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Abstract

Nitrate pollution is a pervasive threat to aquatic species worldwide. • We assessed interactions among nitrate, hypoxia, and elevated temperatures in fish. • Nitrate-exposed fish were more susceptible to both hypoxia and heat stress. • Nitrate pollution heightens fish susceptibility to a changing world. Species persistence in a changing world will depend on how they cope with co-occurring stressors. Stressors can interact in unanticipated ways, where exposure to one stressor may heighten or reduce resilience to another stressor. We examined how a leading threat to aquatic species, nitrate pollution, affects susceptibility to hypoxia and heat stress in a salmonid, the European grayling (Thymallus thymallus). Fish were exposed to nitrate pollution (0, 50 or 200 mg NO 3 − L −1) at two acclimation temperatures (18°C or 22°C) for eight weeks. Hypoxia-and heat-tolerance were subsequently assessed, and the gills of a subset of fish were sampled for histological analyses. Nitrate-exposed fish were significantly more susceptible to acute hypoxia at both acclimation temperatures. Similarly , in 18°C-acclimated fish, exposure to 200 mg NO3-L-1 caused a 1°C decrease in heat tolerance (critical thermal maxima, CTMax). However, the opposite effect was observed in 22°C-acclimated fish, where nitrate exposure increased heat tolerance by~1°C. Further, nitrate exposure induced some histopathological changes to the gills, which limit oxygen uptake. Our findings show that nitrate pollution can heighten the susceptibility of fish to additional threats in their habitat, but interactions are temperature dependent.

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... Histopathological changes of the gill epithelium have been the primary focus because the gills are in direct and constant contact with nitrate in polluted environments (Pereira et al. 2017). The main pathological changes occurring to the gills following exposure to increased nitrate levels include hyperplasia (increase in cell number) and hypertrophy (increase in cell size) of the epithelial cells, hemorrhaging, oedema of secondary epithelium, aneurisms, and necrosis (Figure 9.2; Davidson et al. 2014, Hrubec, Smith, and Robertson 1996, Pereira et al. 2017, Rodgers et al. 2021, Rodrigues et al. 2011. Research on the impacts of elevated nitrate on the integument (skin) and the digestive tract, which is also in direct contact with nitrate, has been poorly studied, and the available evidence is mixed. ...
... Depending on the severity and extent of changes, histopathological damage can compromise the functionality of the affected organs. For example, histopathological changes to the intestine likely cause unwanted effects by decreasing the capacity for nutrient absorption (Xie et al. 2020), whereas histopathological changes to the gills likely impair oxygen uptake (Rodgers et al. 2021). ...
... Moreover, erythrocyte methemoglobin reduction is highly thermally sensitive, such that methemoglobin is reduced to hemoglobin more efficiently at warmer temperatures Nielsen 2018, Ha et al. 2019). However, nitrate exposure has been shown to lower the shortterm heat tolerance of freshwater fishes (Gomez Isaza, Cramp, and Franklin 2020c, Rodgers et al. 2021), which may make fish more susceptible to heatwaves. ...
Chapter
Nitrate is a natural and important component of freshwater ecosystems. Yet, human activities, such as the extensive use of fertilizers, urban wastewater, and aquaculture operations, have significantly increased the concentration of nitrate entering freshwater environments. Nitrate concentrations can nowadays be 10 to 100 times above preindustrial levels, and elevated nitrate concentrations are expected to cause severe consequences to aquatic life. However, the responses of freshwater life to elevated nitrate concentrations are mixed, with some animals showing severe signs of toxicity while others appear unaffected by nitrate, even at extremely high concentrations. This chapter describes the main toxic actions of nitrate to aquatic life and provides a summary of sublethal effects (e.g., impacts on growth, development, histopathology, and endocrine disruption). This chapter also examines how nitrate toxicity is mediated by other environmental variables, such as water temperature, salinity, and other pollutants, to affect aquatic fauna. Finally, this chapter concludes by directing future research on the effects of nitrate on aquatic fauna.
... Moreover, it was documented that plasticity of the gills and ventricle of warm-acclimated silver perch Bidyanus bidyanus neutralized the negative effects of nitrate on the aerobic scope, swimming performance and thermal tolerance (Gomez Isaza et al., 2020c. However, T. thymallus exposed to nitrate (50 mg NO 3 -L − 1 and 200 mg NO 3 -L − 1 ) were found to be more susceptible to acute hypoxia (Rodgers et al., 2021) indicating cross-susceptibility between these stressors and the inefficiency of the current EU maximum nitrate limit (50 mg NO 3 -L − 1 , Nitrates Directive 91/676/EEC) to protect this species against hypoxia. In this study, we determined the interactive effects of nitrate and warm acclimation on haematological parameters (Hb, MetHb, HCT, and mean corpuscular haemoglobin concentrations-MCHC), aerobic performance (AS, SMR, MMR) and acute stress tolerance (heat and hypoxia stress) using a hardy eurythermal fish species, C. carpio, as model species. ...
... After four weeks of treatment exposure, heat tolerance was assessed by measuring the critical thermal maxima (CTMax), a repeatable method to determine maximum thermal limits of organisms (Beitinger et al., 2000). Similar to the method described by Rodgers et al. (2021), eight randomly selected, fasted (24 h) and weighed fish from each treatment were individually placed in cylindrical glass chambers (9 cm diameter × 17 cm height) filled with 900 ml water at corresponding treatment condition ( ± 1 • C; ± 10% mg NO 3 -L − 1 ). Glass chambers were situated in a water bath filled up to maximum level (GFL 1003 model), and were provided with airlines (to ensure DO saturation remain above 80% during trials) and WTW ProfiLine 3310 portable meter (to monitor temperature and DO levels). ...
... Hypoxia tolerance of eight randomly selected fish from each treatment was measured after four weeks of exposure by closed respirometry similar to the method described by Rodgers et al. (2021). Fasted (24 h) and pre-weighed fish were individually placed in a cylindrical glass chamber (7 cm diameter × 13 cm height;) fully filled with water at corresponding treatment condition ( ± 1 • C; ± 10% mg NO 3 -L − 1 ). ...
Article
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 NO 3-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 NO 3-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.
... For example, silver perch (Bidyanus bidyanus) exposed to nitrate pollution (50 or 100 NO 3 − mg l −1 ) for three weeks suffered reduced hypoxia tolerance [29]. Similar findings have also been reported in a freshwater salmonid (Thymallus thymallus), where hypoxia tolerance decreased by 15% in fish exposed to nitrate (50 or 200 NO 3 − mg l −1 ) for eight weeks, compared to controls (0 NO 3 − mg l −1 ) [30]. Heightened hypoxia susceptibility in nitrate-exposed fish is linked to the toxic action of nitrate (and nitrite). ...
... Dissolved nutrients, suspended sediments and contaminants can enter their gills at an increased rate as temperatures rise, but disentangling these interactions can be challenging because chemical availability and organismal detoxification mechanisms are also temperature-dependent [45]. Moreover, fish exposed to nutrients (nitrite or nitrate) generally suffer reduced heat tolerance compared to unexposed fish [30,46]. Specifically, the upper thermal limit of common carp was reduced by 1.2°C following 7 days of nitrite exposure (1 mM) [46], and dropped by 1°C in European grayling following eight weeks of exposure to nitrate (50 or 100 NO 3 − mg l −1 ) [30]. ...
... Moreover, fish exposed to nutrients (nitrite or nitrate) generally suffer reduced heat tolerance compared to unexposed fish [30,46]. Specifically, the upper thermal limit of common carp was reduced by 1.2°C following 7 days of nitrite exposure (1 mM) [46], and dropped by 1°C in European grayling following eight weeks of exposure to nitrate (50 or 100 NO 3 − mg l −1 ) [30]. Nitrate and phosphate exposure can also compromise the resilience of corals to warming [47], and lower coral bleaching thresholds [48]. ...
Article
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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.
... In addition, most investigations on the toxicological effects and processes of Pb 2+ employed inorganic lead salts, especially Pb(NO 3 ) 2 , and neglected the contributions of anions. Earlier it was documented that the anions (e.g., NO 3 -) may affect the fitness of aquatic animals (Kellock et al., 2018;Rodgers et al., 2021) and their presence may either underestimate or exaggerate the effects of Pb 2+ on organisms. Anions are significantly more complicated in natural water systems than they are in laboratory conditions. ...
Article
Water pollution from lead/Pb 2+ poses a significant threat to aquatic ecosystems, and its repercussions on aquatic animals have received considerable attention. Although Pb 2+ has been found to affect numerous aspects of animals, including individual fitness, metabolic status, and symbiotic microbiota, few studies have focused on the associations between Pb 2+-induced variations in fitness, metabolome, symbiotic microbiome, and environmental parameters in the same system, limiting a comprehensive understanding of ecotoxicological mechanisms from a holistic perspective. Moreover, most ecotoxicological studies neglected the potential contributions of anions to the consequences generated by inorganic lead compounds. We investigated the effects of Pb(NO 3) 2 at environmentally relevant concentrations on the Rana omeimontis tadpoles and the water quality around them, using blank and NaNO 3-treated groups as control. Results showed that Pb(NO 3) 2 not only induced a rise in water nitrite level, but exposure to this chemical also impaired tadpole fitness-related traits (e.g., growth and development). The impacts on tadpoles were most likely a combination of Pb 2+ and NO 3-. Tissue metabolomics revealed that Pb(NO 3) 2 exposure influenced animal substrate (i.e., carbohydrate, lipid, and amino acid) and prostaglandin metabolism. Pb(NO 3) 2 produced profound shifts in gut microbiota, with increased Proteobacteria impairing Firmicutes, resulting in higher aerobic and possibly pathogenic bacteria. NaNO 3 also influenced tadpole metabolome and gut microbiome, in a manner different to that of Pb(NO 3) 2. The presence of NO 3-seemed to counteract some changes caused by Pb 2+ , particularly on the microbiota. Piecewise structural equation model and correlation analyses demonstrated connections between tissue metabolome and gut microbiome, and the variations in tadpole phenotypic traits and water quality were linked to changes in tissue metabolome and gut microbiome. These findings emphasized the important roles of gut microbiome in mediating the effects of toxin on aquatic ecosystem. Moreover, it is suggested to consider the influences of anions in the risk assessment of heavy metal pollutions.
... There is a plethora of literature that suggests both human-induced [1][2][3] and natural causes [4][5][6] for the fishkill. While the human-induced kills include known or accidental additions of harmful chemicals, sewage ingress, and fertilizers from agricultural fields into natural waters [7][8][9][10], the natural causes on the contrary are attributed to temperature fluctuations [11,12], anoxic conditions [13,14], cyanobacterial blooms [15,16], and disease outbreak [17,18]. ...
Article
Full-text available
Globally, the frequency of fishkill episodes is increasing, owing to natural and human-induced modification of aquatic ecosystems. A massive fishkill took place on 22 October 2017 along an approximately 1.5 km stretch of the Jhelum River in Srinagar City, India. Thousands of fish died during this specific event, not lasting more than three hours, creating chaos and panic among the local population and government circles. In this context, affected fish were assessed for three morphological parameters, which include skin color, eye appearance, and skin texture. To back our findings, three critical water-quality parameters, including pH, water temperature, and dissolved oxygen essential for the survival of fishes were assessed in the affected river stretch. This study assumes importance given that water-quality observation stations for monitoring the health of the Jhelum River are lacking in the highly urbanized Srinagar City. The morphological examination of fish samples revealed discoloration, bulging eyes, and rough skin texture, indicating chemical contamination of waters in the affected river stretch. The water quality analysis revealed neutral pH (7.2), normal temperature (15.6 °C), and mildly depleted dissolved oxygen (6 mg L−1) levels. While the morphological examination of the affected fish indicated chemical contamination, the physicochemical parameters exhibited a typical scenario of river water. For avoiding any such further incident and to precisely ascertain the cause of such fishkill episodes in future, it is suggested that a few continuous water-quality monitoring sites along Jhelum River should be set up, supplemented with robust ecological modeling simulations.
