Thermal plasticity of the cardiorespiratory system provides cross-tolerance protection to fish exposed to elevated nitrate

Better together: Cross-tolerance induced by warm acclimation and nitrate exposure improved the aerobic capacity and stress tolerance of common carp Cyprinus carpio

Climate warming is a threat of imminent concern that may exacerbate the impact of nitrate pollution on fish fitness. These stressors can individually affect the aerobic capacity and stress tolerance of fish. In combination, they may interact in unexpected ways where exposure to one stressor may heighten or reduce the resilience to another stressor and their interactive effects may not be uniform across species. Here, we examined how nitrate pollution under a warming scenario affects the aerobic scope (AS), and the hypoxia and heat stress susceptibility of a generally tolerant fish species, common carp Cyprinus carpio. We used a 3 × 2 factorial design, where fish were exposed to one of three ecologically relevant levels of nitrate (0, 50, or 200 mg NO₃^- L^-1) and one of two temperatures (18 °C or 26 °C) for 5 weeks. Warm acclimation increased the AS by 11% due to the maintained standard metabolic rate and increased maximum metabolic rate at higher temperature, and the AS improvement seemed greater at higher nitrate concentration. Warm-acclimated fish exposed to 200 mg NO₃^- L^-1 were less susceptible to acute hypoxia, and fish acclimated at higher temperature exhibited improved heat tolerance (critical thermal maxima, CTMax) by 5 °C. This cross-tolerance can be attributed to the hematological results including maintained haemoglobin and increased haematocrit levels that may have compensated for the initial surge in methaemoglobin at higher nitrate exposure.

Use of complex physiological traits as ecotoxicological biomarkers in tropical freshwater fishes

We review the use of complex physiological traits, of tolerance and performance, as biomarkers of the toxicological effects of contaminants in subtropical and tropical freshwater fishes. Such traits are growing in relevance due to climate change, as exposure to contaminants may influence the capacity of fishes to tolerate and perform in an increasingly stressful environment. We review the evidence that the critical oxygen level, a measure of hypoxia tolerance, provides a valuable biomarker of impacts of diverse classes of contaminants. When coupled with measures of cardiorespiratory variables, it can provide insight into mechanisms of toxicity. The critical thermal maximum, a simple measure of tolerance of acute warming, also provides a valuable biomarker despite a lack of understanding of its mechanistic basis. Its relative ease of application renders it useful in the rapid evaluation of multiple species, and in understanding how the severity of contaminant impacts depends upon prevailing environmental temperature. The critical swimming speed is a measure of exercise performance that is widely used as a biomarker in temperate species but very few studies have been performed on subtropical or tropical fishes. Overall, the review serves to highlight a critical lack of knowledge for subtropical and tropical freshwater fishes. There is a real need to expand the knowledge base and to use physiological biomarkers in support of decision making to manage tropical freshwater fish populations and their habitats, which sustain rich biodiversity but are under relentless anthropogenic pressure.

Adding climate change to the mix: responses of aquatic ectotherms to the combined effects of eutrophication and warming

The threat of excessive nutrient enrichment, or eutrophication, is intensifying across the globe as climate change progresses, presenting a major management challenge. Alterations in precipitation patterns and increases in temperature are increasing nutrient loadings in aquatic habitats and creating conditions that promote the proliferation of cyanobacterial blooms. The exacerbating effects of climate warming on eutrophication are well established, but we lack an in-depth understanding of how aquatic ectotherms respond to eutrophication and warming in tandem. Here, I provide a brief overview and critique of studies exploring the cumulative impacts of eutrophication and warming on aquatic ectotherms, and provide forward direction using mechanistically focused, multi-threat experiments to disentangle complex interactions. Evidence to date suggests that rapid warming will exacerbate the negative effects of eutrophication on aquatic ectotherms, but gradual warming will induce physiological remodelling that provides protection against nutrients and hypoxia. Moving forward, research will benefit from a greater focus on unveiling cause and effect mechanisms behind interactions and designing treatments that better mimic threat dynamics in nature. This approach will enable robust predictions of species responses to ongoing eutrophication and climate warming and enable the integration of climate warming into eutrophication management policies.

Harnessing the potential of cross-protection stressor interactions for conservation: a review

Conservation becomes increasingly complex as climate change exacerbates the multitude of stressors that organisms face. To meet this challenge, multiple stressor research is rapidly expanding, and the majority of this work has highlighted the deleterious effects of stressor interactions. However, there is a growing body of research documenting cross-protection between stressors, whereby exposure to a priming stressor heightens resilience to a second stressor of a different nature. Understanding cross-protection interactions is key to avoiding unrealistic 'blanket' conservation approaches, which aim to eliminate all forms of stress. But, a lack of synthesis of cross-protection interactions presents a barrier to integrating these protective benefits into conservation actions. To remedy this, we performed a review of cross-protection interactions among biotic and abiotic stressors within a conservation framework. A total of 66 publications were identified, spanning a diverse array of stressor combinations and taxonomic groups. We found that cross-protection occurs in response to naturally co-occurring stressors, as well as novel, anthropogenic stressors, suggesting that cross-protection may act as a 'pre-adaptation' to a changing world. Cross-protection interactions occurred in response to both biotic and abiotic stressors, but abiotic stressors have received far more investigation. Similarly, cross-protection interactions were present in a diverse array of taxa, but several taxonomic groups (e.g. mammals, birds and amphibians) were underrepresented. We conclude by providing an overview of how cross-protection interactions can be integrated into conservation and management actions and discuss how future research in this field may be directed to improve our understanding of how cross-protection may shield animals from global change.