nutritional source types. Control tanks were studied concurrently. Total counts increased shortly after diuron addition; however, this was followed by a decrease to a level below the control. Reduction in diversity was significantly greater in water from the polluted reservoir, but chromagenic bacteria were significantly reduced in the non‐polluted reservoir. The effects on isolation of specific nutrient types from the two systems following herbicide addition revealed that starch utilizers were the most affected group. Bacteria growing on protein or glucose salts were essentially unaffected. (KEY TERMS: Diuron; Bacteria; Herbicides in Aquatic Environments; Chromagenic Bacteria: Natural Aquatic Bacteria
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