... The divergence of results is explained by the fact that nitrate is more toxic in freshwater environments (Camargo et al., 2005;Kuhn et al., 2010). Therefore, higher nitrate concentrations may confer reduced resilience to fish in freshwater environments (Rodgers et al., 2021). Furthermore, when nitrate concentrations exceed critical levels, the fish welfare and growth performance may be affected (Monsees et al., 2017). ...
Article
This study aimed to evaluate the effects of nitrate (NO 3-N) in juvenile mullet in freshwater. Experiment 1 evaluated the LC 50-96h of fish at five different NO 3-N concentrations (0.01, 85.32, 462.12, 851.25 and 955.25 mg L −1). Experiment 2 evaluated haematological and oxidative stress variables in fish exposed to four NO 3-N concentrations (0.01, 8.16, 33.83 and 52.66 mg L −1) for 5 days. The safe concentration for 96 h was 60.61 mg NO 3-N L −1. In experiment 2, blood glucose and haematological values were lower in fish subjected to 52.66 mg NO 3-N L −1 compared with the other treatments on day 1. On day 5, fish subjected to the two highest NO 3-N concentrations had blood glucose and erythrocytes levels as well as gills and liver ACAP values significantly higher than the control group. Fish exposed to 52.66 mg NO 3-N L −1 showed reduced GST activity in gills, liver and muscle on day 5 compared with day 1. In general , exposure to nitrate did not induce an increase in the TBARS levels in the tissues evaluated. In conclusion, acute exposure to 52.66 mg NO 3-N L −1 impacted the physiological homeostasis of mullets, but it did not cause lipid peroxidation. K E Y W O R D S antioxidant capacity, freshwater, glucose, GST, haemoglobin, lipid peroxidation 2 |
... Conservation Physiology of fishes for tomorrow 11 interaction between nitrate (a pervasive aquatic pollutant), hypoxia and heat stress was recently studied in the European grayling (Thymallus thymallus), with findings showing that nitrate-exposed fish were far more susceptible to both hypoxia and heat stress (Rodgers et al., 2021). Conversely, low oxygen levels were found to help prevent the negative consequences of acute warming on mitochondrial efficiency in the European seabass (Dicentrarchus labrax, Thoral et al., 2021), suggesting that exposure to some stressors may have protective effects on other stressors. ...
Chapter
Marine and freshwater fishes are currently facing complex and extensive challenges, from warming waters and habitat degradation to direct human-wildlife interactions. Within this context, Conservation Physiology has made some important contributions to advance our understanding of the underlying mechanisms leading to these problems, as well as in offering practical solutions. However, there remains much space for the field to grow and significantly expand its impact on real-world conservation of fish biodiversity. As the planet continues to change, so do the problems that fish encounter, and so must the field. Importantly, systemic changes must occur to better represent the diversity of peoples and knowledges who have been historically systemically excluded in this field (and others). In this chapter, we discuss some of the remaining key challenges that conservation physiologists need to overcome to protect and effectively manage fish species around the globe in the coming decades, including those related to diversity, equity, inclusivity and justice. We make suggestions for overcoming these challenges, highlighting examples of how physiological knowledge has been used to conserve fishes and other taxa and providing key resources for understanding and addressing inequities in the field, which may serve as guidance for scientists and practitioners seeking to advance these goals. We finish with a list of priority actions needed to ensure that the field of Conservation Physiology remains relevant and successful in its quest to promote long-term sustainable and equitable conservation solutions.
Article
Aquatic hypoxic events are increasing in frequency and intensity as concentrations of nutrients, such as nitrate, continue to rise from human activities. Many fish species can alter their behavior and physiology to cope with drops in oxygen, but these compensatory strategies may be compromised under high levels of nitrate pollution. Hence, we investigated whether exposure to elevated nitrate concentrations affects key behavioral (avoidance and aquatic surface respiration, ASR) and physiological (hemoglobin and hematocrit levels, ventilation frequency, and burst and prolonged swimming performance) responses of fish to mitigate the impacts of acute hypoxia. Juvenile silver perch (Bidyanus bidyanus) were exposed to one of three nitrate concentrations (0, 50 or 100 mg NO3 − L-1) for three weeks, after which, behavioral and physiological responses of fish to progressive hypoxia were assessed. Fish exposed to nitrate utilized ASR at a higher PO2 threshold during progressive hypoxia compared to control animals but did not alter behavioral avoidance of low oxygen levels. In these nitrate exposed fish, the early onset of ASR behaviors is likely a behavioral, compensatory strategy to cope with nitrate-induced physiological disruptions, namely increases in ventilation frequency and lower levels of hemoglobin and hematocrit. The physiological constraints placed by nitrate and acute hypoxia exposures manifested to lower the swimming performance of silver perch. Collectively, these data suggest that exposure to elevated nitrate is likely to disrupt key behavioral and physiological strategies used by fish to cope with short term hypoxia.
Article
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The marine environment is predicted to become warmer, and more hypoxic, and these conditions may negatively impact the health and survival of coastal fish species, including wild and farmed Atlantic salmon (Salmo salar). Thus, we examined how: (1) moderate hypoxia (∼70% air saturation) at 12°C for 3 weeks; (2) an incremental temperature increase from 12°C to 20°C (at 1°C week-1) followed by 4 weeks at 20°C; and (3) treatment "2" combined with moderate hypoxia affected transcript expression in the liver of post-smolts as compared to control conditions (normoxia, 12°C). Specifically, we assessed the expression of 45 genes related to the heat shock response, oxidative stress, apoptosis, metabolism and immunity using a high-throughput qPCR approach (Fluidigm Biomark™ HD). The expression profiles of 27 "stress"-related genes indicated that: (i) moderate hypoxia affected the expression of several stress genes at 12°C; (ii) their expression was impacted by 16°C under normoxic conditions, and this effect increased until 20°C; (iii) the effects of moderate hypoxia were not additive to those at temperatures above 16°C; and (iv) long-term (4 weeks) exposure to 20°C, with or without hypoxia, resulted in a limited acclimatory response. In contrast, the expression of 15 immune-related genes was not greatly affected until temperatures reached 20°C, and this effect was particularly evident in fish exposed to the added challenge of hypoxia. These results provide valuable information on how these two important environmental factors affect the "stress" physiology and immunity of Atlantic salmon, and we identify genes that may be useful as hypoxia and/or temperature biomarkers in salmonids and other fishes.
Article
The extensively accumulation of nitrate in different water resources is currently regarded as one of the most predominant threats facing aquatic organisms on worldwide scale. In recent years, a growing body of evidences have been attempting to uncover the influences of nitrate on fish growth and health, thereby evaluating its environment security. However, the systematic assessment and intrinsic mechanism of such influences are apparently devoid. Hence, this investigation employed systematic analysis, meta-analysis and bioinformatic analysis to evaluate the nitrate biotoxicity. We first speculated two levels of nitrate concentration according to forty-four published bibliographies. Systematic analysis indicated that the broad variations of fish sensitivity to chronic and acute nitrate exposures were found in juvenile and larval stage, respectively, comparing to egg. Meta-analysis further revealed that survival rate, CF and SGR were significantly improved in low nitrate concentration during chronic exposure. Such improvements were reflected by Total mean differences (TMD) and 95% CIs (Confidence Intervals): Survival rate (-4.06 [-7.67, -0.45]), Fulton's condition factor (CF) (-0.03 [-0.03, -0.02]) and Specific growth rate (SGR) (-0.10 [-0.16, -0.04]). To trace the impact, the alternations of molecular expression and histology in brain, gill, liver, intestine, and blood suggested that the chronic and acute nitrate exposures could result in abnormal tissue structures and molecular dynamics. Moreover, omics analysis via integrating intestinal microbiome (microbial composition; %) and liver transcriptome (Gene Ontology: biological processes) revealed that the low concentration exposure induced a weakly immune response in fish liver and it matched to the intestinal immune response. Overall, current study has filled the gaps in the field of nitrate toxicity. It could also provide a novel insight for the evaluation of pollutant toxicity on aquatic species.
Chapter
An endless list of new chemicals are entering nature, which makes it an impossible task to assess all possible mixture combinations at all possible concentrations and conditions that are leading to the ubiquitous anthropogenic impacts on the aquatic environment resulting from deteriorating water quality. Therefore, ecotoxicology is moving more toward a mechanistic understanding of toxicological processes, using trait-based approaches and sublethal molecular and physiological endpoints to understand the mode of action of pollutants and the adverse outcomes at the organismal and population level. These molecular and physiological endpoints can be used as biomarkers, applicable in the field. This brings ecotoxicological research much closer to conservation physiology. Understanding the relationships between chemical reactivity in the water and in organisms, and assessing the consequences at higher levels, allows conservation physiologists and managers to take the right restoration measures for an optimal improvement of the aquatic habitats of concern. In this chapter we discuss the role which the promising approach of mechanistic-based Adverse Outcome Pathways (AOPs) can play in ecotoxicological research. It studies a pathway of events, from the direct interaction of a chemical with a molecular target, through subsequent intermediate events at cellular, tissue, organ and individual organism levels which then result in an Adverse Outcome (AO) relevant to ecotoxicological risk assessment and regulatory decision-making. In this context, we also discuss the importance of modeling, including bioavailability based and effect based models. Finally, we reflect on the possibilities that meta-analysis has to offer to detect unifying physiological processes, as well as interesting outliers.
Article
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Os compostos nitrogenados (nitrito, nitrato e amônia) são considerados tóxicos para os peixes, pois são capazes de promover alterações histológicas e bioquímicas, diminuir a capacidade de trasnporte de oxigênio no sangue e afetar o fitness (i.e. aptidão do organismo em termos de crescimento e natação). Esses compostos acumulam no ambiente aquático através da excreção de organismos aquáticos, da decomposição de matéria orgânica tanto de origem aquática quanto do sistema terrestre adjacente, e da entrada direta pelo sistema terrestre, afluentes e águas subterrâneas. As chamadas mudanças globais, como as climáticas e de uso do solo, podem potencializar e intensificar essa entrada dos compostos nitrogenados através de atividades humanas, tais como a agricultura e o lançamento de efluentes, como esgoto doméstico. Os peixes estão comumente em contato com diversos poluentes, e a combinação de fatores ambientais como temperatura, pH, disponibilidade de oxigênio e dureza podem ter efeitos sinérgicos ou antagônicos. No entanto, como esses fatores ambientais afetam a toxicidade da amônia, nitrito e nitrato não são totalmente compreendidos. Portanto, nosso objetivo foi revisar a literatura afim de abordar os efeitos dos compostos nitrogenados, e como os parâmetros físico-químicos da água afetam a toxicidade desses compostos.
Article
Climate and land-use changes are expected to increase the future occurrence of wildfires, with potentially devastating consequences for freshwater species and ecosystems. Wildfires that burn in close proximity to freshwater systems can significantly alter the physicochemical properties of water. Following wildfires and heavy rain, freshwater species must contend with complex combinations of wildfire ash components (nutrients, polycyclic aromatic hydrocarbons, and metals), altered light and thermal regimes, and periods of low oxygen that together can lead to mass mortality events. However, the responses of aquatic fauna to wildfire disturbances are poorly understood. Here we provide a systematic review of available evidence on how aquatic animals respond to and recover from wildfire disturbance. Two databases (Web of Science and Scopus) were used to identify key literature. A total of 83 studies from across 11 countries were identified to have assessed the risk of wildfires on aquatic animals. We provide a summary of the main ecosystem-level changes associated with wildfires and the main responses of aquatic fauna to such disturbances. We pay special focus to physiological tools and biomarkers used to assess how wildfires impact aquatic animals. We conclude by providing an overview of how physiological biomarkers can further our understanding of wildfire-related impacts on aquatic fauna, and how different physiological tools can be incorporated into management and conservation plans and serve as early warning signs of wildfire disturbances.
