Toxicological effects of military fog oil obscurant on Daphnia magna and Ceriodaphnia dubia in field and laboratory exposures.

Our purpose was to determine if the acute and sub-lethal effects of fog oil, an obscurant used for military training, could be observed in realistic field exposures. To this end, we exposed Daphnia magna to oil fogs under actual release conditions at a U.S. Army training site. Guided by field investigations, acute toxicity experiments were conducted in the laboratory with the more sensitive species Ceriodaphnia dubia to test the hypothesis that dissolution of fog oil constituents into water is minimal and actual contact by organisms with the water surface is required to cause toxicity. We conducted further experiments to test the hypothesis that vaporization of fog oil alters its chemical composition and toxicity to freshwater invertebrates. In the field, daphnid mortality was minimal more than 5 m from the point of fog generation, but sub-lethal effects were more extensive. Both field and laboratory experiments suggested that physical contact with oils on the water surface was the most important factor driving toxicity. To our knowledge, this is the first attempt to evaluate toxicological endpoints with freshwater invertebrates in field exposures with fog oil.