Presence Of Multiple Stressors Research Articles

Assessing the Role of Sports Participation on Depression, Suicide Ideation, and Suicide Behaviors Among Adolescents Before and During COVID-19.

To assess the association between sports participation, depression, suicide ideation, and suicide behaviors in a nationally representative sample of US adolescents before COVID-19 and during COVID-19. Data from the 2019 Youth Risk Behavior Survey (n = 13,526) and the 2021 Adolescent Behaviors and Experiences Survey (n = 7677) were used to analyze the association between past-year depression/suicide ideation/suicide behaviors and past-year sports participation. The analysis found that 57.4% of adolescents indicated participating in at least 1 sport in 2019; this dropped to 47.7% in 2021. Furthermore, 36.7% of adolescents indicated feeling sad or hopeless almost every day for 2 weeks or more in a row in 2019; this increased to 44.2% in 2021. The percentage of adolescents who indicated considering suicide, making a suicide plan, attempting suicide, and attempted suicide that lead to an injury was similar during 2019 and 2021. Multivariable analysis found that participation in 2 or more sports in 2019 was associated with lower odds of each of the outcomes for depression, suicide ideation, and suicide behaviors, whereas in 2021, participation in 2 or more sports was only associated with lower odds of indicating being sad or hopeless (an indicator for depression) for a 2-week period (adjusted odds ratios = 0.70; 95% confidence interval, 0.58-0.85). Participation in 2 or more sports lowered the risk of feeling sad or hopeless, suicide ideation, and suicide behaviors in 2019, but this effect was absent in 2021. Given the presence of multiple stressors during the COVID-19 pandemic, sports participation alone may not offer sufficient protective effects against suicide behaviors as it did pre pandemic.

Multispecies assemblages and multiple stressors: Synthesizing the state of experimental research in freshwaters

AbstractRecent decades have witnessed a sharp biodiversity decline in freshwaters due to multiple stressors. The presence of multiple stressors is expected to affect community structure and interactions in freshwater ecosystems, with subsequent functional consequences. We synthesized the state of experimental, manipulative multiple‐stressor studies that focused on multispecies assemblages in freshwaters. Compared to rivers and lakes, wetland and groundwater ecosystems have received much less attention in identified multiple‐stressor research. Most of the identified studies investigated combinations of abiotic stressors (e.g., nutrients, pesticides, heavy metals, warming, altered flow and sedimentation) on microbes and invertebrates while biotic stressors and vertebrates have been largely overlooked. The responses of community structure (e.g., alpha diversity, biomass, and abundance), some community/ecosystem functions (e.g., photosynthesis and autotrophic activity, leaf litter degradation), and morphological traits like body size and growth forms were frequently investigated. We observed a clear gap in biotic interactions under multiple‐stressor conditions, which, although difficult to study, could impede a deeper mechanistic understanding of how multiple stressors affect freshwater assemblages and associated ecological processes. Although information on ecosystem recovery pathways following restoration is critical for freshwater management, few studies were designed to provide such information, signifying the disconnections between multiple‐stressor research and environmental practice. To bridge these gaps, researchers and environmental practitioners need to work together to identify key stressors and interactions at different spatial and temporal scales and prioritize stressor management. Such collaborations will enhance the translation of multiple‐stressor research into efficient management strategies to protect and restore freshwater ecosystems.This article is categorized under: Water and Life > Stresses and Pressures on Ecosystems Water and Life > Conservation, Management, and Awareness

Management thresholds shift under the influence of multiple stressors: Eelgrass meadows as a case study

AbstractAs human activities increase in intensity and extent, ecosystems face growing threats from multiple stressors. Successful management requires identifying measurable targets, which is challenging because of data limitations, nonlinear ecosystem responses, and potentially shifting targets under multiple stressors. To identify critical management values and determine whether these values shift in the presence of multiple stressors, we use eelgrass (Zostera marina) meadows as a model system. We reviewed 20 studies that measured the effects of light and temperature on eelgrass performance, providing 109 unique study–site–treatment combinations. We modeled the interactive effect of temperature and light on eelgrass population growth rate (i.e., lateral shoot production rates) using a hierarchical generalized additive model and predicted population growth rates across a range of light levels and temperatures. We found that two critical performance metrics of population growth, zero‐growth and maximum growth rates, shifted across a gradient of light and temperature, suggesting that fixed management targets linked to population growth rates might be unsuitable for managing meadows under multiple stressors. Our approach bridges the gap between data from laboratory and field studies and could be developed into an interactive management tool.

