Interactive effects of multiple stressors vary with consumer interactions, stressor dynamics and magnitude

Predicting the impacts of multiple stressors is important for informing ecosystem management, but is impeded by a lack of a general framework for predicting whether stressors interact synergistically, additively, or antagonistically. Here we use process-based models to study how interactions generalise across three levels of bio-logical organisation (physiological, population, and community) for a simulated two-stressor experiment on a seagrass model system. We found that the same underlying processes could result in synergistic, additive or antagonistic interactions, with interaction type depending on initial conditions, experiment duration, stressor dynamics, and consumer presence. Our results help explain why meta-analyses of multiple stressor experimental results have struggled to identify predictors of consistently non-additive interactions in the natural environment. Experiments run over longer temporal scales, with treatments across gradients of stressor magnitude, are needed to identify the processes that underpin how stressors interact and provide useful predictions to management.

Experimental evolution reveals the synergistic genomic mechanisms of adaptation to ocean warming and acidification in a marine copepod

Metazoan adaptation to global change will rely on selection of standing genetic variation. Determining the extent to which this variation exists in natural populations, particularly for responses to simultaneous stressors, is therefore essential to make accurate predictions for persistence in future conditions. Here, we identify the genetic variation enabling the copepod Acartia tonsa to adapt to experimental ocean warming, acidification, and combined ocean warming and acidification (OWA) conditions over 25 generations. Replicate populations showed a strong and consistent polygenic response to each condition, targeting an array of adaptive mechanisms including cellular homeostasis, development, and stress response. We used a genome-wide covariance approach to partition the genomic changes into selection, drift, and lab adaptation and found that the majority of allele frequency change in warming (56%) and OWA (63%) was driven by selection but acidification was dominated by drift (66%). OWA and warming shared 37% of their response to selection but OWA and acidification shared just 1%. Accounting for lab adaptation was essential for not inflating a shared response to selection between all treatments. Finally, the mechanisms of adaptation in the multiple-stressor OWA conditions were not an additive product of warming and acidification, but rather a synergistic response where 47% of the allelic responses to selection were unique. These results are among the first to disentangle how the genomic targets of selection differ between single and multiple stressors and to demonstrate the complexity that non-additive multiple stressors will contribute to attempts to predict adaptive responses to complex environments.

Stressor interactions under differential exogenous microbial exposure in Daphnia magna_revised

Studies on stressor responses are often performed in controlled laboratory settings. The microbial communities in laboratory settings often differ from the natural environment, which could ultimately be reflected in different stress responses. In this study, we investigated the impact of single versus simultaneous multiple stressor exposure on Daphnia magna life history traits and whether this tolerance was microbiome-mediated. Daphnia individuals were exposed to the toxic cyanobacterium Microcystis aeruginosa and a fungal infection, Aspergillus aculeatus like type. Three genotypes were included to investigate genotype-specific responses. Survival, reproduction and body size were monitored for three weeks and gut microbial communities were sampled and characterized at the end of the experiment. Our study shows survival in Daphnia was microbiome-mediated as survival was only negatively impacted when Daphnia received a lab microbial community. Daphnia which received a natural microbial community have a broader environmental pool of microbiota to randomnly and selectively take up and showed no negative impact on survival. Simultaneous exposure to both stressors also revealed an antagonistic interaction for survival. Fecundity and body size were negatively impacted by exposure to stress, however, responses were here not microbiome-mediated. In addition, genotype specific responses were detected for survival and fecundity, which could be linked with the selective capabilities of the Daphnia genotypes to select beneficial or neutral microbial stains from the environment.

A Brief Perspective on Environmental Science in the Anthropocene: Recalibrating, Rethinking and Re-Evaluating to Meet the Challenge of Complexity

A convincing case has been made that the scale of human activity has reached such pervasiveness that humans are akin to a force of nature. How environmental science responds to the many new challenges of the Anthropocene is at the forefront of the field. The aim of this perspective is to describe Anthropocene as a concept and a time period and discuss its relevance to the contemporary study of environmental science. Specifically, we consider areas in environmental science which may need to be revisited to adjust to complexity of the new era: (a) recalibrate the idea of environmental baselines as Anthropogenic baselines; (b) rethink multiple stressor approaches to recognize a system under flux; (c) re-evaluate the relationship of environmental science with other disciplines, particularly Earth Systems Science, but also social sciences and humanities. The all-encompassing nature of the Anthropocene necessitates the need to revise and reorganize to meet the challenge of complexity.

Another Decade of Marine Climate Change Experiments: Trends, Progress and Knowledge Gaps

Anthropogenic climate change is a significant driver of change in marine ecosystems globally. To improve mechanistic understanding of the impact of climate-related stressors, experimental work on marine organisms has intensified in recent decades. A previous synthesis paper published nearly a decade ago established that Marine Climate Change Experiments (MCCEs) published from 2000–2009 were primarily laboratory-based and focused on single stressors and individual focal temperate species. Using consistent methodology, we compared the 2000–2009 analysis to experiments published in the following decade (i.e. 2010–2019) to assess recent trends in MCCEs and to determine to what extent knowledge gaps and research priorities have been addressed. The search returned 854 papers, vs. 110 from the 2000s, indicating considerable intensification of research effort and output. We found again that single species studies were most common, particularly with benthic invertebrates as model organisms, and that laboratory-based research comprised over 90% of all studies. However, multiple stressor experiments increased substantially, where tests for interaction effects between ocean acidification (i.e., increased pCO2) and warming were particularly common. Furthermore, a wider range of model species were studied and more community-level experiments were conducted in the 2010s compared with the 2000s. In addition, studies on behavioral responses, transgenerational effects, genetic adaptation and extreme climatic events increased markedly. These recent advances in MCCEs have undoubtedly improved understanding of how climate change will affect marine organisms and the communities and ecosystems they underpin. Going forward, biases in the type and distribution of model organisms should be addressed to enhance general understanding of responses to environmental change. Similarly, experiments should manipulate a greater number and range of climate and non-climate factors and increase the number of target organisms to increase realism. Finally, where possible, further research should be combined and contextualized with field-based experiments and observations to better reflect the complexity of marine ecosystems and yield more representative responses to ocean climate change.

