Simulated Heat Wave Research Articles

Simulated Heatwaves Lead to Upregulated Chemical Defense of a Marine Foundation Macrophyte Against Microbial Colonizers

Climate change is characterized not only by increase an mean temperature, but also an increase in the variability around the means causing extreme events like marine heatwaves. These events are expected to have strong influence on the ecology of marine foundation species such as the eelgrass Zostera marina. Bacterial and macroscopic foulers are ubiquitous in the marine environment; they can have detrimental impacts on macrophytes and warming is known to enhance bacterial fouling. Thus, to investigate the consequence of heatwaves on the chemical defence of eelgrass against microbial colonisers, we incubated Z. marina plants in the Kiel Outdoor Benthocosm system under ambient control conditions and two different heatwave treatments: a treatment experiencing two spring heatwaves followed by a summer heatwave, and a treatment only experiencing just the summer heatwave. The capacity to deter microbial colonisers was found to be significantly up-regulated in Z. marina from both heatwave treatments in comparison to Z. marina under control conditions, suggesting defence regulation of Z. marina in response to marine heatwaves. We conclude climate extremes such as heatwaves can trigger an regulation in the defence capacity, which could be necessary for resilience against climate change scenarios. Such dynamics in rapid regulation of defence capacity as found in this study could also apply to other host plant – microbe interactions under scenarios of ongoing climate change or extreme climate events like heatwaves.

Lower bioenergetic costs but similar immune responsiveness under a heat wave in urban compared to rural damselflies.

There is mounting evidence that the widespread phenotypic changes in response to urbanization may reflect adaptations caused by rapid evolutionary processes driven by urban‐related stressors. Compared to increased habitat fragmentation and pollution, adaptations towards another typical urban‐related stressor, that is higher and longer lasting very high temperatures (heat waves), are much less studied. Notably, the sensitivities to heat waves of life‐history traits and important fitness‐related physiological traits such as immune responsiveness and bioenergetic variables (energy availability, energy consumption and their balance) have never been contrasted between urban and rural populations. By conducting a laboratory common‐garden experiment, we compared effects of a simulated heat wave on life history (survival and growth rate), immune responsiveness and bioenergetic variables between three urban and three rural populations of the damselfly Coenagrion puella. Because energy‐mediated trade‐off patterns may only be detected under energetically costly manipulations, all larvae were immune‐challenged by simulating ectoparasitism by water mites. As expected, the simulated heat wave caused negative effects on nearly all response variables. The immune responsiveness, on the other hand, increased under the heat wave, consistent with a trade‐off pattern between immune function and growth, and this similarly between urban and rural populations. A key finding was that urban larvae suffered less from the simulated heat wave compared to the rural larvae in terms of a lower heat wave‐induced depletion in energy availability. This suggests an adaptation of urban populations to better cope with the stronger and more frequent heat waves in cities. Notably, this urbanization‐driven evolution in the bioenergetic variables was not apparent in the absence of a heat wave. Given that changes in energy budgets have strong fitness consequences, our findings suggest that the evolved higher ability to cope with heat waves is fundamental for the survival of urban damselfly populations.

Life History and Immune Challenge Influence Metabolic Plasticity to Food Availability and Acclimation Temperature.

Animals vary in their rates of energy expenditure for self-maintenance (standard metabolic rate [SMR]). Yet we still lack a thorough understanding of the determinants of SMR, potentially because of complex interactions among environmental, life-history, and physiological factors. Thus, we used a factorial design in female sand field crickets (Gryllus firmus) to investigate the independent and interactive effects of food availability (unlimited or limited access), acclimation temperature (control or simulated heat wave), life-history strategy (flight-capable or flight-incapable wing morphology), and immune status (control or chronic immune activation) on SMR (CO2 production rate) measured at 28°C. Both environmental factors independently affected SMR where heat wave and food limitation reduced SMR. Furthermore, wing morphology and immune status mediated the plasticity of SMR to food and temperature. For example, the hypermetabolic effect of food availability was greater in flight-capable crickets and reduced in immune-challenged crickets. Therefore, although SMR was directly affected by food availability and acclimation temperature, interactive effects on SMR were more common, meaning several factors (e.g., life history and immune status) influenced metabolic plasticity to food and temperature. We encourage continued use of factorial experiments to reveal interaction dynamics, which are critical to understanding emergent physiological processes.