Physiological and behavioural strategies of aquatic animals living in fluctuating environments.

Shallow or near-shore environments, such as ponds, estuaries and intertidal zones, are among the most physiologically challenging of all aquatic settings. Animals inhabiting these environments experience conditions that fluctuate markedly over relatively short temporal and spatial scales. Living in these habitats requires the ability to tolerate the physiological disturbances incurred by these environmental fluctuations. This tolerance is achieved through a suite of physiological and behavioural responses that allow animals to maintain homeostasis, including the ability to dynamically modulate their physiology through reversible phenotypic plasticity. However, maintaining the plasticity to adjust to some stresses in a dynamic environment may trade off with the capacity to deal with other stressors. This paper will explore studies on select fishes and invertebrates exposed to fluctuations in dissolved oxygen, salinity and pH. We assess the physiological mechanisms these species employ to achieve homeostasis, with a focus on the plasticity of their responses, and consider the resulting physiological trade-offs in function. Finally, we discuss additional factors that may influence organismal responses to fluctuating environments, such as the presence of multiple stressors, including parasites. We echo recent calls from experimental biologists to consider physiological responses to life in naturally fluctuating environments, not only because they are interesting in their own right but also because they can reveal mechanisms that may be crucial for living with increasing environmental instability as a consequence of climate change.

Growth, metabolism, and dissolved oxygen tolerance of juvenile burbot

The burbot, Lota lota, is being explored as a novel aquaculture species in North America due to its palatability and potential to be grown in areas that currently support rainbow trout, Oncorhynchus mykiss culture. Dissolved oxygen (DO) concentration is a primary limiting factor in fish survival and growth, with tolerance varying among species. We exposed juvenile burbot (19.5 ± 2.2 g) to five dissolved oxygen concentrations (5.0, 5.8, 6.6, 7.4, and 8.3 (control) mg·L −1) for 9 weeks at 15 °C. Size variability was high in all treatments, and food consumption, and growth rates did not differ among DO levels, although fish at 8.3 mg·L−1 grew ca. 29% larger than those at 5.0 mg·L−1. We also measured short-term hypoxia tolerance and resting routine oxygen consumption rates (MO2) of burbot that had been chronically acclimated to the same DO concentrations. Burbot acclimated to 8.3 mg·L−1 lost equilibrium (LOEcrit) at a significantly higher concentration (1.85 ± 0.33 mg·L −1) than that of the 7.4 and 6.6 mg·L −1 acclimated fish (1.50 ± 0.37 and 1.49 ± 0.27; respectively), while all other groups were intermediate (1.67 ± 0.28). The MO2s were not statistically distinguishable among acclimation groups (p-value = 0.25), but MO2 trended lower with decreasing DO concentrations. In summary, juvenile burbot are tolerant to DO concentrations down to 5.0 mg·L −1. We recommend rearing burbot at DO concentrations >7.0 mg·L −1 to optimize growth, especially with the presence of multiple stressors.

Effect of Combined Stressors on C. elegans Lifespan

Cellular stress is a fundamental component of age-associated disease. Cells experience many forms of stress (oxidative, heavy metal, etc.), and as we age the burden of stress and resulting damage increases while our cells’ ability to deal with the consequences becomes diminished due to dysregulation of cellular stress response pathways. By understanding how cells respond to stress we aim to slow age-associated deterioration and develop treatment targets for age-associated disease. The majority of past work has focused on understanding responses to individual stressors. In contrast, how pathology and stress responses differ in the presence of multiple stressors is relatively unknown; we investigate that here. We cultured worms on agar plates with different combinations of arsenic, copper, and DTT (which create oxidative/proteotoxic, heavy metal, and endoplasmic reticulum (ER) stress, respectively) at doses that result in 20% lifespan reduction individually and measured the effect on lifespan. We found that arsenic/copper and arsenic/DTT combinations created additive lifespan reductions while the copper/DTT combination created an antagonistic lifespan reduction when compared to controls (p<0.05). This antagonistic toxicity suggests an interaction either between the mechanisms of toxicity or the cellular response to copper and DTT. We are now evaluating the impact of copper and DTT individually and in combination on unfolded protein and heavy metal response pathways to understand the underlying mechanism of the interaction. Additionally, we are continuing to screen stressors to identify combinations that cause non-additive (synergistic or antagonistic) toxicity to build a comprehensive model of the genetic stress response network in C. elegans.

