Synergism and antagonism among multiple stressors

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Abstract
This study was designed to test for synergism (increased stress) or antagonism (decreased stress) among multiple environmental stressors using additive, multiplicative, and simple comparative effects models. Model predictions were compared to empirical results of laboratory experiments measuring interactions among thermal stress, toxin exposure, and low food on reproduction and survival of two species of cladoceran zooplankton. Stress was defined operationally as a reduction in reproduction or survival relative to optimal conditions over a 7‐d period. These experiments are particularly applicable to episodic stresses such as those associated with short‐term heat waves. Toxin or low food in combination with 30C temperatures were generally more harmful than high temperature alone. However, most multiple stress effects were antagonistic, in that effects in combination were not as severe as predicted based on the sum or the product of their individual effects. In rare cases, interaction among stressors even diminished effects of the worst single stressor. Optimal conditions for reproduction and survival occurred at 25C, high food and 0 mg liter‐1 toxin (a surfactant, sodium dodecyl sulfate). Suppressive effects of stressors examined individually ranked: high temperature (30C) > SDS (10 mg liter‐1 ɥ low food (∼100 mg C liter‐1) > low temperature (20C). Daphnia pulex isolated from a pond which experiences high summer temperatures throughout was more tolerant of 30C conditions than Daphnia pulicaria isolated from a lake with a cold‐water refuge. Differences were observed in individuals exposed as either adults or as 24‐h neonates.

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