Can irrigating more frequently mitigate detrimental heat wave effects on perennial ryegrass growth and persistence?

Heat waves are problematic for grazing livestock systems in regions dependant on temperate (cool-season) pasture species, owing to the often minimal heat tolerance (thermotolerance) of these pastures. A field experiment in north-west Tasmania, Australia, tested the hypothesis irrigating more frequently reduces detrimental heat wave effects on perennial ryegrass (Lolium perenne L.) above-ground growth and short-term persistence (measured via basal frequency). Heat wave conditions were imposed using freestanding infrared heater arrays fitted with a novel control system designed for evaluating heat wave mitigation options. The control system enabled the extent of warming to be regulated, whilst applying an equal infrared flux (heating) to all heated plots. Increasing the frequency of irrigation events suppressed crown temperature (plant-soil interface) elevation under heaters. When irrigation was applied every 2, 5 or 10 days, median crown temperature elevation under heaters equalled 5.5 °C, 6.5 °C and 7.0 °C, respectively. However, detrimental effects of imposed heat wave conditions on perennial ryegrass growth were not mitigated by more frequent irrigation events. This resulted from the small differences in crown temperature elevation between irrigation frequency treatments, combined with supraoptimal crown temperatures occurring <10% of the time during the simulated 10-day heat wave events. The infrequent occurrence of supraoptimal crown temperatures: (i) resulted from mild ambient conditions (mean ambient air temperature, 17.2 °C); and (ii) explains why applied heating only reduced perennial ryegrass growth by 13 kg DM ha−1 day−1 (23%) when heaters were operated, with no residual effect in the accompanying recovery regrowth cycle. Applied heating also had no appreciable effect on perennial ryegrass basal frequency. Under more extreme heat wave conditions, irrigation frequency may have a larger effect on perennial ryegrass tissue temperature and subsequent growth, especially in environments characterised by higher crown temperatures.

Projection of heat wave in China under global warming targets of 1.5 °C and 2 °C by the ISIMIP models

Heat waves in China were projected under 1.5 °C and 2 °C of warming targets using the bias-adjusted simulations of 4 GCMs obtained from ISIMIP2b. The simulations of GCMs on heat wave over China during historical period of 1986–2005 were examined against daily gridded observation CN05 from National Meteorological Information Center of China. In this evaluation, three heat wave definitions respectively with absolute, relative and combined threshold were used. The results indicated that the spatial distribution of heat wave indices, including heat wave days and heat wave magnitude, could be reasonably reproduced by the simulations obtained from ISIMIP2b, and the centers over Xinjiang and Jianghuai areas could be well captured by the multi-model ensemble (MME). Among three definitions based on different thresholds, the heat wave definition based on combined threshold outperformed other two definitions with the higher correlation coefficient with observation, particularly the heat wave days (0.89) and heat wave duration (0.89). In addition, the temporal evolutions of simulated heat waves indices were generally in good agreement with observations, particular the heat wave magnitude, but with lower trend of heat wave days (0.72 days/decade) and duration (0.17 days/decade) than those in observation (1.73 days/decade and 0.41 days/decade) due to the positive bias before 1990s. The features of heat waves over China under 1.5 and 2 °C warming targets projected by the ISIMIP were explored further. The periods of 2020–2039 under RCP2.6 scenario and 2040–2059 under the RCP4.5 scenario were selected to represent the 1.5 °C and 2 °C warming targets, respectively. The MME suggested that the heat wave will increase significantly in terms of days, duration and magnitude over China. The averaged heat wave days and heat wave duration were projected to reach 10.8 days and 3.9 days under 1.5 °C warming target, which was around 1.86 and 1.56 times relative to those during historical period. Notably, the increases of heat wave were at a higher rate under 2 °C of warming target than 1.5 °C of warming target, with the heat wave days and heat wave duration were about 2.53 and 2.00 times of those during historical period. Moreover, the heat wave magnitude was projected to reach 35.2 °C and 35.4 °C, with the highest frequency fall in the range of 34.9–35.2 °C and 35.5–35.8 °C under 1.5 °C and 2 °C of warming targets, respectively.