Microplastics shift impacts of climate change on a plant-microbe mutualism: Temperature, CO2, and tire wear particles

Anthropogenic stressors can affect individual species and alter species interactions. Moreover, species interactions or the presence of multiple stressors can modify the stressor effects, yet most work focuses on single stressors and single species. Plant-microbe interactions are a class of species interactions on which ecosystems and agricultural systems depend, yet may be affected by multiple global change stressors. Here, we use duckweed and microbes from its microbiome to model responses of interacting plants and microbes to multiple stressors: climate change and tire wear particles. Climate change is occurring globally, and microplastic tire wear particles from roads now reach many ecosystems. We paired perpendicular gradients of temperature and carbon dioxide (CO2) treatments with factorial manipulation of leachate from tire wear particles and duckweed microbiomes. We found that tire leachate and warmer temperatures enhanced duckweed and microbial growth, but caused effects of microbes on duckweed to become negative. However, induced negative effects of microbes were less than additive with warming and leachate. Without tire leachate, we observed that higher CO2 and temperature induced positive correlations between duckweed and microbial growth, which can strengthen mutualisms. In contrast, with tire leachate, growth correlations were never positive, and shifted negative at lower CO2, again suggesting leachate disrupts this plant-microbiome mutualism. In summary, our results demonstrate that multiple interacting stressors can affect multiple interacting species, and that leachate from tire wear particles could potentially disrupt plant-microbe mutualisms.

The power of large-scale community science in addressing anthropogenic change.

The human imprint on the global environment has become so pervasive that it rivals the forces of nature in its impact on ecosystem function. We are now living in the Anthropocene with increased air, light, noise, and heavy metal pollution (Foley et al., 2005). Understanding the link between anthropogenic stressors and functional trait characteristics will be vital to predict species response and adaptability to environmental change. Especially as urban areas increase, where anthropogenic changes are highly concentrated and overlapping, investigating how multiple stressors interact to affect individuals and populations is a current research priority. Wilson et al. (this issue) levy the power of human interest in wildlife to study the effects of human-generated pollution (2021). The authors provide a timely and novel continent-level study that addresses the interaction of light and noise pollution on species abundance and response using community science.

Managing the trifecta of disease, climate, and contaminants: Searching for robust choices under multiple sources of uncertainty

Wood frogs, like other amphibian species worldwide, are experiencing population declines due to multiple stressors. In the northeastern United States, wood frog declines are thought to result from a reduction in successful metamorphosis in part due to climate change, disease (specifically ranavirus), and contaminant exposure. The presence of multiple stressors can increase uncertainty in characterizing the main effects of each stressor, as well as understanding the degree to which their effects interact (additively or synergistically) to impact populations. This uncertainty adds inherent challenges to selecting appropriate management actions for conserving populations. Finding solutions that are robust to these uncertainties can improve management amid absent or equivocal knowledge. We used a Bayesian Belief Network (BBN), a quantitative tool that allowed us to evaluate how potential management actions might mitigate the effects of increasingly frequent and severe droughts, ranavirus exposure, and methylmercury on wood frog populations in the northeastern U.S. In our system, successful wood frog recruitment was largely driven by hydroperiod regardless of other stressors. Our modelling indicated that increased hydroperiod lowered the probability of complete metamorphosis failure from 0.6 to 0.37, suggesting that under the conditions tested in the model, pond hydrology is more important for successful recruitment than either methylmercury or ranavirus exposure. As more information becomes available on stressor interactions, model scenarios could be re-evaluated, and management options reconsidered.

