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First immature instar and adult egg-carrying Daphnia pulex Leydig acclimated from 5 to 30°C, at 5°C intervals, and Daphnia magna Straus acclimated from 5 to 25°C, at 5°C intervals, were instantaneously immersed at 30 and 35°C and observed for mortality. Observations were made at 0.5, 1, 2, 4, and 8 h after immersion and every 8 h thereafter until 95 percent mortality occurred. Twenty organisms were used in each test, and the tests were carried out in temperature-controlled water baths. Filtered lake water was used for culture and testing.
Adult egg-bearing D. pulex and D. magna were more sensitive to the upper lethal temperatures than their respective immature forms, which indicates that mature egg-carrying forms should be used for determining temperature tolerance levels. The results also indicated that organisms held in laboratory cultures for extended time periods at constant temperatures have decreased upper temperature tolerance limits, in comparison with organisms adapted to the fluctuating temperatures of natural environments, which have been laboratory acclimated for shorter time periods.
Shortcomings in the methodologies of previous thermal tolerance studies on zooplankton are discussed, and recommendations are made as to how these methods can be improved.
aquatic toxicology, temperature, thermal tolerance, Daphnia, zooplankton, acclimation
Chief of environmental assessment and support staff, Tennessee Valley Authority, Chattanooga, Tenn.