Heatwave effects on the swimming behaviour of a Mediterranean freshwater fish, the Iberian barbel Luciobarbus bocagei

Heatwaves, which can be defined as increases of at least 5 °C in air temperature for more than five consecutive days for a specified reference period, are expected to become more frequent under the ongoing climate change, with freshwater organisms being particularly vulnerable to high temperature fluctuations. In Mediterranean-climate areas, depending on the extent of summer droughts and loss of longitudinal connectivity, river segments may become isolated, maintaining fish populations confined to a series of disconnected pools, with no possibility to move to thermal refugia and thus becoming more prone to thermal stress. In this study, we evaluated the effect of a simulated heatwave on the swimming behaviour of juvenile stages of a potamodromous native cyprinid fish, the Iberian barbel Luciobarbus bocagei, under experimental mesocosm conditions. Behavioural traits included fish activity, boldness and shoal cohesion and were continuously measured at a constant flow velocity of 18 cm s−1, which is typical of riffle habitats. Overall, results show that the behaviour of juvenile Iberian barbel is likely to be affected by heatwaves, with fish displaying lower activity and boldness, while no clear difference was observed in shoal cohesion. This study highlights the importance of managing thermal refugia that are crucial for fish to persist in intermittent rivers. Future studies should focus on the interaction of heatwaves with other stressors, such as oxygen depletion, for a broader understanding of the perturbation affecting freshwater fishes under a changing climate.

Thermoregulatory Responses of Heat Acclimatized Buffaloes to Simulated Heat Waves.

Simple SummaryThe simulated heat wave triggered a relevant thermoregulatory response in heat-acclimatized buffaloes. During the simulated heat wave the high respiratory rate and seating rate limited the heat storage and the levels of hyperthermia. Moderate and transient hyperthermia combined with complete recovery of homeothermy during the night prevented changes in blood parameters, except for ion potassium. The high sweating rates seem to have been crucial for the homeothermy’s maintenance. For the first time, early in the morning, adaptive hypothermia was recorded in buffaloes.Climate change is seen as a significant threat to the sustainability of livestock production systems in many parts of the world, particularly in tropical regions. Extreme meteorological events can result in catastrophic production and death of livestock. Heat waves in particular can push vulnerable animals beyond their survival threshold limits. However, there is little information about buffalo responses to sudden changes in the thermal environment, specifically the heat waves. This study aimed to quantify the thermoregulatory and blood biochemical responses of heat-acclimatized buffaloes to a simulated heat wave. The experiment was designed in a climatic chamber with two periods of 4 days each. Twelve heat acclimated buffalo heifers aged 18 months were used. The climatic chamber environment was set as follows: 4-day period (P1) simulating the same weather conditions of a summer in humid tropical climate used as a baseline, with daily cycle with Ta and RH at 27 ± 1 °C and 76% from 0600 h to 1900 h and 24 ± 1 °C and 80% from 1900 h to 0600 h, and 4-day period (P2), simulating a daily heat wave cycle, from 0600 h to 1900 h with Ta and RH kept at 36 °C and 78% and from 1900 h to 0600 h, 27 °C and 74%. All animals were subject to both treatments and data were analyzed by a repeated measure analysis of variance, with post-hoc pooling comparison performed by Tukey’s test. In P2, there was observed a significant increase in respiratory frequency (p < 0.01), found four times in P1. The sweating rates were quite high in both periods; still, there were significant increases in P2 compared to P1 (p < 0.01) (4931 and 3201 g/m2/h, respectively). A slight but significant increase in rectal temperature was observed during the day (p < 0.01), with a rising until 1900 h. The simulated heat wave in P2 did not affect the values of the erythrogram or leukogram, excluding the significant reduction in K+ (p < 0.05). The low heat storage and the subsequent fast and full recovery of the thermal balance late afternoon appear to be related to the high sweating rate values. The massive sweating rate emphasizes its relevance in the maintenance of buffalo homeothermy. The absence of changes in hematological parameters has revealed the considerable physiological resilience of buffaloes toward simulated heat waves.

