Czechia

Increasing drought frequency and duration pose a significant threat to fish species in dryland river systems. As ectotherms, thermal and hypoxia tolerances directly determine the capacity of fish species to persist in these environments during low flow periods when water temperatures are high and waterbodies become stratified. Managing the ecological function of dryland river refugia requires an understanding of how chronically elevated temperatures and hypoxia affect fish physiological performance, and their capacity to compensate performance in response. We studied the effects of chronic high temperature (30oC) or chronic hypoxia (50% air saturation) on two species of freshwater fish, golden perch Macquaria ambigua, and Murray cod, Maccullochella peelii. In both species, cross tolerance resulted in improved hypoxia and thermal tolerance limits following acclimation to either chronic hypoxia or high temperature. Physiological plasticity of the heart also resulted following acclimation to hypoxic waters in golden perch, increasing in size to support an elevated oxygen transport capacity. In Murray cod, body size significantly affected thermal and hypoxia responses. Small M. peelii were significantly less hypoxia tolerant than larger individuals, while larger fish were significantly less thermally tolerant than smaller fish. Our data suggests that acclimation to elevated temperature or elevated hypoxia can improve both hypoxia and thermal tolerance limits in some fish species, but that responses man differ across size classes. This information will be critical in developing habitat suitability models to inform policy decisions that serve to balance competing demands on freshwater resources in dryland rivers.

SEB Conference 2024

Between a hot place and hypoxia: Physiological responses of fish to high temperature and hypoxia in dryland river systems

drought

cross tolerance

fish

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SEB Conference 2025

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Coastal and estuary ecosystems are increasingly subjected to compound events, where multiple extreme events such as marine heatwaves (MHW) and hypoxia occur simultaneously. Given the fundamental role of temperature and oxygen in biological processes, the combined alteration of these parameters is often expected to have non-linear effects on organisms. However, the potential for an extreme temperature event to elicit a different response than gradual warming when combined with hypoxia remains understudied. Therefore, we examined the within-generational response to simultaneous hypoxia and MHW, and the transgenerational plastic responses during the recovery phase of the foundational marine copepod species, Acartia tonsa. In more detail, we measured survival, egg production rates, metabolic rates and upper thermal limits on mature males and females (F1) following a five-day exposure to the isolated and combined effects of hypoxia and a MHW, and on their offspring (F2) placed back in a control condition for their whole development, once they reached adulthood. Surprisingly, we observed only synergistic effects for F1 males’ upper thermal limits and no strong carry-over effects in the recovering F2 coming from the parental combined condition. However, MHW had a dominant effect on the physiology of A. tonsa, decreasing survival, increasing metabolic rates and causing strong carry-over effects on F2 individuals’ survival. These results indicate that potential cross-protection mechanisms between the two stressors were at play, enabling copepods to face the deletarious effects of MHW when exposed simultaneously to hypoxia. This supports the importance of looking at the impact of compound events on marine organisms.

Two extreme events are better than one? Physiological responses following simultaneous exposure to hypoxia and marine heatwave in Acartia tonsa 

The physiological response to thermal fluctuations has been a subject of investigation for decades. These studies have been fundamental in unraveling the underlying mechanisms that allow organisms to cope with temperature variations. However, when attempting to generalize organism responses to environmental changes in natural settings, we confront the inconvenient truth: the natural environment is inherently complex, characterized by diverse and variable stressors. To address this complexity, a transition from a single stressor approach to a multiple stressor scenario becomes essential. We must explore how different thermal fluctuations interact with co-variability in other environmental factors, shaping the phenotypic responses of wild organisms. While recent research has focused on predictable co-variation between stressors (e.g., heat and hypoxia), the effects of simultaneous exposure to emerging and human-driven environmental changes—such as heatwaves and toxicant releases—remain unclear. In this context, I present two case studies examining the phenotypic plasticity of thermal tolerance in adult and offspring generations of two fish species. Specifically, we investigate their responses to environmentally relevant concentrations of copper, a widely used and released metal, both independently and in conjunction with a heatwave event. Contrary to our initial hypothesis, the data revealed unexpected results: thermal plasticity was not negatively affected by copper exposure, to some extent. Instead, we observed a potential cross-tolerance effect. This suggests that exposure to one stressor may enhance an organism’s ability to cope with a second stressor. These findings offer new perspectives and inform future research directions.

