Aquatic organisms are exposed to a multitude of contaminants and to fully understand the impact of multiple stressors on fish populations, we must first understand the mechanism of action for each toxicant and how the combined effects manifest at the level of the individual. 17a-ethinylestradiol has been known to cause adverse reproductive effects including reduced fecundity and fertility, intersex and sex change in fish by mimicking naturally produced estrogen at low concentrations. Ammonia has been known to cause adverse reproductive and mortality effects in individual fish through effects or damage to the central nervous system. A flow-through diluter system was used to test the individual effects of environmentally relevant concentrations of these two contaminants as well as their mixture on fathead minnow reproduction. Significant findings from this study suggest that U.S. EPA water quality criterion for ammonia in waters where non-salmonids are present (8.4mg/L) is above the lowest observable effect concentration for fathead minnow reproduction. The lowest un-ionized ammonia concentration (5mg/L) tested during this study resulted in a 29% decrease in cumulative fecundity. The mixture of both 17a-ethinylestradiol and ammonia at their respective environmentally relevant concentration resulted in increased mortality, but did not to show adverse effects on reproduction. This study demonstrated the need for toxicity testing with multiple stressor scenarios and the importance of re-evaluating current U.S. EPA water quality criteria.
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