Gene expression differences between abalone that are susceptible and resilient to a simulated heat wave event

Sudden increases in water temperature can lead to mortality of cultured and wild mollusc communities during the summer months. The frequency and severity of mortality events are expected to increase as climate changes. The molecular basis for individual survival ability is poorly understood. To better understand signatures and mechanisms affecting survival we analyzed the gene expression profiles of greenlip abalone (Haliotis laevigata) classified as susceptible or resilient to laboratory induced heat stress both prior to, and at three time points throughout, the heat stress event. We detected 487 genes that were significantly differentially expressed between resilient and susceptible abalone (28 of these were significantly differentially expressed over all three time points and 26 were significantly differentially expressed throughout two of the three heat stress trial sampling points). These 54 genes also demonstrated a similar relative expression level difference between resilient and susceptible abalone in samples collected six months prior to the heat stress event. Three distinct co-expression networks incorporating these genes were also identified. The most informative transcriptomic differences between resilient and susceptible abalone throughout the heat stress trial were associated with metabolism (e.g. Mitofusin 1) and immune process (e.g. Multiple epidermal growth factor-like domain 10, Lysozyme). These functional mechanisms may enable resilient individuals to endure heat wave events. These signatures could be used to predict an individual's resilience. The findings have application in aiding stock selection for the aquaculture industry, to provide greater resilience to global climate change.

Heat Waves Produce Ecologically Relevant Expression Profiles for Temperature‐responsive Genes

The red‐eared slider turtle (Trachemys scripta elegans), exhibits temperature‐dependent sex determination (TSD), where sex‐specific genetic profiles are produced in response to incubation temperature. Despite over 50 years of study on this topic, we still do not fully understand how temperature affects the molecular mechanisms underlying TSD, and prior studies aimed at characterizing genes in the sex‐determining pathway have primarily used constant incubation temperatures. Such studies fail to capture the thermal variability that developing embryos naturally experience, but have been used to identify potential candidate genes in the sex‐determining pathway. One candidate, cold‐inducible RNA‐binding protein (Cirp), has sex‐specific expression in several species of turtles and other reptiles under constant incubation conditions. We characterized Cirp expression in the gonads of T. s. elegans embryos exposed to temperatures that are a better mimic of natural nest conditions. All eggs were incubated for the first 24 days at male‐producing temperatures (MPT; 25.0 ± 3°C) before a subset were shifted to female‐producing temperatures (FPT; 29.5 ± 3°C) to induce ovarian development via a simulated heatwave. Embryonic gonads were sampled on day 24, then sampled from both FPT and MPT for five consecutive days. In contrast to results from constant temperature studies, we found that Cirp expression did not differ in embryos at MPT and FPT. We then submitted a subset of our samples for RNAseq to identify genes exhibiting differential expression (DE) in the gonads of embryos at FPT and MPT. A preliminary analysis found 74 genes to be upregulated and 102 genes to be downregulated by day 5 of the heatwave. Similar work using constant temperature incubation found 1887 DE genes in the gonads of embryos at the same stage of development. Only 25 DE genes were shared between the constant and fluctuating temperature datasets. Our initial comparison of gene expression under constant and fluctuating conditions suggests that while constant incubation conditions result in many more DE genes, many of these genes may not be differentially expressed under more natural incubation conditions. Further, these findings highlight the importance of using ecologically relevant conditions to identify genes expressed during early gonadogenesis in TSD species.Support or Funding InformationAmerican Physiological Society William Townsend Porter Pre‐Doctoral Fellowship, Society for Integrative and Comparative Biology Grants‐in‐Aid‐of‐Research, Beta Lambda Phi Sigma Biological Honors Society Weigel Grant

Impact of a simulated marine heatwave in the hematological profile of a temperate shark (Scyliorhinus canicula)