Cyanobacteria Microcystis aeruginosa Contributes to the Severity of Fish Diseases: A Study on Spring Viraemia of Carp

Fish are exposed to numerous stressors in the environment including pollution, bacterial and viral agents, and toxic substances. Our study with common carps leveraged an integrated approach (i.e., histology, biochemical and hematological measurements, and analytical chemistry) to understand how cyanobacteria interfere with the impact of a model viral agent, Carp sprivivirus (SVCV), on fish. In addition to the specific effects of a single stressor (SVCV or cyanobacteria), the combination of both stressors worsens markers related to the immune system and liver health. Solely combined exposure resulted in the rise in the production of immunoglobulins, changes in glucose and cholesterol levels, and an elevated marker of impaired liver, alanine aminotransferase (ALT). Analytical determination of the cyanobacterial toxin microcystin-LR (MC-LR) and its structurally similar congener MC-RR and their conjugates showed that SVCV affects neither the levels of MC in the liver nor the detoxification capacity of the liver. MC-LR and MC-RR were depurated from liver mostly in the form of cysteine conjugates (MC-LR-Cys, MC-RR-Cys) in comparison to glutathione conjugates (LR-GSH, RR-GSH). Our study brought new evidence that cyanobacteria worsen the effect of viral agents. Such inclusion of multiple stressor concept helps us to understand how and to what extent the relevant environmental stressors co-influence the health of the fish population.

Multiple stressor null models frequently fail to detect most interactions due to low statistical power

1. Most ecosystems are subject to co-occurring, anthropogenically driven changes and understanding how these multiple stressors interact is a pressing concern. Stressor interactions are typically studied using null models, with the additive and multiplicative null expectation being those most widely applied. Such approaches classify interactions as being synergistic, antagonistic, reversal, or indistinguishable from the null expectation. Despite their wide-spread use, there has been no thorough analysis of these null models, nor a systematic test of the robustness of their results to sample size or sampling error in the estimates of the responses to stressors. 2. We use data simulated from food web models where the true stressor interactions are known, and analytical results based on the null model equations to uncover how (i) sample size, (ii) variation in biological responses to the stressors and (iii) statistical significance, affect the ability to detect non-null interactions. 3. Our analyses lead to three main results. Firstly, it is clear the additive and multiplicative null models are not directly comparable, and over one third of all simulated interactions had classifications that were model dependent. Secondly, both null models have weak power to correctly classify interactions at commonly implemented sample sizes (i.e., ≤6 replicates), unless data uncertainty is unrealistically low. This means all but the most extreme interactions are indistinguishable from the null model expectation. Thirdly, we show that increasing sample size increases the power to detect the true interactions but only very slowly. However, the biggest gains come from increasing replicates from 3 up to 25 and we provide an R function for users to determine sample sizes required to detect a critical effect size of biological interest for the additive model. 4. Our results will aid researchers in the design of their experiments and the subsequent interpretation of results. We find no clear statistical advantage of using one null model over the other and argue null model choice should be based on biological relevance rather than statistical properties. However, there is a pressing need to increase experiment sample sizes otherwise many biologically important synergistic and antagonistic stressor interactions will continue to be missed.

Atypical, high-diversity assemblages of foraminifera in a mangrove estuary in northern Brazil

Mangrove forests are extremely productive ecosystems, are sources and sinks of organic carbon, and provide essential services both to the marine environment and people. We have studied the composition and species richness of modern benthic foraminiferal assemblages from mangrove swamps along the Mamanguape River in Paraíba, northern Brazil. Sampling points for foraminifera were selected to acquire information on the composition of foraminiferal assemblages from dense mangrove stands collected along a river transect. Almost 100 species of benthic foraminifera were identified within the shallow mangrove habitats. The large number of identified mangrove taxa is the highest recorded so far for true mangrove habitats. The high species richness rivals shallow-water assemblages recorded from nearby offshore and reef environments and indicates that a particularly large number of species is capable of growing and flourishing under multiple stressor conditions. Numerical analysis of the faunal assemblages shows that specific taxa, which were previously known to be uncommon in mangrove environments, are abundant in the Mamanguape River estuary. The atypical foraminiferal fauna found in the Mamanguape River estuary resembles shallow-water offshore assemblages, is characterized by high percent abundances of perforate and miliolid taxa, and contains only very few of the otherwise typical and numerically abundant agglutinated mangrove taxa. The unusual structure of the assemblages recorded provides insight into what combination of environmental variables controls their composition and novel perspectives to reconstruct past mangrove environments. Distribution, diversity, and species-specific analysis will provide guidance on the use of Brazilian mangrove foraminifera as indicators for the strength of tidal activity, pollution, and anoxia in coastal waters and sea-level reconstructions.