One is not enough: the importance of multiple-stressor scenarios in thermal plasticity studies

Migratory flight is an intensive exercise that requires birds to maintain high aerobic capacities for many hours or days. Maintaining oxygen supply to flight muscles is therefore important during migration, especially since tracking studies have shown that songbirds will ascend to altitudes of 6,000 m during migratory flight where oxygen is less readily available (hypoxia). Whether there are adaptations or seasonal plasticity along the oxygen cascade and hypoxic chemoreflex that allow songbirds to fly at such high altitudes during the migratory season is unknown. Research during my postdoc has shown that migratory songbirds exhibit seasonal plasticity along the oxygen cascade which enhance oxygen uptake and movement to the flight muscle during migration. This includes changes in breathing pattern, haemoglobin-oxygen binding affinity, and muscle fiber size and phenotype. We have also shown that the magnitude of seasonal plasticity may be dictated by migratory distance and/or whether the songbird species is migratory or resident. Additionally, in yellow-rumped warblers there appears to be a blunted hypoxic chemoreflex in response to flight at high altitude (~3,000 m above sea level). Measurements of adrenaline concentration increased with flight, but were not significantly higher when flight was conducted at high altitude. Together, these findings show that songbirds exhibit seasonal plasticity along the oxygen cascade which would be important for maintain oxygen movement to the flight muscle during high altitude flight.

Seasonal flexibility along the oxygen cascade allows for high-altitude flights in migratory songbirds

Chromosomal fusions have been hypothesized to facilitate evolutionary adaptation, but empirical evidence has been scarce. We examined patterns of genome evolution in the copepod Eurytemora affinis species complex, which are numerically dominant and highly invasive, with the extraordinary capacity to rapidly invade novel salinities. Prior studies from my laboratory (Lee Lab) revealed parallel selection acting on the same sets of ion transporter genes during independent saline to freshwater invasions by populations from genetically distinct clades. Our chromosome-level genomes of three genetically divergent clades of this species complex revealed peculiar genome architectures that might contribute to their remarkable capacity to acclimate and evolve during salinity invasions. Among the three genetically distinct clades of E. affinis complex, we discovered independent chromosomal fusions in two different clades, with the Europe clade (E. affinis proper) having 15 chromosomes, fusing independently into 7 chromosomes in the Gulf clade (E. gulfia) and 4 chromosomes in the Atlantic clade (E. carolleeae). Notably, for the highly invasive Atlantic clade (E. carolleeae), chromosomal fusion sites, especially the centromeres, were significantly enriched with signatures of selection associated with contemporary shifts from saline to freshwater habitats. These chromosomal fusions joined functionally-related ion transporter genes, forming "supergenes" at the centromeres, where recombination is low. This study uncovers novel patterns of genome architecture evolution with potentially important implications for mechanisms of adaptive evolution in response to radical environmental change.

Genome Architecture Evolution in the copepod Eurytemora affinis species complex

Heatwaves and hypoxic events are major stressors for marine fishes in an era of global change. We investigated impacts of these extreme events on growth and indicators of metabolism in the three genetically distinct populations of European seabass, Atlantic (AT), West and East-Mediterranean (WM, EM). Fish were reared in common garden in three seasonal thermal regimes that reflect their natural environments (eAT 10.2-18.4°C; eWM 12.5-23.0°C; eEM 16.7-28.0°C) and, at peak summer temperature, subsamples were weighed, measured and exposed in triplicate for 10 days to a heatwave (+4°C), hypoxia (50% saturation), heatwave+hypoxia, or control. At the end of the events, weight and length were measured, and blood samples were collected to measure plasma indicators of energy and hydromineral homeostasis (glucose, lactate, triglycerides, protein, calcium, chloride, sodium, osmotic pressure). By comparison to the control, in eAT warming from 18.4 to 22.4°C stimulated feeding and growth, an effect abolished by hypoxia and hypoxia+heatwave. In eWM, there was little effect of heatwave or hypoxia on growth, but heatwave+hypoxia caused some inhibition. In eEM, all the extreme events caused loss of appetite and weight loss. Despite evolving in different thermal environments, the three populations did not differ in the effects of the extreme events on growth, plasma indicators are being analysed. The results demonstrate that heatwaves can be beneficial for growth in cold environments, that seabass are resilient to moderate hypoxia even when in combination to a heatwave, but extreme events clearly have negative effects in warmer environments.