• A simulated heatwave altered hematological parameters in temperate catsharks. • It increased leukocyte and thrombocyte counts, along with ventilation rates. • Erythrocyte counts and size ratio decreased in response to this treatment. As Earth’s temperature continues to rise, sudden warming events, designated as marine heatwaves (MHWs), are becoming more frequent and longer. This phenomenon is already shown to significantly impact marine ecosystems and respective fauna. While experimental acclimation to higher temperatures is known to affect predatory behavior, metabolism and overall fitness of sharks, the effects of short-term exposure to high temperatures on sharks’ physiology has yet to be investigated in a MHW context. Thus, the aim of our work was to study the impact of a category II MHW (Δ3 °C, 15 days) on: i) hematological parameters (total erythrocytes, leukocytes, thrombocytes, erythrocyte nuclear abnormalities (ENAs) counts, and nucleus to cytoplasmic ratio), ii) heart and spleen to body ratios, and iii) ventilation rates of juvenile catsharks ( Scyliorhinus canicula ). We found that MHW exposure led to significant changes in normal blood cell counts, by lowering erythrocyte counts and nucleus to cytoplasm ratio, and increasing leukocyte and thrombocyte counts. Moreover, ventilation rates increased consistently over the course of the MHW. However, there were no changes regarding the presence of ENA, as well as spleen and heart to body ratios. Our findings indicate limited capabilities for coping with sudden warming events, suggesting potential disruption in shark physiological homeostasis as the frequency, duration and intensity of MHWs are expected to be strengthened.

Evidence of trans-generational developmental modifications induced by simulated heat waves in an arthropod

Heat waves are considered to pose a greater risk to arthropods with their limited thermoregulation abilities than the increase of mean temperatures. Theoretically, within- and trans-generational modifications may allow populations to keep pace with rapidly occurring heat waves. Here, we evaluated this assumption using individuals of predatory mite Amblydromalus limonicus from the F1 and F2 generation, which were exposed to summer or simulated heat wave conditions during juvenile development. Independent of generation, survival and male body size were insensitive to heat waves. Heat stress elongated juvenile development of F1 males and females, and lowered the F1 female size at maturity indicating non-adaptive within-generational effects. Trans-generational modifications speeded up the development of F2 males and females and resulted in larger body size of F2 females deriving from the heat wave-experienced F1 generation. Faster F2 development should be adaptive, because it reduces the exposure time to heat waves and promotes an early beginning of mating activities. Being large at extreme high temperatures maybe a benefit for the F2 females, because large individuals are less vulnerable to dehydration and overheating. Thus, the potential fitness loss from reduced F1 growth should be compensated by increased fitness in the F2 indicating adaptive trans-generational modifications.

Simulation of extreme heat waves with empirical importance sampling

Abstract. Simulating ensembles of extreme events is a necessary task to evaluate their probability distribution and analyze their meteorological properties. Algorithms of importance sampling have provided a way to simulate trajectories of dynamical systems (like climate models) that yield extreme behavior, like heat waves. Such algorithms also give access to the return periods of such events. We present an adaptation based on circulation analogues of importance sampling to provide a data-based algorithm that simulates extreme events like heat waves in a realistic way. This algorithm is a modification of a stochastic weather generator, which gives more weight to trajectories with higher temperatures. This presentation outlines the methodology using European heat waves and illustrates the spatial and temporal properties of simulations.

Impact of marine heat waves and cold spell events on the bivalve Anomalocardia flexuosa: A seasonal comparison

The effects of increasing or decreasing extreme temperatures on bivalves depend on their physiological and biochemical capacity to respond to changes in ambient temperature. We tested the response of the clam Anomalocardia flexuosa to simulated marine heat waves and cold spells, under summer and winter experimental conditions. We sought information about physiological and biochemical parameters, as well as survival rates during two bioassays of 43 days each. The winter cold spell simulations showed that extreme temperatures acted as a physiological and biochemical stimulus, linked to an increase in metabolic rates, and consequently higher maintenance costs, as acclimatory strategies. On the other hand, the summer heat wave extreme temperatures exceeded the individuals’ thermal tolerance limits, resulting in an inability to acclimate and a high mortality. These experiments suggest that A. flexuosa can be considered as a sensitive indicator of heat wave events.