Nutrient management and structural shifts in fish assemblages: Lessons learned from an Area of Concern in Lake Ontario

Abstract While total phosphorus (TP) is a critical determinant of freshwater ecosystem productivity, multiple stressors can induce shifts in energy pathways, with profound implications for ecosystem and fishery restoration. The Bay of Quinte (Lake Ontario, Canada) is a Great Lakes nearshore ecosystem that has been historically subjected to a variety of environmental perturbations: cultural eutrophication, low dissolved oxygen, reduced fisheries, climatic extremes, phosphorus (P) abatement, and aquatic invasive species. We used the Bay of Quinte to study how trophic state alterations affect fish assemblages in Great Lakes nearshore environments by examining the response of fish biomass to TP concentration variability in the presence of multiple stressors. Our analysis is based on a 42‐year (1972–2013) dataset from the Bay of Quinte for water quality through the food web to fishes. We employed a series of statistical tools that can offer insights into the structural changes induced by the events examined. We first used dynamic linear modelling to detect temporal trends in fish biomass, while accounting for year‐to‐year TP variability over three spatial segments of the bay. We then developed piecewise regression models to assess the extent to which specific ecological events induced distinct shifts in the fish assemblage. Multiple regression modelling was used to quantify the relative importance of TP, zooplankton, and surface water temperature on fish biomass. Based on gillnets, there were consistent fish biomass changes across the bay with increased biomass before P control (1972–1977), declines after P control followed by the establishment of a steady state or modest increase (1978–1994), and a declining trajectory during the recent period (1995–2013). Even when accounting for the role of water temperature and zooplankton, TP still had a significant effect on fish biomass. However, the strength and nature of the relationship varied among fish groups, and overall, the effect of TP on fish biomass has weakened in recent years. Our models show that fish biomass in the Bay of Quinte is shaped by the year‐to‐year TP variability. However, the relationship between P and fish abundance has been modulated by various ecological events with the consequence that the Bay of Quinte fish assemblage has changed and the food web now produces less fish biomass per unit of TP. A projected reduction of mean ambient TP levels from 30 to 25 μg/L, is expected to induce a 24% decline in total fish biomass, and further shift the fish assemblage with the biomass of planktivores and walleye declining by &gt;60% and 30%, respectively. Recreational fishing provides important economic benefits in the Bay of Quinte, through tourism and other local business operations. Recognising the economic importance of fishing, our analysis provides critical insights regarding the on‐going management efforts to reduce external nutrient loadings (point and non‐point sources, urban storm water) and further lower ambient TP levels and primary productivity. The potential implications of already low nutrient concentrations for fish productivity represent a major challenge in effectively balancing water quality with fisheries management in the area.

Stem Cell Regulation During Chronic Infection

Infections and chronic inflammatory conditions affect blood production and are frequently associated with peripheral cytopenias. Common examples include tuberculosis and atypical mycobacterial infections, HIV, hepatitis C, rheumatoid arthritis, and inflammatory bowel disease. Low blood counts amplify morbidity and mortality in each of these conditions. Recent studies have contributed to a better understanding of the pathophysiologic conditions linking inflammation and bone marrow suppression. While studies to investigate the effects of chronic infection on human hematopoietic stem and progenitor cells (HSPCs) are limited in number and scope, we and others have used animal models to evaluate the effects of infection and inflammation on HSPCs. During persistent Mycobacterium avium infection of mice, hematopoietic stem cells (HSCs) exited from quiescence and differentiated at a high rate. The total number of HSCs decreased by approximately 95% in a period of 4 months, even in the absence of widespread myelofibrosis, and the animals developed pancytopenia. Physiologic changes during infection, including loss of quiescence, diminished self-renewal, and increased differentiation, could largely be recapitulated by administration of the inflammatory cytokine interferon gamma (IFNg) alone. Indeed, studies from our lab and others collectively indicate that inflammatory cytokines are key drivers of hematopoietic changes in chronic infections and inflammatory conditions. Cytokines including interferon alpha, IFNg, IL1, IL6 and TNFalpha and pathogen-associated molecular patterns such as LPS have all been shown to increase HSC division and reduce their self-renewal. In our system, the major mechanism of stem cell loss was through excessive terminal differentiation; however, the presence of multiple stressors may also increase replication stress and propensity for apoptosis in these cells. Recent studies further indicate that inflammatory signaling may alter the number and function of cells that are produced by HSPCs, through selective activation of HSPC subtypes or epigenetic reprogramming. Collectively, the effects of inflammatory signaling on HSPC self-renewal and differentiation have important clinical implications in a number of disease states. Excessive inflammatory signaling is associated with acquired aplastic anemia and myelodysplastic syndrome; therefore, identifying the molecular mechanisms by which inflammatory cytokines such as IFNg drive terminal differentiation and HSC loss may yield strategies to modify disease progression for these syndromes. Recent work showed that inflammation critically drives Tet2-associated clonal hematopoiesis; the role of inflammation in clonal hematopoiesis and malignant transformation more broadly remains an active area of investigation. Finally, the concept that inflammatory signaling may alter the lineage output of HSPCs, either through selective or directive differentiation, may aid in the understanding of immune responses to infectious diseases, with potential therapeutic implications. DisclosuresNo relevant conflicts of interest to declare.