Impacts of heatwaves and hypoxia on three European seabass Dicentrarchus labrax populations reared in three seasonal thermal regimes

In ecological communities, body mass and species abundance create size spectra that reflect metabolic demands varying with size. These patterns are generally predictable; however, how temperature and predation alter these predictions at the community level has not been extensively explored. In this study, we recreated 24 stream ecosystems in mesocosms to test the effects of two temperatures (ambient and elevated) and two predation scenarios. After 30 days, we analyzed macroinvertebrate samples for changes in the ontogenetic scaling of their resting metabolic rates. Our study investigates the impact of temperature on the ontogenetic scaling of resting metabolic rate in freshwater macroinvertebrate communities, particularly in relation to predation regimes. We observe that rising temperatures cause a decrease in the metabolic scaling exponent in environments with predation pressure (e.g., from approximately 0.3 to 0.5, instead of the predicted 0.75 by metabolic scaling theory), resulting in steeper size spectra (e.g., from approximately -1.2 to -1.5, instead of the predicted -2 by metabolic scaling theory). These findings highlight the complex interplay between abiotic and biotic factors in shaping energy dynamics and community structure, underscoring the need to integrate individual energetics into size-based community models. This underscores the interactive influence of temperature and predation on ecophysiological processes, potentially altering the slope of size spectra and affecting energy exchange equilibrium within ecological communities. Our work paves the way for further investigation into the ecological consequences of metabolic scaling deviations in the context of global environmental changes.

Temperature and predation affect metabolic scaling: Do individual metabolic constraints shape community size spectra?

Fishes are fantastically plastic creatures, capable of altering their morphology, physiology, life-history, and even offspring responses to changes in the environment. A key question in freshwater ecology is how fishes will respond through plasticity to continued changes in climate - especially to predicted increases in the variability of temperature. Here, we describe lessons learned from long-term, multi-generational experiments examining thermal variability's plastic effects in a model organism, zebrafish (Danio rerio). We found that long-term exposure to thermal variability confers relative benefits in a number of fitness-related traits, but these benefits were not without costs. We highlight the advantages of using thermally variable regimes in experiments and discuss important knowledge gaps that remain in the study of thermal variability and plasticity.

Plasticity to thermal variability within and across generations

Confidence and competence in skills pertaining to data manipulation, analysis, interpretation, and presentation are increasingly sort and valued across numerous sectors but improving these competencies in students can be challenging. There is a disconnect between the steps taken to understand and analyse data and then present findings in a professional setting and students frequently struggle with this. In response to this, I have created fresh resources, introduced new teaching tools, modified assessment strategy and reinforced key concepts in data literacy across the curriculum with an aim of improving student engagement and attainment in the data sciences. This process, as with many advances in improving pedagogical practice, has largely been ad hoc and evidence for initial success and impact began as observational and qualitative. However, long-term cohort wide data sets can come into effect with consistent use of assessment and marking strategy. Here, I show how different evidence can be used to measure impact of pedagogical practice to students and then how this evidence can be applied to guide course curriculums, and more broadly, influence best practice beyond the institution.

Scholarship in data literacy; establishing evidence of pedagogical efficacy and measuring impact and influence within and beyond the institution.

In West Africa, cereals production per capita is declining due to the fast growth in population outpacing the increase in food production. Moreover, models predict that global changes will reduce cereals yield in this region. Improve crop productivity and resilience is therefore a major challenge. Pearl millet is a key cereal for food security in sub-Saharan Africa, mostly grown in areas with limited agronomic potential characterized by low rainfall and low-fertility soils. 
Root traits represent potential new targets for breeding pearl millet varieties more adapted to future climate scenarios. The aim of this work was to characterize the main water stress pattern faced by pearl millet in West Africa and to identify root traits that contribute to tolerance to this stress. 
Crop modelling revealed that vegetative drought stress is a major constraint in the Sahel. Subsequently, a panel of 160 newly re-sequenced inbred lines was grown under irrigated and vegetative drought stress over two years in field conditions in Senegal and was phenotyped for root anatomical traits, as well as yield components traits. We observed a large diversity of response to drought stress for root traits and yield stress tolerance indexes. We found that some root vasculature traits were associated with tolerance to vegetative drought stress. Further physiological studies suggest a link between metaxylem traits and transpiration efficiency and suggest an overall strategy to deal with vegetative stress. QTLs controlling these traits were identified by association genetics. Our results could provide new avenues to improve drought tolerance in pearl millet.


Targeting root traits to improve tolerance to vegetative drought episodes in pearl millet (Pennisetum glaucum L.) 

Reproductive senescence is increasingly reported to occur in vertebrate populations in the wild but how the onset and rate of reproductive senescence vary in response to environmental fluctuations remains poorly understood. Taking benefit on a long-term detailed individual monitoring of reproductive performance of known-aged females for over 35 years in a forest roe deer population in western France, we first provide firm evidence that both pregnancy rates and litter size display senescence, with different age at onset and different rate of senescence. Then, we show that cohort quality markedly influences reproductive senescence patterns. Moreover, condition-dependence of reproductive performance differs both across female ages and among cohorts of different quality.

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Two extreme events are better than one? Physiological responses following simultaneous exposure to hypoxia and marine heatwave in Acartia tonsa

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