Insect herbivory antagonizes leaf cooling responses to elevated temperature in tomato

As global climate change brings elevated average temperatures and more frequent and extreme weather events, pressure from biotic stresses will become increasingly compounded by harsh abiotic stress conditions. The plant hormone jasmonate (JA) promotes resilience to many environmental stresses, including attack by arthropod herbivores whose feeding activity is often stimulated by rising temperatures. How wound-induced JA signaling affects plant adaptive responses to elevated temperature (ET), however, remains largely unknown. In this study, we used the commercially important crop plant Solanum lycopersicum (cultivated tomato) to investigate the interaction between simulated heat waves and wound-inducible JA responses. We provide evidence that the heat shock protein HSP90 enhances wound responses at ET by increasing the accumulation of the JA receptor, COI1. Wound-induced JA responses directly interfered with short-term adaptation to ET by blocking leaf hyponasty and evaporative cooling. Specifically, leaf damage inflicted by insect herbivory or mechanical wounding at ET resulted in COI1-dependent stomatal closure, leading to increased leaf temperature, lower photosynthetic carbon assimilation rate, and growth inhibition. Pharmacological inhibition of HSP90 reversed these effects to recapitulate the phenotype of a JA-insensitive mutant lacking the COI1 receptor. As climate change is predicted to compound biotic stress with larger and more voracious arthropod pest populations, our results suggest that antagonistic responses resulting from a combination of insect herbivory and moderate heat stress may exacerbate crop losses.

Leaf Gas Exchange Performance of Ten Quinoa Genotypes under a Simulated Heat Wave.

Quinoa (Chenopodium quinoa Willd.) is a highly nutritious crop that is resilient to a wide range of abiotic stresses; however, sensitivity to high temperatures is regarded as an impediment to adoption in regions prone to heat waves. Heat stress is usually associated with a decrease in crop reproductive capacity (e.g., pollen viability), yet little is known about how leaf physiological performance of quinoa is affected by high temperatures. Several trials were conducted to understand the effect of high temperatures, without confounding stressors such as drought, on ten selected quinoa genotypes considered to encompass heat sensitive and heat tolerant plant material. Plants were grown under favorable temperatures and exposed to two temperature treatments over four consecutive days. The heat treatment simulated heat waves with maximum and minimum temperatures higher during the day and night, while the control treatment was maintained under favorable temperatures (maximum and minimum temperatures for ‘Heat’: 45/30 °C and ‘Control’: 20/14 °C). Leaf gas exchange (day), chlorophyll fluorescence (predawn and day) and dark respiration (night) were measured. Results show that most quinoa genotypes under the heat treatment increased their photosynthetic rates and stomatal conductance, resulting in a lower intrinsic water use efficiency. This was partly corroborated by an increase in the maximum quantum yield of photosystem II (Fv/Fm). Dark respiration decreased under the heat treatment in most genotypes, and temperature treatment did not affect aboveground biomass by harvest (shoot and seeds). These results suggest that heat stress alone favors increases in leaf carbon assimilation capacity although the tradeoff is higher plant water demand, which may lead to plant water stress and lower yields under non-irrigated field conditions.

Targeted model evaluations for climate services: A case study on heat waves in Bangladesh

Though not a sufficient condition, the ability to reproduce key elements of climate variability over the historical record should be a minimum requirement for placing any confidence in a model’s climate forecasts or projections of climate change. When projections are used to guide practical adaptation, model evaluations should focus on the weather and climate events of interest to decision-makers, their physical drivers in the climate system and their variability on decision-relevant timescales. This paper argues for a greater emphasis on such targeted model evaluations to enable useful climate services. We illustrate this approach through a case study on heat waves in Bangladesh, but draw wider conclusions that are applicable to climate services development more broadly.The simulation of heat waves in Bangladesh is evaluated in several climate models, focusing on timescales relevant to the long-term viability of a heat action plan: the average, interannual variability and seasonality of temperature and heat-wave frequency. Where the physical drivers of variability are broadly captured, a considered interpretation of the models could provide insights into future heat-wave behaviour. However, substantial biases are found in the statistics and in some physical drivers of heat, raising questions about the suitability of some of the models for determining certain aspects of future risk. Specifically, simple bias corrections cannot be used to make inferences about possible future changes in various weather statistics such as timing of heat waves during the year. Results emphasize the potential pitfalls of performing only perfunctory climatological evaluations and highlight areas for model improvement in the simulation of South Asian climate variability.

