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 30ЊC 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 25ЊC, high food and 0 mg liter Ϫ1 toxin (a surfactant, sodium dodecyl sulfate). Suppressive effects of stressors examined individually ranked: high temperature (30ЊC) Ͼ SDS (10 mg liter Ϫ1 Ն low food (ϳ100 g C liter Ϫ1) Ͼ low temperature (20ЊC). Daphnia pulex isolated from a pond which experiences high summer temperatures throughout was more tolerant of 30ЊC 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.In recent years, the list of chemical, physical, and biological stressors considered to be potentially dangerous to the environment has grown rapidly. Scientific and conservation organizations have urged scientists, managers, and policy makers to consider the ecological effects of stressors for proper regulation and management of natural resources (e.g., Naiman et al. 1995;Arnell et al. 1996). Evaluating systems that are impacted anthropogenically is particularly challenging, because there are many possible stressors eliciting a variety of potential effects. Importantly, effects of environmental stressors are usually tested individually (Vouk et al. 1987), but in nature, organisms often are exposed to several stressors simultaneously (Schindler et al. 1996;Yan et al. 1996). This raises the fundamental question addressed by our study: Are stressors more harmful in combination than alone?To answer this question we compared three models, com- AcknowledgmentsWe are grateful to Ashley Mattoon, Heather Hamilton, Christy Jackson, Regina Henry, Janis Hall, Karen Baumgartner, and Matt Royer for assisting with the life table experiments and maintaining cultures. We also thank Jim Dykes for statistical assistance, Clyde Goulden for his helpful discussions about a number of relevant issues, Denise Breitburg and two anonymous reviewers for useful comments on the manuscript, Paul Pickhardt fo...

High-resolution data collected over the past 60 years by a single family of Siberian scientists on Lake Baikal reveal significant warming of surface waters and long-term changes in the basal food web of the world's largest, most ancient lake. Attaining depths over 1.6 km, Lake Baikal is the deepest and most voluminous of the world's great lakes. Increases in average water temperature (1.21 °C since 1946), chlorophyll a (300% since 1979), and an influential group of zooplankton grazers (335% increase in cladocerans since 1946) may have important implications for nutrient cycling and food web dynamics. Results from multivariate autoregressive (MAR) modeling suggest that cladocerans increased strongly in response to temperature but not to algal biomass, and cladocerans depressed some algal resources without observable fertilization effects. Changes in Lake Baikal are particularly significant as an integrated signal of long-term regional warming, because this lake is expected to be among those most resistant to climate change due to its tremendous volume. These findings highlight the importance of accessible, long-term monitoring data for understanding ecosystem response to large-scale stressors such as climate change.

BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research.

No abstract

No abstract