Article
The progression of climate warming will expose ectotherms to transient heatwave events and temperatures above their tolerance range at increased frequencies. It is therefore pivotal that we understand species' physiological limits and the capacity for various controls to plastically alter these thresholds. Exercise training could have beneficial impacts on organismal heat tolerance through improvements in cardio-respiratory capacity, but this remains unexplored. Using juvenile Chinook salmon (Oncorhynchus tshawytscha), we tested the hypothesis that exercise training improves heat tolerance through enhancements in oxygen-carrying capacity. Fish were trained once daily at 60% of their maximum sustainable swim speed, UCRIT, for 60 min. Tolerance to acute warming was assessed following three weeks of exercise training, measured as the critical thermal maximum (CTMAX). CTMAX measurements were coupled with examinations of the oxygen carrying capacity (haematocrit, haemoglobin concentration, relative ventricle size, and relative splenic mass) as critical components of the oxygen transport cascade in fish. Contrary to our hypothesis, we found that exercise training did not raise the CTMAX of juvenile Chinook salmon with a mean CTMAX increase of just 0.35 °C compared to unexercised control fish. Training also failed to improve the oxygen carrying capacity of fish. Exercise training remains a novel strategy against acute warming that requires substantial fine-tuning before it can be applied to the management of commercial and wild fishes.
Article
The stress history of an ectotherm may be a pivotal predictor of how they cope with rapid spikes in environmental temperature. An understanding of how stressors in habitats and commercial operations affect ectotherm heat tolerance is urgently required so that management actions can be informed by thermal physiology. We hypothesised that brief exposure to mild stress would heighten tolerance to subsequent heat stress, indicative of a cross-tolerance interaction, whereas exposure to severe stress would reduce heat tolerance, reflecting a cross-susceptibility interaction. To test this hypothesis, we assessed how three acute stressors (salinity shock [10 or 33 ppt for 2 h]), air exposure (1 or 5 min) and crowding [95.6 kg m⁻³ for 2 h]), commonly experienced by fish, affected the heat tolerance (measured as critical thermal maximum, CTMAX) in juvenile Chinook salmon (Oncorhynchus tshawytscha). Fish were exposed to one of the three stressors and left for 24 h of recovery prior to measuring CTMAX. Heat tolerance was improved by ∼0.6 °C in fish exposed to salinity shock (10 ppt) and air exposure (5 min) compared to unstressed controls, demonstrating cross-tolerance. However, the development of cross-tolerance was non-linear with stressor severity, and crowding stress had no effect on CTMAX. Together these results show that some forms of stress can heighten acute heat tolerance in ectotherms, but the development of cross-tolerance is highly specific to both stressor type and stressor severity.
Article
Nitrate (NO3⁻) pollution of waterbodies has attracted significant global attention as it poses a serious threat to aquatic organisms and human beings. This study aimed to evaluate the role of NO3⁻, an end product of biological nitrification processes, in immune status and lipid metabolism to have a comprehensive understanding of its toxic effects on fishes. Therefore, in this work, juvenile turbot (Scophthalmus maximus) were subjected to four nominal concentrations of NO3⁻ (i.e., 0, 50, 200, 400 mg/L of NO3⁻-N) for a 60-day period. The results indicated that increased exposure to NO3– (200 and/or 400 mg/L) enhanced the concentrations of plasma heat shock protein concentrations (HSP70), complement component 3 (C3), complement component 4 (C4), immunoglobulin M (IgM) and lysozyme (LYS), which meant that NO3⁻ caused fluctuations in the plasma immune system. Higher exposure to NO3– (200 and/or 400 mg/L) also caused significant enhancements in plasma glutamic pyruvic transaminase (GPT), as well as glutamic oxaloacetic transaminase (GOT) activity. Furthermore, NO3– exposure resulted in upregulation of liver TNF-α, IL-1β, HSP70, HSP90, and LYS. Additionally, the results suggested that NO3⁻ exposure caused a certain degree of histological damage and inflammation in the liver and activated the immune defense processes of juvenile turbot. Furthermore, the mRNA expression levels of certain genes associated with lipid metabolism (peroxisome proliferator-activated receptor-alpha [PPAR-α], carnitine palmitoyltransferase 1[CPT1], liver X receptor [LXR] together with sterol regulatory element binding protein-1 [SREBP-1]) increased significantly within fish liver exposed to 200/400 mg/L NO3⁻-N treatments. Finally, the results obtained from the analysis of the integrated biological responses version 2 (IBRv2) also confirmed the toxic effects of NO3⁻ on juvenile turbot. According to these findings, it can be found that NO3⁻ emission in the aquatic environment needs to be strictly controlled, as it may cause immune and lipid metabolism disorders in fish.
Thesis
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The acceleration of anthropogenic activity has caused novel or extreme environmental challenges for species to contend with. Species must now contend with complex combinations of environmental threats which include habitat degradation, pollution, and climate change. Yet, the lack of available data on how species cope when confronted with multiple environmental challenges poses a significant challenge to conservation. Nutrient pollution is regarded as one of the most common and widespread forms of habitat degradation. Anthropogenic activities have caused a rampant increase in nitrate (NO3-) concentrations, peaking at concentrations above 100 mg NO3- L-1. Aquatically respiring organisms (amphibians, amphipods, fish) are particularly vulnerable to elevated nitrate concentrations, causing significant physiological and behavioural alterations. These alterations may be exacerbated by the presence of additional threats, but information on the interactive effects of nitrate and other environmental stressors is scarce. Therefore, the first aim of this thesis was to quantify the impacts of elevated nitrate exposure on key fitness related traits using meta-analytical tools and examine whether organismal survival is affected by nitrate and its interaction with other stressors. Across studies, exposure to elevated nitrate decreased the activity, growth, and survival of aquatic taxa. Further, antagonistic interactions between nitrate and other stressors were most predominant, indicating that future research should focus on interacting stressors that act on the same physiological mechanism (e.g. pH, elevated temperature and hypoxia) as they represent the greatest likelihood for “ecological surprises”. The meta-analysis also revealed that data on fish and crustaceans is limited and these taxa were therefore the focus of subsequent experimental chapters. Environmental pH is one factor that may modify the toxicity of nitrate by exacerbating its uptake and disrupting key physiological performance traits. To test this prediction, blueclaw crayfish (Cherax destructor) were exposed to one of two pH levels (pH 5.0 and 7.0) and three nitrate concentrations (0, 50 and 100 mg NO3- L-1). Aerobic scope (maximal minus standard oxygen uptake rates) was measured at six time points and crayfish performance (chelae strength and righting response) was assessed after 28 days. Aerobic scope was compromised by the interaction between low pH and nitrate and resulted in prolonged elevations of standard oxygen uptake. Declines in aerobic scope corresponded with a lowering of chelae strength and righting capacity. Similarly, combined exposure to nitrate and low pH reduced the aerobic scope and swimming performance of spangled perch (Leiopotherapon unicolor), an effect underpinned by an accumulation of nitrate within the blood and reductions to blood-oxygen carrying capacity. Nitrate-induced reductions to oxygen transport were expected to lower species’ tolerance of, and impede their capacity to compensate for prolonged exposure to, elevated temperatures. To test this prediction, silver perch (Bidyanus bidyanus) were exposed to 28 or 32oC and simultaneously exposed to one of three nitrate concentrations (0, 50 or 100 NO3- L-1). Indicators of performance, aerobic scope and upper thermal tolerance (CTMAX) were assessed after 8-weeks. The aerobic scope of 28oC-acclimated fish declined with increasing temperature, and the effect was more pronounced in nitrate-exposed individuals. Declines in aerobic scope corresponded with poorer swimming performance and a 0.8oC decrease in CTMAX. In contrast, acclimation to 32oC masked the effects of nitrate; swimming performance was thermally insensitive, aerobic scope was maintained, and CTMAX was increased by ~1oC. These results are suggestive of a cross-tolerance interaction and potential mechanisms underlying this interaction were explored by measuring attributes of the heart, gills and blood. Plasticity of the ventricle (increased myocardial thickness) and gill structures (decreased lamellar thickness, interlamellar cell mass) following high temperature acclimation were uncovered, which potentially provide overlapping protection to elevated nitrate concentrations. Lastly, the impact of elevated nitrate on behavioural and physiological responses of silver perch to acute hypoxia were investigated. Fish were exposed to one of three nitrate treatments (0, 50 or 100 mg NO3- L-1) for three weeks, then, behavioural avoidance and aquatic surface respiration (ASR) responses to progressive hypoxia were quantified. Physiological changes evoked under progressive hypoxia were also assessed, including haematological changes, ventilation frequency (VF) and swimming performance. Exposure to elevated nitrate did not alter behavioural avoidance of low oxygen but nitrate-exposed fish did utilise ASR at a higher PO2 threshold during progressive hypoxia. Nitrate exposure had small impacts on key physiological responses; haemoglobin and haematocrit levels were reduced and the VF of nitrate-exposed fish were elevated both at rest and under hypoxic conditions. These physiological disturbances during nitrate exposure had pronounced effects on the swimming performance and hypoxia tolerance of fish and indicate that nitrate pollution is likely to increase the susceptibility of fish to aquatic hypoxia. Overall, presence of nitrate and additional stressors impaired energy homeostasis, such that aerobic scope is reduced and compromised the functioning of aerobically supported traits (e.g. swimming, righting, growth), due to disruptions of the blood-oxygen carrying capacity. Physiological compensation can offset the effects of nitrate, possibly explaining the predominance for antagonistic interactions. This body of work highlights the unpredictability of stressor interactions and underscores the importance of experimental assessments in addressing the eco-physiological constraints of species in our ever-changing world.
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Human activities present aquatic species with numerous of environmental challenges, including excessive nutrient pollution (nitrate) and altered pH regimes (freshwater acidification). In isolation, elevated nitrate and acidic pH can lower the blood oxygen-carrying capacity of aquatic species and cause corresponding declines in key functional performance traits such as growth and locomotor capacity. These factors may pose considerable physiological challenges to organisms but little is known about their combined effects. To characterise the energetic and physiological consequences of simultaneous exposure to nitrate and low pH, we exposed spangled perch (Leiopotherapon unicolor) to a combination of nitrate (0, 50 or 100 mg L-1) and pH (pH 7.0 or 4.0) treatments in a factorial experimental design. Blood oxygen-carrying capacity (haemoglobin concentration, methaemoglobin concentrations and oxygen equilibrium curves), aerobic scope and functional performance traits (growth, swimming performance and post-exercise recovery) were assessed after 28 days of exposure. The oxygen-carrying capacity of fish exposed to elevated nitrate (50 and 100 mg L-1) was compromised due to reductions in haematocrit, functional haemoglobin levels and a 3-fold increase in methaemoglobin concentrations. Oxygen uptake was also impeded due to a right shift in oxygen-haemoglobin binding curves of fish exposed to nitrate and pH 4.0 simultaneously. A reduced blood oxygen-carrying capacity translated to a lowered aerobic scope, and the functional performance of fish (growth and swimming performance and increased post-exercise recovery times) was compromised by the combined effects of nitrate and low pH. These results highlight the impacts on aquatic organisms living in environments threatened by excessive nitrate and acidic pH conditions.
