Assessing the Impact of Pesticide Mixtures on the Survival and Fecundity of Daphnia pulex
This study was undertaken because little is known about the sub-lethal effects of pesticides on aquatic organisms. To answer the question, "What are the long term effects of pesticides on non-target organisms?", several tests were conducted on multiple generations of the common water flea, Daphnia pulex. The results indicate that mixtures of pesticides were more toxic than any individual pesticide, and population growth was adversely affected at sub-lethal doses. Acute toxicity tests may underestimate the long term toxicity of both individual pesticides and mixtures of pesticides.
Abstract
Aquatic ecosystems are frequently exposed to mixtures of chemicals, particularly agricultural chemicals, that may adversely affect non-target animals. Laboratory bioassays were conducted to determine the lethal and sub-lethal effects of several pesticides commonly found in surface water, and of a mixture of these pesticides together, on the Cladoceran, Daphnia pulex (Leydig). The pesticides tested included diazinon, chlorpyrifos, malathion, carbaryl, and 2,4-D. Tests consisted of either a) 10-day population growth studies, with starting populations of 10 D. pulex neonates per cup, that measured the instantaneous rate of increase (ri), or b) life table chronic exposure studies, starting with one individual D. pulex neonate per cup, that measured the intrinsic rate of increase (ro). In situ field studies were also conducted to determine the impact of fluxes of multiple contaminants under natural conditions. Results indicated that chlorpyrifos was the single most toxic pesticide tested in the population growth studies in the laboratory. Diazinon, however, was the single most toxic pesticide tested in the life table studies in the laboratory. In both the population growth studies and the life table chronic exposure studies, the mixture of all five pesticides together was more toxic to D. pulex than any of the pesticides tested individually. In the in situ tests little difference was seen in the survival of the original D. pulex exposed to the different water types, regardless of the location of the test, however, fecundity was affected by both the water type and location of the test.
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