Site-specific ammonia toxicity to fish of the Red and Assiniboine Rivers and implications for Manitoba water quality objectives

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Partridge, Amy D.
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Site-specific acute- and chronic-exposure toxicity tests were conducted on five fish species (i.e., channel catfish, fathead minnow, northern pike, walleye and white sucker) resident to the Red and Assiniboine Rivers to evaluate survival and growth effects of un-ionized ammonia (NH3). Site-specificity was established using Red River water as the control and dilution water for all toxicity tests. 96-hour LC50 values ranged from 0.22 mg NH3/L for larval white sucker to >0.76 mg NH3/L for juvenile fathead minnows. End-of-Test LC20 values ranged from 0.13 mg NH3/L for larval northern pike to >0.58 mg NH3/L for juvenile fathead minnows. Growth was impaired in one group of juvenile fathead minnows at 0.52 mg NH3/L. Results of these tests, expressed as total ammonia-nitrogen, were used to compare the sensitivities of resident fish species to ammonia concentrations meeting current Manitoba Surface Water Quality Objectives (MSWQOs). Acute criteria established by Manitoba Conservation for pHs and temperatures common in the Red and Assiniboine Rivers (i.e., pH = 7.8 - 8.4, temperature = OoC - 25oC) range between 3.9 and 12.1 mg total ammonia-nitrogen/L. Results of this study suggest that these criteria are protective to only one of the five fish species tested (i.e., channel catfish). Any changes to acute criteria should reflect the heightened sensitivity of fish tested under site-specific conditions and should be lowered to provide an appropriate level of protection to cool water aquatic life. Conversely, chronic criteria range between 1.6 and 12.9 mg total ammonia-nitrogen/L and are adequately protective or overprotective for all fish species tested except possibly northern pike at water temperatures below 15oC. Further testing or in situ monitoring of northern pike is recommended to ensure that Provincial chronic objectives support the maintenance and propagation of this species. Finally, tests were conducted to determine whether NH3 is the sole toxicant in treated effluent discharged into the Red and Assiniboine Rivers a municipal wastewater treatment facility in Winnipeg, Manitoba (i.e, the North End Water Pollution Control Centre). White sucker and fathead minnows were consistently more sensitive to NH3 in the presence of effluent suggesting that another constituent of the effluent may increase ammonia toxicity or is itself toxic.