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Despite the European Nitrate Directive (ND) being issued almost 30 years ago, groundwater nitrate contamination is still a serious threat to ecosystems and human health. In one of the areas most affected by nitrates, the Lombardy Plain (Italy), the effectiveness of the ND and the capacity of governance to support its application correctly was assessed using a socio-hydrogeological approach. Nitrate trends over 11 years show that most regions present steady or increasing concentrations, highlighting how contamination can affect previously impaired situations and supposedly resistant and resilient aquifers. Stakeholder network analysis reveals that the governance framework does not support knowledge dissemination and changes in farmers’ attitudes, hindering water quality improvements. Nitrogen input needs to be reduced and manure relocation monitored. The local governance scale has a key role in enhancing ND dissemination. Reports to the EU Commission should integrate multi- or interdisciplinary evaluation of trends, including governance dynamics, alongside hydrochemical information.
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We evaluated the acute and chronic toxicity of nitrate to juvenile fat greenling Hexagrammos otakii. The 24‐, 48‐, 72‐, and 96‐hr LC50s of nitrate to 1.91 ± 0.7 g greenlings were 2,741, 2,413.5, 2,357.6, or 2,339.2 mg/L nitrate‐N, respectively. Greenlings (6.55 ± 1.83 g) were exposed to 5 mg/L (control) and 157 mg/L for 4 weeks in a recirculating aquaculture system. After 4 weeks, length, weight, feed conversion ratio, and specific growth rate were significantly (p < 0.05) lower for nitrate‐exposed fish than for control fish. Elevated nitrate exposure was associated with decreased plasma hemoglobin concentration and red blood cell count. Our results demonstrate that nitrate poses a threat to greenlings and provide information that is useful for establishing water quality criteria for early life stages of this cultured fish. The sensitivity of greenlings to elevated NO3− should be evaluated at other life stages to determine how chronic exposure might impact survival, growth, health, reproductive success, and harvest quality.
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1.Respirometry is a ubiquitous practice in experimental biology, but there is a lack of standard practiceswhen analysing the resulting data, limiting transparency and reproducibility. As respirometry datasets become increasingly large and analytical approaches more complex, manipulating the data remains a challenge and often intractable with existing tools. 2.Here we describe the respR R package, a collection of functions that implement a workflow‐based approach to automate the analysis and visualisation of respirometry data. The package can be used for closed, intermittent flow, flow‐through and open‐tank respirometry and uses well‐defined sets of rules to reliably and rapidly generate reproducible results. 3.We demonstrate how respR uses novel computing methods such as rolling regressions and kernel density estimates to reliably detectmaximum, minimum and most linear sections of the data, and critical oxygen tension, Pcrit. 4.Although designed specifically with aquatic respirometry in mind, the object‐oriented approach of the package and the unit‐less nature of its analytical functions mean that parts of the package can easily be used to estimate linear relationships from a range of applications in many research disciplines.
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Marine heatwaves (MHWs) are periods of extreme warm sea surface temperature that persist for days to months1 and can extend up to thousands of kilometres2. Some of the recently observed marine heatwaves revealed the high vulnerability of marine ecosystems3-11 and fisheries12-14 to such extreme climate events. Yet our knowledge about past occurrences15 and the future progression of MHWs is very limited. Here we use satellite observations and a suite of Earth system model simulations to show that MHWs have already become longer-lasting and more frequent, extensive and intense in the past few decades, and that this trend will accelerate under further global warming. Between 1982 and 2016, we detect a doubling in the number of MHW days, and this number is projected to further increase on average by a factor of 16 for global warming of 1.5 degrees Celsius relative to preindustrial levels and by a factor of 23 for global warming of 2.0 degrees Celsius. However, current national policies for the reduction of global carbon emissions are predicted to result in global warming of about 3.5 degrees Celsius by the end of the twenty-first century16, for which models project an average increase in the probability of MHWs by a factor of 41. At this level of warming, MHWs have an average spatial extent that is 21 times bigger than in preindustrial times, last on average 112 days and reach maximum sea surface temperature anomaly intensities of 2.5 degrees Celsius. The largest changes are projected to occur in the western tropical Pacific and Arctic oceans. Today, 87 per cent of MHWs are attributable to human-induced warming, with this ratio increasing to nearly 100 per cent under any global warming scenario exceeding 2 degrees Celsius. Our results suggest that MHWs will become very frequent and extreme under global warming, probably pushing marine organisms and ecosystems to the limits of their resilience and even beyond, which could cause irreversible changes.
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Fishes faced with novel thermal conditions often modify physiological functioning to compensate for elevated temperatures. This physiological plasticity (thermal acclimation) has been shown to improve metabolic performance and extend thermal limits in many species. Adjustments in cardiorespiratory function are often invoked as mechanisms underlying thermal plasticity because limitations in oxygen supply have been predicted to define thermal optima in fishes, however few studies have explicitly linked cardiorespiratory plasticity to metabolic compensation. Here we quantify thermal acclimation capacity in the commercially harvested Nile perch (Lates niloticus) of East Africa, and investigate mechanisms underlying observed changes. We reared juvenile Nile perch for 3 months under two temperature regimes, and then measured a series of metabolic traits (e.g., aerobic scope, AS) and critical thermal maximum (CTmax) upon acute exposure to a range of experimental temperatures. We also measured morphological traits of heart ventricles, gills, and brains to identify potential mechanisms for compensation. We found that long-term (3-months) exposure to elevated temperature induced compensation in upper thermal tolerance (CTmax) and metabolic performance (SMR, MMR and AS), and induced cardiac remodeling in Nile perch. Furthermore, variation in heart morphology influenced variations in metabolic function and thermal tolerance. These results indicate that plastic changes enacted over longer exposures lead to differences in metabolic flexibility when acutely exposed to temperature variation. Furthermore, we established functional links between cardiac plasticity, metabolic performance, and thermal tolerance, providing evidence that plasticity in cardiac capacity may be one mechanism for coping with climate change.
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Critical thermal maximum (CTmax) is a commonly and increasingly used measure of an animal’s upper thermal tolerance limit. However, it is unknown how consistent CTmax is within an individual, and how physiologically taxing such experiments are. We addressed this by estimating the repeatability of CTmax in zebrafish, and measured how growth and survival were affected by multiple trials. The repeatability of CTmax over four trials was 0.22 (0.07–0.43). However, CTmax increased from the first to the second trial, likely because of thermal acclimation triggered by the heat shock. After this initial acclimation response individuals became more consistent in their CTmax, reflected in a higher repeatability measure of 0.45 (0.28–0.65) for trials 2–4. We found a high innate thermal tolerance led to a lower acclimation response, whereas a high acclimation response was present in individuals that displayed a low initial CTmax. This could indicate that different strategies for thermal tolerance (i.e. plasticity vs. high innate tolerance) can co-exist in a population. Additionally, repeated CTmax trials had no effect on growth, and survival was high (99%). This validates the method and, combined with the relatively high repeatability, highlights the relevance of CTmax for continued use as a metric for acute thermal tolerance.
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Hypoxia is a common occurrence in aquatic habitats, and it is becoming an increasingly frequent and widespread environmental perturbation, primarily as the result of anthropogenic nutrient enrichment and climate change. An in-depth understanding of the hypoxia tolerance of fishes, and how this varies among individuals and species, is required to make accurate predictions of future ecological impacts and to provide better information for conservation and fisheries management. The critical oxygen level (Pcrit) has been widely used as a quantifiable trait of hypoxia tolerance. It is defined as the oxygen level below which the animal can no longer maintain a stable rate of oxygen uptake (oxyregulate) and uptake becomes dependent on ambient oxygen availability (the animal transitions to oxyconforming). A comprehensive database of Pcrit values, comprising 331 measurements from 96 published studies, covering 151 fish species from 58 families, provides the most extensive and up-to-date analysis of hypoxia tolerance in teleosts. Methodologies for determining Pcrit are critically examined to evaluate its usefulness as an indicator of hypoxia tolerance in fishes. Various abiotic and biotic factors that interact with hypoxia are analysed for their effect on Pcrit, including temperature, CO2, acidification, toxic metals and feeding. Salinity, temperature, body mass and routine metabolic rate were strongly correlated with Pcrit; 20% of variation in the Pcrit data set was explained by these four variables. An important methodological issue not previously considered is the inconsistent increase in partial pressure of CO2 within a closed respirometer during the measurement of Pcrit. Modelling suggests that the final partial pressure of CO2 reached can vary from 650 to 3500 µatm depending on the ambient pH and salinity, with potentially major effects on blood acid–base balance and Pcrit itself. This database will form part of a widely accessible repository of physiological trait data that will serve as a resource to facilitate future studies of fish ecology, conservation and management.
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Fish gills, owing to their status as a multifunctional organ, have always fascinated researchers. In spite of the intense work done on the morphologic examination of gills, the organ is relatively under-used in health evaluation in fish. The existing literature on this topic is reviewed here. Our review summarises important diagnostic guidelines for the examination of gill structure and describes the morphological lesions that develop under the influence of different biological and physicochemical factors. The picture that should emerge is that of an organ that is extremely sensitive to all types of handling and unfavourable changes in the external and internal environments. We conclude that studying the morphology of the fish gills provides an opportunity to assess fish health status as well as information on possible health hazards coming from their environment.
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Interactions between multiple ecosystem stressors are expected to jeopardize biological processes, functions and biodiversity. The scientific community has declared stressor interactions—notably synergies—a key issue for conservation and management. Here, we review ecological literature over the past four decades to evaluate trends in the reporting of ecological interactions (synergies, antagonisms and additive effects) and highlight the implications and importance to conservation. Despite increasing popularity, and ever-finer terminologies, we find that synergies are (still) not the most prevalent type of interaction, and that conservation practitioners need to appreciate and manage for all interaction outcomes, including antagonistic and additive effects. However, it will not be possible to identify the effect of every interaction on every organism’s physiology and every ecosystem function because the number of stressors, and their potential interactions, are growing rapidly. Predicting the type of interactions may be possible in the near-future, using meta-analyses, conservation-oriented experiments and adaptive monitoring. Pending a general framework for predicting interactions, conservation management should enact interventions that are robust to uncertainty in interaction type and that continue to bolster biological resilience in a stressful world. © 2016 The Author(s) Published by the Royal Society. All rights reserved.
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Global climate change is impacting organisms, biological communities and ecosystems around the world. While most research has focused on characterizing how the climate is changing, including modeling future climatic conditions and predicting the impacts of these conditions on biodiversity, it is also the case that climate change is altering the environmental impacts of chemical pollution. Future climate conditions are expected to influence both the worldwide distribution of chemicals and the toxicological consequences of chemical exposures to organisms. Many of the environmental changes associated with a warming global climate (e.g., increased average – and possibly extreme – temperatures; intense periods of drier and wetter conditions; reduced ocean pH; altered salinity dynamics in estuaries) have the potential to enhance organism susceptibility to chemical toxicity. Additionally, chemical exposures themselves may impair the ability of organisms to cope with the changing environmental conditions of the shifting climate. Such reciprocity in the interactions between climate change and chemicals illustrates the complexity inherent in predicting the toxicological consequences of chemical exposures under future climate scenarios. Here, we summarize what is currently known about the potential reciprocal effects of climate change and chemical toxicity on wildlife, and depict current approaches and ongoing challenges for incorporating climate effects into chemical testing and assessment. Given the rapid pace of new man-made chemistries, the development of accurate and rapid methods to evaluate multiple chemical and non-chemical stressors in an ecologically relevant context will be critical to understanding toxic and endocrine-disrupting effects of chemical pollutants under future climate scenarios.
