Synergism and antagonism among multiple stressors

This study was designed to test for synergism (increased stress) or antagon ism (decreased stress) among multiple environmental stressors using additiv e, multiplicative, and simple comparative effects models. Model predictions were compared to empirical results of laboratory experiments measuring int eractions among thermal stress, toxin exposure, and low food on reproductio n and survival of two species of cladoceran zooplankton. Stress was defined operationally as a reduction in reproduction or survival relative to optim al conditions over a 7-d period. These experiments are particularly applica ble to episodic stresses such as those associated with short-term heat wave s. Toxin or low food in combination with 30 degrees C temperatures were genera lly more harmful than high temperature alone. However, most multiple stress effects were antagonistic, in that effects in combination were not as seve re 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 oc curred at 25 degrees C, high food and 0 mg liter(-1) toxin (a surfactant, s odium dodecyl sulfate). Suppressive effects of stressors examined individua lly ranked: high temperature (30 degrees C) > SDS (10 mg liter(-1) greater than or equal to low food (similar to 100 mu g C liter(-1)) > low temperatu re (20 degrees C). Daphnia pulex isolated from a pond which experiences hig h summer temperatures throughout was more tolerant of 30 degrees C conditio ns than Daphnia pulicaria isolated from a lake with a cold-water refuge. Di fferences were observed in individuals exposed as either adults or as 24-h neonates.