Adult exposure to ocean acidification is maladaptive for larvae of the Sydney rock oyster Saccostrea glomerata in the presence of multiple stressors.

Parental effects passed from adults to their offspring have been identified as a source of rapid acclimation that may allow marine populations to persist as our surface oceans continue to decrease in pH. Little is known, however, whether parental effects are beneficial for offspring in the presence of multiple stressors. We exposed adults of the oyster Saccostrea glomerata to elevated CO2 and examined the impacts of elevated CO2 (control = 392; 856 µatm) combined with elevated temperature (control = 24; 28°C), reduced salinity (control = 35; 25) and reduced food concentration (control = full; half diet) on their larvae. Adult exposure to elevated CO2 had a positive impact on larvae reared at elevated CO2 as a sole stressor, which were 8% larger and developed faster at elevated CO2 compared with larvae from adults exposed to ambient CO2 These larvae, however, had significantly reduced survival in all multistressor treatments. This was particularly evident for larvae reared at elevated CO2 combined with elevated temperature or reduced food concentration, with no larvae surviving in some treatment combinations. Larvae from CO2-exposed adults had a higher standard metabolic rate. Our results provide evidence that parental exposure to ocean acidification may be maladaptive when larvae experience multiple stressors.

Additive effects of the herbicide glyphosate and elevated temperature on the branched coral Acropora formosa in Nha Trang, Vietnam

The combined effects of the herbicide glyphosate and elevated temperature were studied on the tropical staghorn coral Acropora formosa, in Nha Trang bay, Vietnam. The corals were collected from two different reefs, one close to a polluted fish farm and one in a marine-protected area (MPA). In the laboratory, branches of the corals were exposed to the herbicide glyphosate at ambient (28 °C) and at 3 °C elevated water temperatures (31 °C). Effects of herbicide and elevated temperature were studied on coral bleaching using photography and digital image analysis (new colorimetric method developed here based on grayscale), chlorophyll a analysis, and symbiotic dinoflagellate (Symbiodinium, referred to as zooxanthellae) counts. All corals from the MPA started to bleach in the laboratory before they were exposed to the treatments, indicating that they were very sensitive, as opposed to the corals collected from the more polluted site, which were more tolerant and showed no bleaching response to temperature increase or herbicide alone. However, the combined exposure to the stressors resulted in significant loss of color, proportional to loss in chlorophyll a and zooxanthellae. The difference in sensitivity of the corals collected from the polluted site versus the MPA site could be explained by different symbiont types: the resilient type C3u and the stress-sensitive types C21 and C23, respectively. The additive effect of elevated temperatures and herbicides adds further weight to the notion that the bleaching of coral reefs is accelerated in the presence of multiple stressors. These results suggest that the corals in Nha Trang bay have adapted to the ongoing pollution to become more tolerant to anthropogenic stressors, and that multiple stressors hamper this resilience. The loss of color and decrease of chlorophyll a suggest that bleaching is related to concentration of chloro-pigments. The colorimetric method could be further fine-tuned and used as a precise, non-intrusive tool for monitoring coral bleaching in situ.