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Anthropogenic activities are greatly altering the habitats of animals, whereby fish are already encountering several stressors simultaneously. The purpose of the current study was to investigate the capacity of fish to respond to two different environmental stressors (high temperature and overnight hypoxia) separately and together. We found that acclimation to increased temperature (from 7.7±0.02°C to 14.9±0.05°C) and overnight hypoxia (daily changes from normoxia to 63-67% oxygen saturation), simulating climate change and eutrophication, had both antagonistic and synergistic effects on the capacity of fish to tolerate these stressors. Thermal tolerance of Arctic char (Salvelinus alpinus) and landlocked salmon (Salmo salar m. Sebago) increased with warm acclimation by 1.3°C and 2.2°C, respectively, but decreased when warm temperature was combined with overnight hypoxia (by 0.2°C and 0.4°C, respectively). In contrast, the combination of the stressors more than doubled hypoxia tolerance in salmon and also increased the tolerance in char by 22%. Salmon had 1.2°C higher thermal tolerance than char, but char tolerated much lower oxygen levels than salmon at a given temperature. The changes in hypoxia tolerance were connected to the responses of the oxygen supply and delivery system. The relative ventricle mass was higher in cold than warm acclimated salmon but the thickness of compact layer of ventricle increased with combination of warm and hypoxia acclimation in both species. Char had also significantly larger hearts and thicker compact layers than salmon. The results illustrate that while fish can have protective responses when encountering single environmental stressor, the combination of stressors can have unexpected species-specific effects which will influence their survival capacity. © 2015. Published by The Company of Biologists Ltd.
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Temporary ponds, where many amphibians from temperate regions breed, show an annual cycle with a maximum water volume in spring followed by a progressive desiccation throughout late spring and summer. This desiccation leads to a decrease in dissolved oxygen and an increase in nitrogen levels, which can additionally increase because of anthropogenic sources such as chemical fertilizers. We analyzed the toxicity posed by environmentally relevant levels of a common nitrogenous fertilizer, ammonium nitrate, at different conditions of oxygen availability to Bufo calamita tadpoles, which typically develop in ephemeral ponds. Ammonium nitrate (90.3 mg N-NO3NH4/l) and hypoxic conditions (initial dissolved oxygen 4.53 ± 0.40 mg/l) caused significant lethal effects after 7 and 12 days of exposure, respectively. At the end of experiment (16 days), mortality rates were 32.5 % in individuals exposed to the fertilizer and 15 % in those growing under hypoxic conditions. When both stressors were combined, they showed an additive effect on tadpole survival. Malformations, such as oedemas and spinal curvatures, and locomotory abnormalities, were detected after 12 days of experiment in >90 % of individuals exposed to 45.2 mg N-NO3NH4/l under hypoxic conditions, whereas none of these stressors by separate related to abnormality rates >35 %. Delayed development was also observed in tadpoles exposed to ammonium nitrate with hypoxia affecting developmental rate only after 12 days of exposure. The results are discussed in terms of potential mechanisms linking negative effects of both factors as well as in terms of potential alterations of the ecological plasticity that often allows amphibians to survive in unpredictable environments.
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To address how capacity for oxygen transport influences tolerance of acute warming in fishes, we investigated whether a reduction in haematocrit, by means of intra-peritoneal injection of the haemolytic agent phenylhydrazine, lowered upper critical temperature of sea bass. A reduction in haematocrit from 42±2 to 20±3% (mean ± s.e.m.) caused a significant but minor reduction in upper critical temperature, from 35.8 ± 0.1 to 35.1±0.2°C, with no correlation between individual values for haematocrit and upper thermal limit. Anaemia did not influence the rise in oxygen uptake between 25 and 33°C, because the anaemic fish were able to compensate for reduced blood oxygen carrying capacity with a significant increase in cardiac output. Therefore, in sea bass the upper critical temperature, at which they lost equilibrium, was not determined by an inability of the cardio-respiratory system to meet the thermal acceleration of metabolic demands.
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The pink shrimp Farfantepenaeus brasiliensis is native in southern Brazil and is potentially suited for aquaculture. Under intensive culture, the accumulation of nitrogenous compounds results from excretion by the shrimp and from the processes of feed decomposition and nitrification. The objective of this study was to evaluate ammonia, nitrite, and nitrate toxicity effects on oxygen consumption of juvenile pink shrimp. Shrimps (initial weight 0.7 ± 0.15 g) were exposed over a period of 30 days to 50%, 100%, and 200% of the safe levels of total ammonia (TAN = 0.88 mg/L), nitrite (NO2− = 10.59 mg/L), and nitrate (NO3− = 91.20 mg/L) for the species. The specimens were individually collected and placed in respirometry chambers, where the oxygen consumption was measured over a period of two hours. Throughout the experiment there was no significant difference (p > 0.05) among treatments in terms of survival and growth. The pink shrimp juveniles exposed to nitrogen concentrations of 200% of the nitrite and nitrate safe level showed the highest oxygen consumption (p < 0.05).
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Previous research indicates that rainbow trout Oncorhynchus mykiss begin to exhibit health and welfare problems when cultured within water recirculating aquaculture systems (WRAS) operated at low exchange (6.7 days hydraulic retention time) and a mean feed loading rate of 4.1 kg feed/m3 daily makeup flow. These studies could not conclusively determine the causative agent of the health and welfare issues, but accumulation of mean nitrate nitrogen (NO3-N) to approximately 100 mg/L was determined to be a potential cause of abnormal swimming behaviors such as “side swimming” and rapid swimming velocity. A subsequent controlled, 3-month study was conducted to determine if NO3-N concentrations of 80-100 mg/L resulted in chronic health issues for rainbow trout. Equal numbers of rainbow trout (16.4 ± 0.3 g) were stocked within six replicated 9.5 m3 WRAS. Three WRAS were maintained with a mean NO3-N concentration of 30 mg/L (“low”) resulting from nitrification, and three WRAS were maintained with a mean concentration of 91 mg/L (“high”) via continuous dosing of a sodium nitrate stock solution in addition to nitrification. All six WRAS were operated with equal water exchange (1.3 days mean hydraulic retention time) and mean feed loading rates (0.72 kg feed/m3 daily makeup flow), which provided enough flushing to limit the accumulation of other water quality concentrations. Rainbow trout growth was not significantly impacted by the high NO3-N treatment. Cumulative survival for fish cultured within the high NO3-N WRAS was lower and bordered statistical significance, which resulted in total rainbow trout biomass that was significantly lower for this group at study's end. In addition, a significantly greater prevalence of side swimming rainbow trout occurred in the high NO3-N treatment, as was observed during previous research. Swimming speeds were generally greater for rainbow trout cultured in the high NO3-N treatment, but were not always significantly different. Although most water quality variables were controlled, significant differences between treatments for the concentrations of other water quality parameters inhibited definitive conclusions regarding the effect of NO3-N. However, due to the unlikely toxicity of confounding water quality parameters, study results provided strong evidence that relatively low NO3-N levels, 80-100 mg/L, were related to chronic health and welfare impacts to juvenile rainbow trout under the described conditions.
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Population response to global change will depend on responses to a multivariate set of changes in abiotic habitat characteristics and biotic interactions. Organismal biologists seeking to make ecological inferences about the impacts of global change by studying physiological performance have traditionally performed carefully controlled experimental studies that examine one variable at a time. Those studies, while of high value, may not lead to accurate predictions of organismal responses in the natural habitat, where organisms experience concomitant changes in multiple environmental factors. The symposium "Physiological Responses to Simultaneous Shifts in Multiple Environmental Stressors: Relevance in a Changing World" focused on physiological studies in which multiple environmental variables were simultaneously examined and brought together an international group of early-career and established speakers with unique perspectives on studies of multistressors. In doing so, the objective of the symposium was to frame the necessary next steps for increasing predictive capacity of organismal responses to environmental shifts in the natural habitat, establish novel collaborations among researchers actively investigating physiological responses to a multivariate environment, and broaden the number of researchers conducting such studies.
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Presenting the first continental-scale assessment of reactive nitrogen in the environment, this book sets the related environmental problems in context by providing a multidisciplinary introduction to the nitrogen cycle processes. Issues of upscaling from farm plot and city to national and continental scales are addressed in detail with emphasis on opportunities for better management at local to global levels. The five key societal threats posed by reactive nitrogen are assessed, providing a framework for joined-up management of the nitrogen cycle in Europe, including the first cost-benefit analysis for different reactive nitrogen forms and future scenarios. Incorporating comprehensive maps, a handy technical synopsis and a summary for policy makers, this landmark volume is an essential reference for academic researchers across a wide range of disciplines, as well as stakeholders and policy makers. It is also a valuable tool in communicating the key environmental issues and future challenges to the wider public.
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Accumulation of metabolic CO2 can acidify marine waters above and beyond the ongoing acidification of the ocean by anthropogenic CO2. The impact of respiration on carbonate chemistry and pH is most acute in hypoxic and anoxic basins, where metabolic CO2 accumulates to high concentrations. The bottom waters of the Lower St. Lawrence Estuary (LSLE), where persistently severe hypoxia has developed over the last 80 years, is one such case. We have reconstructed the evolution of pH in the bottom waters from historical and recent data, and from first principles relating the stoichiometry of CO2 produced to oxygen consumed during microbial degradation of organic matter. Based on the value of the atmospheric partial pressure of CO2 that best reproduces the preformed dissolved inorganic carbon concentration in the bottom waters, we estimate the average ventilation age of the bottom waters to be 16 ± 3 years. The pH of the bottom waters has decreased by 0.2 to 0.3 over the last 75 years, which is four to six times greater than can be attributed to the uptake of anthropogenic CO2. The pH decrease is accompanied by a decline in the saturation state with respect to both calcite and aragonite. As of 2007, bottom waters in the LSLE are slightly supersaturated with respect to calcite (Ωc ≈ 1.06 ± 0.04) but are strongly undersaturated with respect to aragonite (Ωa ≈ 0.67 ± 0.03).
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SYNOPSIS. Organisms living in coastal waters, and especially estuaries, have long been known to have behavioral or physiological mechanisms that enable them to exist in water containing low amounts of oxygen. However, the respiratory con-sumption of oxygen that generates hypoxia is also responsible for producing sig-nificant amounts of carbon dioxide. An elevation of carbon dioxide pressure in water will cause a significant acidosis in most aquatic organisms. Thus, the com-bination of low oxygen and elevated carbon dioxide that occurs in estuaries rep-resents a significant environmental challenge to organisms living in this habitat. Organisms may maintain oxygen uptake in declining oxygen conditions by using a respiratory pigment and/or by making adjustments in the convective flow of water and blood past respiratory surfaces (i.e., increase cardiac output and ven-tilation rate). Severe hypoxia may result in an organism switching partially or completely to anaerobic biochemical pathways to sustain metabolic rate. There is also evidence to suggest that organisms lower their metabolism during hypoxic stress. Elevated water CO 2 (hypercapnia) produces an acidosis in the tissues of organisms that breathe it. This acidosis may be wholly or partially compensated (i.e., mechanisms return pH to pre-exposure levels), or may be uncompensated. Some studies have examined the effects on organisms of exposure simultaneously to hypoxia and hypercapnia. This article reviews some of the specific adaptations and responses of organisms to low oxygen, to high carbon dioxide, and to the co-occurrence of low oxygen and high carbon dioxide.
