Survival, Respiration, and Food Assimilation of Daphnia magna Exposed to Petroleum- and Coal-Derived Oils at Three Temperatures

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Abstract
Acute toxicity tests with the water-soluble fractions (WSFs) of both oils showed that mortality of Daphnia magna was directly related to temperature and the WSF concentration. The coal-derived oil was more toxic than the diesel fuel oil. The 48-h LC50s for the former were 0.70, 0.44, and 0.18% at 10, 20, and 25 °C, respectively; the values for the diesel oil WSF were 87, 32, and 9.7% at 10, 20, and 25 °C, respectively. Respiration rates of animals exposed to sublethal concentrations of the WSFs at 10, 20, and 25 °C were significantly affected by the WSF concentration of the oils and temperature. The interaction between the concentration of each oil with temperature was statistically significant. Low concentrations of diesel oil WSF stimulated respiration rates at each temperature, but higher concentrations depressed them. At all temperatures, oxygen consumption rates of animals exposed to the coal-derived oil WSF decreased with increased concentration. The exposure to sublethal concentrations of both oils affected food assimilation of the animals in a manner similar to that observed when respiration was the criterion of toxic effect. The interaction between diesel oil WSF concentrations and temperature, but not between coal oil WSF concentrations and temperature, was statistically significant. At 20 and 25 °C, low concentrations of diesel oil WSF stimulated assimilation rates, whereas low concentrations of the coal oil WSF depressed them. Generally, high concentrations of both WSFs depressed assimilation rates below control values. These results emphasize the importance of natural stresses, such as temperature, in enhancing toxicity of synthetic chemicals to aquatic organisms, and they reinforce the need to consider such variables as season of the year in evaluating biotic effects of oil spills.Key words: temperature, Daphnia magna, petroleum, coal oil, synthetic oil, respiration, food assimilation, acute toxicity, chronic toxicity