Counseling People Displaced by War: Experiences of Refugees from the Former Yugoslavia

The purpose of this qualitative study was to explore the lived experiences of refugees from the former Yugoslavia who migrated to the United States as a result of the civil wars in the 1990s. The present research utilized a phenomenological method, in which the researchers collected data using in-depth interviews with 10 participants; analyzed the data themes relating to the pre- and post-migration experiences; and documented high rates of exposure to war-related violence and the presence of multiple stressors during resettlement. The study offers an integration of the collective essence and meaning of refugees' experiences. Findings suggested that being a refugee and resettling in a new country constitute a complex and life-changing process. Overall, the results indicated that the migration process for refugees from the former Yugoslavia was modulated by stressors during the war, migration and resettlement. The study concludes with a discussion of implications for counseling practice and counselor education.Keywords: civil wars, refugees, migration, Yugoslavia, resettlement, stressorsViolent conflicts throughout the world have left millions of people displaced, some within their own country (the internally displaced) and some across international borders (refugees). The United Nations High Commissioner for Refugees (UNHCR) reported that in 2010,43.7 million people from over 125 countries were forcibly displaced from their homes by civil or interstate war (2011), and that the expense of meeting needs was nearly $2 billion. The psychological impact of war has been widely acknowledged and well documented (Miller & Rasco, 2004; Miller, Weine, et al, 2002; Mollica, 2006; Murthy & Lakshminarayana, 2006; Porter & Haslam, 2001; Summerfield, 2003; van den Heuvel, 1998).The most frequently reported consequence of war exposure is post-traumatic stress disorder (PTSD), followed by depression, recurrent nightmares, insomnia, chronic hyperarousal, impaired concentration and irritability (Miller & Rasco, 2004). It has been estimated that 50% of refugees experienced higher levels of PTSD, depression and other psychiatric problems (de Jong, Scholte, Koeter, & Hart, 2000), with the lifetime prevalence of PTSD among specific groups of trauma survivors ranging from 15%-24%, compared to 8% in the general United States population (de Jong et al., 2001). PTSD is associated with long-term physical health problems, higher mortality rates and heart disease (Hamblen & Schnurr, 2007).One country in particular, the former Socialist Federative Republic of Yugoslavia, experienced similar issues. The breakup of Yugoslavia in the 1990s produced an estimated 992,200 refugees and 1,203,000 internally displaced people, which constituted 3% of the total population (U.S. Department of Health & Human Services [USDHHS], 1999). From 1983-1999, the United States accepted an estimated 200,000 refugees from the former Yugoslavia (USDHHS, 1999). Furthermore, in 1998 and 1999, refugees from the former Yugoslavia were the largest refugee group admitted into the United States, representing 36% of all arrivals (USDHHS, 1999).Consistent with other wars, the hardships in the former Yugoslavia were particularly acute for women, children and the elderly (Weiss & Pasic, 1998). Thousands of people suffered through war trauma, persecution, torture, abrupt and sometimes repeated displacements, physical violence against themselves or their family, rape and other forms of sexual violence. As a result of this exposure, refugees from the former Yugoslavia have reported high rates of depressive symptoms, PTSD and other trauma-related issues (Porter & Haslam, 2001; Vojvoda, Weine, McGlashan, Becker, & Southwick, 2008; Weine et al., 1998).Purpose of StudyResettling in the United States is not an easy process, and many refugees experience numerous problems in their new host country. Some of these problems include poverty, illiteracy, prolonged dependence on government aid, cultural differences, social isolation, the language barrier, and loss of status (Carlson & Rosser-Hogan, 1993, p. …

On the incongruity between developmental plasticity and methodological rigidity

OPINION article Front. Behav. Neurosci., 17 January 2013Sec. Pathological Conditions https://doi.org/10.3389/fnbeh.2012.00093

Trade-offs between drug toxicity and benefit in the multi-antibiotic resistance system underlie optimal growth of E. coli