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This paper contrasts the natural and anthropogenic controls on the conversion of unreactive N2 to more reactive forms of nitrogen (Nr). A variety of data sets are used to construct global N budgets for 1860 and the early 1990s and to make projections for the global N budget in 2050. Regional N budgets for Asia, North America, and other major regions for the early 1990s, as well as the marine N budget, are presented to Highlight the dominant fluxes of nitrogen in each region. Important findings are that human activities increasingly dominate the N budget at the global and at most regional scales, the terrestrial and open ocean N budgets are essentially disconnected, and the fixed forms of N are accumulating in most environmental reservoirs. The largest uncertainties in our understanding of the N budget at most scales are the rates of natural biological nitrogen fixation, the amount of Nr storage in most environmental reservoirs, and the production rates of N2 by denitrification.
Article
Rising temperatures are set to imperil freshwater fishes as climate change ensues unless compensatory strategies are employed. However, the presence of additional stressors, such as elevated nitrate concentrations, may affect the efficacy of compensatory responses. Here, juvenile silver perch (Bidyanus bidyanus) were exposed to current-day summer temperatures (28oC) or a future climate-warming scenario (32oC) and simultaneously exposed to one of three ecologically relevant nitrate concentrations (0, 50 or 100 mg L-1). We measured indicators of fish performance (growth, swimming), aerobic scope (AS) and upper thermal tolerance (CTMAX) to test the hypothesis that nitrate exposure would increase susceptibility to elevated temperatures and limit thermal compensatory responses. After 8 weeks of acclimation, the thermal sensitivity and plasticity of AS and swimming performance were tested at three test temperatures (28, 32, 36oC). The AS of 28oC-acclimated fish declined with increasing temperature, and the effect was more pronounced in nitrate exposed individuals. In these fish, declines in AS corresponded with poorer swimming performance and a 0.8oC decrease in CTMAX compared to unexposed fish. In contrast, acclimation to 32oC masked the effects of nitrate; fish acclimated to 32oC displayed a thermally insensitive phenotype whereby locomotor performance remained unchanged, AS was maintained and CTMAX was increased by ∼1oC irrespective of nitrate treatment compared to fish acclimated to 28oC. Growth was however markedly reduced in 32oC-acclimated compared to 28oC-acclimated fish. Our results indicate that nitrate exposure increases the susceptibility of fish to acute high temperatures, but thermal compensation can override some of these potential detrimental effects.
Article
Climate warming and nitrate pollution are pervasive aquatic stressors that endanger the persistence of fishes prevailing in anthropogenically disturbed habitats. Individually, elevated nitrate and temperature can influence fish energy homeostasis by increasing maintenance costs and impairing oxygen transport capacity. However, it remains unknown how fish respond to simultaneous exposure to elevated temperature and nitrate pollution. Hence, we examined the combined effects of nitrate and elevated temperatures on aerobic scope (AS, maximum-standard metabolic rates) and cardiorespiratory attributes (haemoglobin HB, haematocrit HCT, relative ventricle mass RVM, and somatic spleen index SSI) in a freshwater salmonid, Thymallus thymallus. A 3 × 2 factorial design was used, 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 22 °C) for 6 weeks. Elevated temperature increased AS by 36% and the improvement was stronger when coupled with nitrate exposure, indicating a positive synergistic interaction. HB was reduced by nitrate exposure, while HCT was independent of nitrate pollution and temperature. Stressor exposure induced remodeling of key elements of the cardiorespiratory system. RVM was 39% higher in fish exposed to 22 °C compared to 18 °C but was independent of nitrate exposure. SSI was independent of temperature but was 85% and 57% higher in fish exposed to 50 and 200 mg NO3- l⁻¹, respectively. Taken together, these results highlight that simultaneous exposure to elevated temperatures and nitrate pollution offers cross-tolerance benefits which may be underscored by cardiorespiratory remodeling.
Article
Nutrient effluents from urban and agricultural inputs have resulted in high concentrations of nitrate in freshwater ecosystems. Exposure to nitrate can be particularly threatening to aquatic organisms, but a quantitative synthesis of the overall effects on amphibians, amphipods and fish is currently unavailable. Moreover, in disturbed ecosystems, organisms are unlikely to face a single stressor in isolation, and interactions among environmental stressors can enhance the negative effects of nitrate on organisms. Here, the effects of elevated nitrate on activity level, deformity rates, hatching success, growth and survival of three taxonomic groups of aquatically respiring organisms are documented. Effect sizes were extracted from 68 studies and analysed using meta-analytical techniques. The influence of nitrate on life-stages was also assessed. A factorial meta-analysis was conducted to examine the effect of nitrate and its interaction with other ecological stressors on organismal survival. Overall, the impacts of nitrate are biased towards amphibians (46 studies) and fish (13 studies), and less is known about amphipods (five studies). We found that exposure to nitrate translates to a 79% decrease in activity, a 29% decrease in growth, and reduces survival by 62%. Nitrate exposure also increases developmental deformities but does not affect hatching success. Nitrate exposure was found to influence all life-stages except embryos. Differences in the sensitivity of nitrate among taxonomic groups tended to be negligible. The factorial meta-analysis (14 amphibians and two amphipod studies) showed that nitrate in combination with other stressors affects survival in a non-additive manner. Our results indicate that nitrate can have strong effects on aquatic organisms and can interact with other environmental stressors which compound the negative effects on survival. Overall, the impacts of nitrate and additional stressors are complex requiring a holistic approach to better conserve freshwater biodiversity in the face of ongoing global change.
Article
The critical O2 tension (Pcrit) is the lowest PO2 at which an animal can maintain some benchmark rate of O2 uptake (ṀO2 ). This PO2 has long served as a comparator of hypoxia tolerance in fishes and aquatic invertebrates, but its usefulness in this role, particularly when applied to fishes, has recently been questioned. We believe that Pcrit remains a useful comparator of hypoxia tolerance provided it is determined using the proper methods and hypoxia tolerance is clearly defined. Here, we review the available methods for each of the three steps of Pcrit determination: (1) measuring the most appropriate benchmark ṀO2 state for Pcrit determination (ṀO2,std, the ṀO2 required to support standard metabolic rate); (2) reducing water PO2 ; and (3) calculating Pcrit from the ṀO2 versus PO2 curve. We make suggestions on best practices for each step and for how to report Pcrit results to maximize their comparative value. We also discuss the concept of hypoxia tolerance and how Pcrit relates to a fish's overall hypoxia tolerance. When appropriate methods are used, Pcrit provides useful comparative physiological and ecological information about the aerobic contributions to a fish's hypoxic survival. When paired with other hypoxia-related physiological measurements (e.g. lactate accumulation, calorimetry-based measurements of metabolic depression, loss-of-equilibrium experiments), Pcrit contributes to a comprehensive understanding of how a fish combines aerobic metabolism, anaerobic metabolism and metabolic depression in an overall strategy for hypoxia tolerance.
Article
Nitrite is a widespread form of pollution which directly lowers the blood oxygen carrying capacity of aquatically respiring species. It is unknown if this impairment of oxygen transport translates into an increased susceptibility to elevated temperatures. We hypothesised that nitrite exposure would lower blood oxygen carrying capacity and decrease both aerobic scope (maximum - standard metabolic rate) and heat tolerance. To test these hypotheses, juvenile European carp (Cyprinus carpio) were exposed to two levels of nitrite (0 mM or 1 mM) for seven days and haematological parameters, critical thermal maxima (CTMax) and aerobic scope were assessed. Nitrite exposure reduced total haemoglobin by 32.9%. Aerobic scope remained unchanged in fish exposed to nitrite; however, marked declines in CTMax (1.2°C reduction) were observed in nitrite-exposed fish. These findings demonstrate that nitrite exposure can significantly impair heat tolerance, even when aerobic capacity is maintained.
Article
Natural systems are threatened by a variety of anthropogenic stressors and so understanding the interactive threats posed by multiple stressors is essential. In this study we focused on urban stressors that are ubiquitous to urban estuarine systems worldwide: elevated nutrients, toxic chemical contaminants, built infrastructure and non-indigenous species (NIS). We investigated structural (abundance, diversity and species richness) and functional endpoints (productivity, primary production (chlorophyll-a) and metabolism) commonly used to determine responses to these selected stressors. Through a systematic review of global literature, we found 579 studies of our selected stressors; 93% measured responses to a single stressor, with few assessing the effects of multiple stressors (7%). Structural endpoints were commonly used to measure the effects of stressors (49% of the total 579 studies). Whereas, functional endpoints were rarely assessed alone (10%) but rather in combination with structural endpoints (41%). Elevated nutrients followed by NIS were the most studied single stressors (43% and 16% of the 541 single stressor studies), while elevated nutrients and toxic contaminants were overwhelmingly the most common stressor combination (79% of the 38 multiple stressor studies); with NIS and built infrastructure representing major gaps in multi-stressor research. In the meta-analysis, structural endpoints tended to decrease, while functional endpoints increased and/or decreased in response to different types of organisms or groups. We predicted an antagonistic effect of elevated nutrients and toxic contaminants based on the opposing enriching versus toxic effects of this stressor combination. Of note, biodiversity was the only endpoint that revealed such an antagonistic response. Our results highlight the continuing paucity of multiple stressor studies and provide evidence for opposing patterns in the responses to single and interacting stressors depending on the measured endpoint. The latter is of significant consequence to understanding relevant impacts of stressors in coastal monitoring and management.
Article
In this study, we measured the interactive effect of temperature (22 °C and 28 °C) and waterborne copper (Cu) contamination (9 μg/L and 20 μg/L) on the killifish Poecilia vivipara. Endpoints analyzed included parameters involved in Cu-accumulation, antioxidant capacity (antioxidant capacity against peroxyl radicals [ACAP] and total antioxidant capacity [TAC]), oxidative damage (lipid peroxidation [LPO]) and upper thermal tolerance (critical thermal maximum [CTMax]). Results show that Cu hepatic accumulation was elevated in 28 °C in comparison to 22 °C in both exposure groups. For gills, this was true only in 20 μg/L. Moreover, hepatic and brachial accumulation were concentration-dependent in both acclimation temperatures. Additionally, Hepatic ACAP and TAC were elevated in animals acclimated to 28 °C and only the animals kept at this temperature had reduced ACAP and TAC levels facing metal exposure (9 and 20 μg/L). Similarly, the combination of elevated temperature and Cu exposure raised hepatic LPO levels. Finally, animals acclimated to 28 °C had higher CTMax levels in comparison to fish acclimated to 22 °C both in control and exposed animals, however, CTMax of contaminated fish were only reduced in comparison to control in animals kept at 28 °C. Concluding, we show that the physiological mechanism besides the potentiating effect of elevated temperature in Cu toxicity is related to higher hepatic and branchial metal accumulation and elevated oxidative stress in the liver, outlined by reduced antioxidant capacity and elevated oxidative damage. We also show that these outcomes lead to compromised organismal performance, characterized by reduced CTMax. Finally, it is concluded that Cu exposure in warmer periods of the year or within global warming predictions may be more hazardous to fish populations.
Article
• Limited food availability and altered thermal regimes (e.g. cold water releases from dams) are two common stressors threatening the persistence of fishes inhabiting anthropogenically disturbed freshwater systems. Yet, the combined effects of these stressors remain poorly characterised. • To remedy this, we examined the isolated and combined effects of low temperature exposure and food restriction on specific growth rate (SGR, % body mass/day) and upper thermal tolerance (critical thermal maxima, CTMax) in larval white sturgeon (Acipenser transmontanus [Acipenseridae], 32 days post‐hatch, body mass: 0.25 ± 0.03 g, mean ± standard deviation). A 2 × 2 factorial design was implemented with fish exposed to one of two ecologically‐relevant acclimation temperatures (cold exposure: 11°C or a control temperature: 18°C) and one of two food restriction treatments designed to emulate observed declines in food availability (100% or 40% optimal feed rate) for 6 weeks (N: 3 replicate tanks/treatment, 50 fish/tank). • Specific growth rate was affected by both low temperature exposure and food restriction in isolation; low temperature exposure reduced SGR by 56.5% and food restriction reduced SGR by 30.6%. Simultaneous exposure to low temperature and food restriction resulted in a greater but less than additive reduction in SGR (80.6%), indicating that the stressors interacted antagonistically. • Critical thermal maxima were c. 2°C higher in 18°C‐acclimated fish (CTMax = 30.7 ± 0.4°C, mean ± standard error) compared to 11°C‐acclimated fish (CTMax = 28.6 ± 0.2°C, mean ± standard error); however, CTMax was independent of food restriction in both 11°C‐ and 18°C‐acclimated fish. • These data highlight the unpredictability of stressor interactions and may guide holistic conservation strategies, which target co‐occurring stressors in freshwater habitats.
Article
Pcrit - generally defined as the PO2 below which the animal can no longer maintain a stable rate of O2 consumption (ṀO2 ), such that ṀO2 becomes dependent upon PO2 - provides a single number into which a vast amount of experimental effort has been invested. Here, with specific reference to water-breathers, I argue that this focus on the Pcrit is not useful for six reasons: (1) calculation of Pcrit usually involves selective data editing; (2) the value of Pcrit depends greatly on the way it is determined; (3) there is no good theoretical justification for the concept; (4) Pcrit is not the transition point from aerobic to anaerobic metabolism, and it disguises what is really going on; (5) Pcrit is not a reliable index of hypoxia tolerance; and (6) Pcrit carries minimal information content. Preferable alternatives are loss of equilibrium (LOE) tests for hypoxia tolerance, and experimental description of full ṀO2 versus PO2 profiles accompanied by measurements of ventilation, lactate appearance and metabolic rate by calorimetry. If the goal is to assess the ability of the animal to regulate ṀO2 from this profile in a mathematical fashion, promising, more informative alternatives to Pcrit are the regulation index and Michaelis-Menten or sigmoidal allosteric analyses.
Article
Freshwater organisms are increasingly exposed to combinations of stressors. However, since it is time‐consuming and costly, research on the interaction of stressors, such as compound toxicity and global warming on vertebrates, is scarce. Studies on multigenerational effects of these combined stressors are almost non‐existent. Here, we tested the combined effects of 4 °C warming and cadmium exposure on life history traits, biomarkers, bio‐accumulation and multigenerational tolerance in the turquoise killifish Nothobranchius furzeri. The extremely short life cycle of this vertebrate model allows for assessment of sublethal and multigenerational effects within four months. The applied cadmium concentrations had only limited effects on the measured endpoints, which suggests that N. furzeri is more resistant to cadmium than fathead minnow and rainbow trout. In contrast, the temperature increase of 4 °C was stressful: it delayed female maturation and lowered adult mass and fecundity. Finally, indications of synergistic effects were found on peak fecundity and embryonic survival. Overall, these results indicate the importance of studying chronic and multigenerational effects of combined stressors. This article is protected by copyright. All rights reserved
Article
Multiple environmental stressors, including nutrient effluents (i.e. nitrates [NO3-]) and altered pH regimes, influence the persistence of freshwater species in anthropogenically disturbed habitats. Independently, nitrate and low pH affect energy allocation by increasing maintenance costs and disrupting oxygen uptake, which ultimately results in impacts upon whole animal performance. However, the interaction between these two stressors has not been characterised. To address this, the effects of nitrate and pH and their interaction on aerobic scope and physiological performance were investigated in the blueclaw crayfish, Cherax destructor. Crayfish were exposed to a 2 × 3 factorial combination, with two pH levels (pH 5.0 and 7.0) and three nitrate concentrations (0, 50 and 100 mg L-1NO3-). Crayfish were exposed to experimental conditions for 65 days and growth and survival were monitored. Aerobic scope (i.e. maximal - standard oxygen uptake) was measured at six time points (1, 3, 5, 7, 14, and 21 days) during exposure to experimental treatments. Crayfish performance was assessed after 28 days, by measuring chelae strength and whole animal activity capacity via the righting response. Survival was reduced in crayfish exposed to pH 5.0, but there was no exacerbation of this effect by exposure to high nitrate levels. Aerobic scope was compromised by the interaction between low pH and nitrate and resulted in prolonged elevations of standard oxygen uptake rates. Exposure to nitrate alone affected aerobic scope, causing a 59% reduction in maximum oxygen uptake. Reduced aerobic capacity translated to reduced chelae strength and righting capacity. Together, these data show that low pH and elevated nitrate levels reduce aerobic scope and translate to poorer performance in C. destructor, which may have the potential to affect organismal fitness in disturbed habitats.
Article
Pollution and predation are two omnipresent stressors in aquatic systems that can interact in multiple ways, thereby challenging accurate assessment of the effects of pollutants in natural systems. Despite the widespread occurrence of morphological antipredator mechanisms, no studies have tested how these can affect the sensitivity of prey to pesticides. Sensitivity to pesticides is typically measured via reductions in growth rates and survival, but also reductions in heat tolerance are to be expected and are becoming increasingly important in a warming world. We investigated how autotomy, a widespread morphological antipredator mechanism where animals sacrifice a body part (here the caudal lamellae) to escape when attacked by a predator, modified the sensitivity to the insecticide chlorpyrifos in larvae of the damselfly Coenagrion puella. Exposure to chlorpyrifos reduced the growth rate and heat tolerance (measured as CTmax). A key finding was that the pesticide had a greater impact on growth rates of intact animals, i.e. those that retained their lamellae. This reduced sensitivity to chlorpyrifos in animals without lamellae can be explained by the reduced outer surface area which is expected to result in a lower uptake of the pesticide. Larvae that underwent autotomy exhibited a lower heat tolerance, which may also be explained by the reduced surface area and the associated reduction in oxygen uptake. There is a wide diversity of morphological antipredator mechanisms, suggesting that there will be more examples where these mechanisms affect the vulnerability to pollutants. Given the importance of pollution and predation as structuring forces in aquatic food webs, exploring the potential interactions between morphological antipredator mechanisms and sensitivity to pollutants will be crucial for risk assessment of pollutants in aquatic systems.
Article
To evaluate whether oxygen-carrying capacity influences thermal tolerance in fishes, we reared four Chinook salmon families in present-day (+0°C) and possible future (+4°C) temperatures and assessed the response of hematocrit (Hct) to acute temperature stress. In the +4°C treatment, Hct increased above control levels when juvenile fish were exposed to their critical thermal maximum (CTmax). Conversely, no effect of temperature stress on Hct was found in the +0°C treatment. Hct was positively associated with CTmax (r2=0.12; n=66), contributing to the CTmax of the +4°C treatment being significantly higher than that of the +0°C treatment (mean ± SD, 26°±0.6°C and 25°±0.5°C, respectively). The association between CTmax and Hct found here supports the hypothesis that thermal tolerance is affected by oxygen supply to tissue. Moreover, the developmental plasticity of CTmax and Hct could represent an adaptive mechanism for salmon faced with climate change.
Article
Changes in the Earth's environment are now sufficiently complex that our ability to forecast the emergent ecological consequences of ocean acidification (OA) is limited. Such projections are challenging because the effects of OA may be enhanced, reduced or even reversed by other environmental stressors or interactions among species. Despite an increasing emphasis on multifactor and multispecies studies in global change biology, our ability to forecast outcomes at higher levels of organization remains low. Much of our failure lies in a poor mechanistic understanding of nonlinear responses, a lack of specificity regarding the levels of organization at which interactions can arise, and an incomplete appreciation for linkages across these levels. To move forward, we need to fully embrace interactions. Mechanistic studies on physiological processes and individual performance in response to OA must be complemented by work on population and community dynamics. We must also increase our understanding of how linkages and feedback among multiple environmental stressors and levels of organization can generate nonlinear responses to OA. This will not be a simple undertaking, but advances are of the utmost importance as we attempt to mitigate the effects of ongoing global change.
Article
Zebrafish is currently intensively reared in laboratories around the world due to its increasing importance as an experimental model in biomedical research. Recirculating aquaculture systems (RAS) have been generally used for zebrafish rearing, allowing the maintenance of high densities of fish with minimal water renewal. Biofilters play a pivotal role in RAS by converting the highly toxic ammonia excreted by fish into nitrate. Due to its much lower acute toxicity, nitrate accumulation in RAS is usually neglected, and fish can be exposed to relatively high levels of this compound for extended periods. This study evaluated the impact of chronic exposure to relevant levels of nitrate on the zootechnical performance and histology of selected organs (gills, integument, kidney, liver, and intestine) of juvenile zebrafish, with the aim to define safety levels of this nitrogenous compound for zebrafish rearing. For that, groups of 30-day-old zebrafish were exposed to < 7 (control), 100, 200, and 400 mg L− 1 nitrate-N for 28 days. No mortality was registered in fish exposed up to 200 mg L− 1 nitrate, and all individuals seemed externally healthy; however, in fish exposed to the highest nitrate concentration mortality reached 47% at the end of the trial, and many individuals showed lethargy, abnormal swimming, emaciation, lordosis, and/or superficial lesions. Although final growth was not significantly different among groups, growth parameters tend to decrease with increasing levels of nitrate, and a significant negative correlation was found between weight gain and nitrate levels, suggesting a dose-dependent negative effect of nitrate on growth. Except for the lowest nitrate concentration (100 mg L− 1 nitrate-N), the histological survey revealed significant changes induced by nitrate in all examined organs, and a dose-dependent effect of nitrate on the overall histopathological changes is suggested. In conclusion, this study shows that the chronic exposure of zebrafish juveniles to nitrate induces histopathological changes that would lead to a negative impact on the general health condition of fish. Fish growth tended to decrease and the overall histological damages tended to increase with increasing nitrate levels, particularly above 100 mg L− 1 (the lowest tested value). Thus, we recommend that this limit of 100 mg L− 1 nitrate-N should not be exceeded in RAS during rearing of juvenile zebrafish.
Article
Studies on chronic or acute toxicity of nitrogen species on fish in recirculating aquaculture systems (RAS) usually focused on adverse effects of total ammonia nitrogen (TAN: sum of NH3 + NH4+) and nitrite (), while underestimating the potential effects of high nitrate accumulation on growth and health status of fish. In our study, Nile tilapia (Oreochromis niloticus) were exposed to five different nitrate concentrations (0, 10, 100, 500 and 1000 mg L−1 -N) over 30 days. Growth parameters (feed conversion ratio (FCR), specific growth rate (SGR), hepatosomatic index (HSI)), blood samples (concentrations of haemoglobin, methaemoglobin, plasma /) and the histology of the gills were studied to evaluate growth and health status of the fish. At the highest nitrate concentration, the fish showed significantly reduced growth and impaired health status (SGR, FCR, plasma /, haemoglobin and methaemoglobin concentration), demonstrating that too high nitrate concentrations can negatively influence tilapia production in RAS. Here, we recommend not exceeding concentrations of 500 mg L−1 -N in juvenile tilapia culture to ensure an optimal health and growth status of the fish, as below that concentration no effects on the tilapia have been observed.