BackgroundEfflux is a widespread mechanism of reversible drug resistance in bacteria that can be triggered by environmental stressors, including many classes of drugs. While such chemicals when used alone are typically toxic to the cell, they can also induce the efflux of a broad range of agents and may therefore prove beneficial to cells in the presence of multiple stressors. The cellular response to a combination of such chemical stressors may be governed by a trade-off between the fitness costs due to drug toxicity and benefits mediated by inducible systems. Unfortunately, disentangling the cost-benefit interplay using measurements of bacterial growth in response to the competing effects of the drugs is not possible without the support of a theoretical framework.ResultsHere, we use the well-studied multiple antibiotic resistance (MAR) system in E. coli to experimentally characterize the trade-off between drug toxicity (“cost”) and drug-induced resistance (“benefit”) mediated by efflux pumps. Specifically, we show that the combined effects of a MAR-inducing drug and an antibiotic are governed by a superposition of cost and benefit functions that govern these trade-offs. We find that this superposition holds for all drug concentrations, and it therefore allows us to describe the full dose–response diagram for a drug pair using simpler cost and benefit functions. Moreover, this framework predicts the existence of optimal growth at a non-trivial concentration of inducer. We demonstrate that optimal growth does not coincide with maximum induction of the mar promoter, but instead results from the interplay between drug toxicity and mar induction. Finally, we derived and experimentally validated a general phase diagram highlighting the role of these opposing effects in shaping the interaction between two drugs.ConclusionsOur analysis provides a quantitative description of the MAR system and highlights the trade-off between inducible resistance and the toxicity of the inducing agent in a multi-component environment. The results provide a predictive framework for the combined effects of drug toxicity and induction of the MAR system that are usually masked by bulk measurements of bacterial growth. The framework may also be useful for identifying optimal growth conditions in more general systems where combinations of environmental cues contribute to both transient resistance and toxicity.

Variability in responses to nutrients and trace elements, and transmission of stressor effects through an estuarine food web

Aquatic systems are increasingly exposed to multiple stressors from anthropogenic sources. These stressors can vary in the consistency and magnitude of responses they elicit in biota and in how the presence of additional stressors modifies their effects. Understanding how the biological environment and temporal dynamics influence responses to stressors, and how stressors interact, is important to predicting their effects in the natural environment. We examined temporal variability in responses of an experimental estuarine food web to elevated trace elements and nutrients, as well as non‐additive effects of the combination of these two stressors. Experiments were conducted four times during spring through autumn 1996 in 20 l‐m3 mesocosms. We measured a range of system‐, population‐, and individual‐level parameters to quantify responses of phytoplankton, bacterioplankton, heterotrophic nanoflagellates, copepods, fish, and benthic invertebrates to trace element and nutrient additions. The response to trace element additions was more variable both temporally and among phytoplankton and higher trophic level taxa than was the response to nutrient additions. Most taxa increased, either significantly or showed a trend toward increasing, in response to nutrient additions in all four mesocosm runs. In contrast, the direction as well as the magnitude of responses to trace element additions varied considerably among taxa and experimental runs. Two distinct types of nutrient3trace element interactions were important. First, temporal dynamics of nutrient ratios appeared to affect the temporal pattern of toxicity of trace elements to phytoplankton. Second, in the June mesocosm run when trace element additions reduced production, abundance, or growth of many organisms, these reductions were often proportionately greater in nutrient addition tanks than where no nutrients were added. Our results suggest that considerable temporal and taxonomic variation in responses to trace element loadings are likely to be seen in field settings even under constant loadings to the system and that trace elements may mask the magnitude of the response to high nutrient loadings in eutrophic systems. More generally, the presence of multiple stressors may either increase or dampen the temporal and spatial variability seen in aquatic systems, depending on the interactions among stressors and the influence of background environmental conditions and sensitive species on the expression of stressor effects.

A holistic approach to ecosystem health assessment using fish population characteristics

The status of a fish population is a reflection of the overall condition of the aquatic environment in which that population resides. As such, fish population characteristics can be used as indicators of environmental health. Simple and inexpensive methods to follow fish population responses to environmental degradation are lacking. This paper outlines a protocol whereby environmental impacts on fish populations are classified by five patterns based on characteristics such as mean age, fecundity and condition factor. The patterns summarize population changes and describe responses to exploitation, recruitment failure, the presence of multiple stressors, food limitation and niche shifts. Classification is best based on the selection, and appropriate sampling, of a comparable reference population. Population characteristics can be used to examine ecosystems exposed to stressors for evidence of long-term damage, and when used with biochemical indicators, can be a powerful tool for ecosystem health assessment. The five responses are illustrated using published data on a number of species challenged by increased predation pressure, acidification, eutrophication, mine waste and reservoir impoundment. Application of this scheme will aid in directing and focusing research efforts on crucial aspects impacted by changing conditions.