Article
Human activities are increasing both the frequency of hypoxic episodes and the mean temperature of aquatic ecosystems, but few studies have considered the possibility that acclimation to one of these stressors could improve the ability to cope with the other stressor. Here, we used Atlantic killifish, Fundulus heteroclitus, to test this hypothesis. Hypoxia tolerance was measured as time to loss of equilibrium in hypoxia (LOEhyp) at 0.4 kPa oxygen. Time to LOEhyp declined from 73.3±6.9 min at 15°C to 2.6±3.8 min at 23°C, and at 30°C no fish could withstand this level of hypoxia. Prior acclimation to warm temperatures significantly increased time to LOEhyp. Hypoxia tolerance of the southern subspecies of killifish, F. heteroclitus heteroclitus, was greater than that of the northern subspecies, F. heteroclitus macrolepidotus, measured both as critical oxygen tension (Pcrit) and as time to LOEhyp. Warm acclimation offset the negative effects of temperature on time to LOEhyp to a similar extent in the two subspecies. Warm acclimation increased total lamellar surface area of the gill in both subspecies as a result of regression of an interlamellar cell mass (ILCM). However, differences in total lamellar surface area could not explain differences in time to LOEhyp between the subspecies, suggesting that the lower time to LOEhyp of northern fish is related to their higher routine metabolic rate. These data suggest that thermal plasticity in gill morphology can improve the capacity of this species to tolerate hypoxia, and shows how existing plasticity may help organisms to cope with the complex interacting stressors that they will encounter with increasing frequency as our climate changes.
Article
The accelerating rate of global change has focused attention on the cumulative impacts of novel and extreme environmental changes (i.e., stressors), especially in marine ecosystems. As integrators of local catchment and regional processes, freshwater ecosystems are also ranked highly sensitive to the net effects of multiple stressors, yet there has not been a large-scale quantitative synthesis. We analysed data from 88 papers including 286 responses of freshwater ecosystems to paired stressors, and discovered that overall, their cumulative mean effect size was less than the sum of their single effects (i.e., an antagonistic interaction). Net effects of dual stressors on diversity and functional performance response metrics were additive and antagonistic, respectively. Across individual studies, a simple vote-counting method revealed that the net effects of stressor pairs were frequently more antagonistic (41%) than synergistic (28%), additive (16%) or reversed (15%). Here, we define a reversal as occurring when the net impact of two stressors is in the opposite direction (negative or positive) from that of the sum of their single effects. While warming paired with nutrification resulted in additive net effects, the overall mean net effect of warming combined with a second stressor was antagonistic. Most importantly, the mean net effects across all stressor pairs and response metrics were consistently antagonistic or additive, contrasting the greater prevalence of reported synergies in marine systems. Here, a possible explanation for more antagonistic responses by freshwater biota to stressors is that the inherent greater environmental variability of smaller aquatic ecosystems fosters greater potential for acclimation and co-adaptation to multiple stressors. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.
Article
Six species of darters (Etheostoma) were studied in the field and laboratory to relate respiration to habitat selection. The species ranged in habitat preference from a fast-water form (Etheostoma rufilineatum) to a slow-water form (Etheostoma fusiforme). Field measurements showed that winter O_2 tensions were not limiting the distribution of the darters when these tensions were compared with the critical O_2 determinations made in the laboratory at a low temperature (10 degrees C). However, summertime ambient O_2 levels were as low as 3.3 ppm [= 7.33 kilopascals or 55 mm Hg], a level that was 1.0-1.7 ppm below the wintertime critical O_2 tension (P_c) of several of these species. The fishes compensated for this by lowering P_c in the summer. This finding is contrary to previously reported results on the relationship between P_c and temperature. While not all darters selected habitats such that the downward shift of P_c in the summer was of ecological consequence, this was the case in 1 of the slow-water stream forms (E. boschungi) which inhabits waters in the summer that would be uninhabitable without such a shift in P_c. Additionally, one of the darters that typifies fast-water habitats (E. rufilineatum) had a P_c of 6.1 ppm [= 14.0 kilopascals or 105 mm Hg), which prevented it from occupying slow waters in the summertime. However, such respiratory exclusion from available habitats is not the case in the other stream forms studied (Etheostoma flabellare, Etheostoma squamiceps, and Etheostoma duryi), nor do these other stream forms occupy waters as slow as they are capable of on the basis of respiratory abilities. Therefore, a limited tolerance to low O_2 levels at summertime temperatures is not the general cause of the preference of Etheostoma for streams, although this is the case in some specific instances. In addition, the stillwater from (E. fusiforme) does not have any respiratory advantage over many of the stream forms in terms of a low metabolic rate or low P_c. However, darters as a group do show a rather low metabolic rate when compared to freshwater fishes in general.
Article
Climate change, sea level rise, and human freshwater demands are predicted to result in elevated temperature and salinity variability in upper estuarine ecosystems. Increasing levels of environmental stresses are known to induce the cellular stress response (CSR). Energy for the CSR may be provided by an elevated overall metabolic rate. However, if metabolic rate is constant or lower under elevated stress, energy for the CSR is taken from other physiological processes, such as growth or reproduction. This study investigated the examined energetic responses to the combination of temperature and salinity variability during a multigenerational exposure of partheogenetically reproducing Daphnia pulex. We raised D. pulex in an orthogonal combination of daily fluctuations in temperature (15, 15–25, 15–30 °C) and salinity (0, 0–2, 0–5). Initially metabolic rates were lower under all variable temperature and variable salinity treatments. By the 6th generation there was little metabolic variation among low and intermediate temperature and salinity treatments, but metabolic suppression persisted at the most extreme salinity. When grown in the control condition for the 6th generation, metabolic suppression was only observed in D. pulex from the most extreme condition (15–30 °C, 0–5 salinity). Generation time was influenced by acclimation temperature but not salinity and was quickest in specimens reared at 15–25 °C, likely due to Q10 effects at temperatures closer to the optima for D. pulex, and slowest in specimens reared at 15–30 °C, which may have reflected elevated CSR. Acute tolerance to temperature (LT50) and salinity (LC50) were both highest in D. pulex acclimated to 15–30 °C and salinity 0. LT50 and LC50 increased with increasing salinity in specimens raised at 15 °C and 15–25 °C, but decreased with increasing salinity in specimens raised at 15–30 °C. Thus, increasing temperature confers cross-tolerance to salinity stress, but the directionality of synergistic effects of temperature and salinity depend on the degree of environmental variability. Overall, the results of our study suggest that temperature is a stronger determinant of metabolism, growth, and tolerance thresholds, and assessment of the ecological impacts of environmental change requires explicit information regarding the degree of environmental variability.
Article
RNA pieces in the spliceosome, has a domain V counterpart, containing a 2-nucleotide bulge located 5 base pairs away from an AGC triad (10). Formation of an analogous metal-binding platform in this region of U6 (11) may explain the apparent ability of spliceosomal RNAs to retain catalytic activity in the complete absence of the many protein components that usually accompany splicing (12). A domain V-like element could have played a major role during the RNA world era of evolution , serving as the catalytic center for RNA cleavage, transesterification, and polymeriza-tion reactions. The new structure provides a powerful starting point for future investigations of group II introns and the spliceosome. The lack of electron density for domain VI, which is important for the first step of splicing in many group II introns, and the absence of exons from the structure preclude us from seeing how these elements dock onto the surface created by domains I to V. Thus, the structural details of substrate recognition and catalysis remain undefined. The nature of the conformational change known to separate the two steps of splicing (13) also remains unclear. Finally, it will be important for our understanding of group II intron self-splicing to capture the structures of the other intermediates along the splicing pathway and to pursue experiments that link features of these structures with functionally defined interactions.
Article
Cobia Rachycentron canadum is a fast growing fish with world-wide potential for aquaculture, and has been considered for rearing in recirculating aquaculture systems (RAS). Nitrate is considered the least toxic nitrogenous product in the ammonia nitrification process, but as it may accumulate in RAS, toxic levels can be reached. The objective of this study was to evaluate the acute toxicity and the histopathological effects of nitrate on juvenile cobia. Juveniles (6.87±0.36g; 11.8±0.19cm) were acutely exposed to six concentrations of nitrate (500–3000ppm NO3−-N) plus a control during 96h. At the end of this period of exposure, juvenile cobia were sampled for histopathological evaluation. The estimated LC50 of nitrate to juvenile cobia was equal to 2407 and 1829mg/L NO3−-N at 24 and 96h, respectively. Cobia exposed to sub-lethal nitrate concentrations showed histopathologic alterations in the gills, esophagus and brain. The gills revealed epithelial hyperplasia with complete lamellar fusion, telangiectasia, and lamellar shorting induced by necrosis, and the esophagus presented hyperplasia of epithelium and mucus cells. In the brain, glial cells proliferation, satellitosis (microglial cells surrounding neurons with swollen and prenecrotic neurons), and Virchow-Robin spaces (enlarged perivascular spaces, EPVS) were observed. The results of the present study indicate that juvenile cobia have a high tolerance to acute exposure of nitrate. However, assorted histopathological responses were observed for cobia at sub-lethal nitrate concentrations. Therefore, further studies are needed to estimate safe chronic nitrate levels for juvenile cobia culture.
Article
Hypoxia is an ever increasing threat to aquatic systems. While fluctuating levels of dissolved oxygen (DO) can be a natural phenomenon, hypoxia caused by eutrophication and organic pollution is now considered to be amongst the most pressing and critical water pollution problems in the world, particularly in densely populated regions. The effects of low DO on fishes are an area of great concern and thriving study. Researchers have examined the effects of low DO on fishes from the cellular to community level. The purpose of the current paper is to review the effects of low DO on complex fish behaviour, community and fish physiology. Our review will also highlight studies in which DO is known to interact with a known contaminant. Throughout the paper we will highlight areas in need of future research such as chronic exposure, interactive effects of DO and contaminants, an increased understanding of how hypoxia affects communities of organisms, and finally a need for an increase in freshwater studies.
Article
For aquatic ectotherms, increasing water temperatures cause an exponential increase in metabolic rate and decreasing oxygen solubility. Fish species that regulate their metabolic rate to low dissolved oxygen concentrations are understood to be hypoxia tolerant whereas salmonid fish are considered to be classic metabolic conformers and their metabolic rate is dependent on the environmental oxygen concentration. This study examined Atlantic salmon, Salmo solar, undergoing a progressive hypoxia at optimal temperatures and at temperatures nearing the upper thermal tolerance limit for the species to determine if metabolic regulation occurred. Oxygen consumption was measured on individual Atlantic salmon (150.7 +/- 40.8 g) in 66-L static respirometers; oxygen measurements were taken every 5 min until the fish lost equilibrium. Metabolic regulation was observed at all temperatures and occurred in 67, 50 and 50% of the fish at 14, 18 and 22 degrees C. respectively. The plateau metabolic rate (VO(2PL)) was 293.4 +/- 24.5 mg.kg.h(-1) at 22 degrees C which was significantly higher than in the 14 and 18 degrees C treatments (191.1 +/- 24.5 and 203.9 +/- 12.6 mg.kg.h(-1), respectively). This difference was also reflected in the critical oxygen threshold (P(crit)) where the value for the 22 degrees C treatment (4.59 +/- 0.32 mg.L(-1)) was significantly higher than those of the 14 and 18 degrees C treatments (3.46 +/- 0.14 and 3.39 +/- 0.26 mg.L(-1) respectively). These results indicate that some fish from the Tasmanian population of Atlantic salmon have the ability to regulate metabolic rate to low oxygen concentrations and therefore show a relatively high degree of hypoxia tolerance. (C) 2011 Elsevier B.V. All rights reserved.
Article
Water pollution induces pathological changes in fish. As an indicator of exposure to contaminants, histology represents a useful tool to assess the degree of pollution, particularly for sub-lethal and chronic effects. However, a standardized method for the description and assessment of histological changes, mainly for use in freshwater fish, is still lacking. In this paper, the present authors propose a standardized tool for the assessment of histological findings which can be applied to different organs. The methodology is based on two factors: (1) the extension of a pathological change is rated with a ‘score value’; and (2) the pathological importance of this alteration is defined as an ‘importance factor’. The sum of the multiplied score values and importance factors of all diagnosed changes results in different indices. With these indices, statistical analysis can be carried out. Assessment methods for the gills, liver, kidney and skin are described.