Facultative hyperthermia during a heatwave delays injurious dehydration of an arboreal marsupial.

Heatwaves negatively impact wildlife populations and their effects are predicted to worsen with ongoing global warming. Animal mass mortality at extremely high ambient temperature (Ta) is evidence for physiological dysfunction and, to aid conservation efforts, improving our understanding of animal responses to environmental heat is crucial. To address this, I measured the water loss, body temperature and metabolism of an Australian marsupial during a simulated heatwave. The body temperature of the common ringtail possum Pseudocheirus peregrinus increased passively by ∼3°C over a Ta of 29-39°C, conveying water savings of 9.6 ml h-1 When Ta crossed a threshold of 35-36°C, possums began actively cooling by increasing evaporative water loss and thermal conductance. It is clear that facultative hyperthermia is effective up to a point, but once this point is surpassed - the frequency and duration of which are increasing with climate change - body water would rapidly deplete, placing possums in danger of injury or death from dehydration.

Ad libitum water consumption off-sets the thermal and cardiovascular strain exacerbated by dehydration during a 3-h simulated heatwave.

To assess whether ad libitum water ingestion of different temperatures is sufficient to prevent dehydration-related exacerbations of thermal and cardiovascular strain, during exposure to conditions representative of a heatwave. Twelve participants (mean ± SD; 25 ± 4years) exercised for 180min at 3 METs in 40.1 ± 0.6°C, 40.4 ± 2.1%RH four times: (i) consuming 20°C water ad libitum (AL20); (ii) consuming 4°C water ad libitum (AL4); (iii) replacing no fluids (NOFR); (iv) replacing sweat losses (FULLFR). Fluid consumption (FC), dehydration (%DEH), rectal temperature (Tre), rate-pressure product (RPP), forearm blood flow (FBF), mean skin temperature (Tsk), and local sweat rate (LSR) were measured/determined. FC was greater in AL20 (1.30 ± 0.41 L) than AL4 (1.03 ± 0.32 L; P = 0.003). %DEH was lower (P < 0.001) in AL20 (0.11 ± 0.76%), AL4 (0.43 ± 0.64%), and FULLFR (0.01 ± 0.12%) compared to NOFR (1.93 ± 0.28%). %DEH was lower in AL20 than AL4 (P = 0.003). In NOFR, end-trial changes in Tre were greater (P < 0.001) (1.05 ± 0.27°C) compared to all other trials, but similar among AL20 (0.72 ± 0.30°C), AL4 (0.76 ± 0.25°C) and FULLFR (0.74 ± 0.35°C). End-trial RPP was higher (P < 0.001) in NOFR (12,389 ± 1578mmHg·bpm) compared to all other trials, but similar among FULLFR (11,067 ± 1292mmHg·bpm), AL20 (11,214 ± 2078mmHg·bpm) and AL4 (11,089 ± 1795mmHg·bpm). No differences in Tsk or LSR were observed among trials, but FBF was lower in NOFR compared to FULLFR (2.84 ± 0.69 vs. 3.52 ± 0.96ml/100ml/min; P = 0.029). 4°C or 20°C ad libitum water ingestion prevented dehydration levels that exacerbate thermal/cardiovascular strain, despite blunted fluid intake with 4°C water. Higher core temperatures with NOFR are attributed to impaired internal heat distribution secondary to a lower FBF.

Elevated CO2 and heatwave conditions affect the aerobic and swimming performance of juvenile Australasian snapper

As climate change advances, coastal marine ecosystems are predicted to experience increasingly frequent and intense heatwaves. At the same time, already variable CO2 levels in coastal habitats will be exacerbated by ocean acidification. High temperature and elevated CO2 levels can be stressful to marine organisms, especially during critical early life stages. Here, we used a fully cross-factored experiment to test the effects of simulated heatwave conditions (+ 4 °C) and elevated CO2 (1000 µatm) on the aerobic physiology and swimming performance of juvenile Australasian snapper, Chrysophrys auratus, an ecologically and economically important mesopredatory fish. Both elevated temperature and elevated CO2 increased resting metabolic rate of juvenile snapper, by 21–22% and 9–10%, respectively. By contrast, maximum metabolic rate was increased by elevated temperature (16–17%) and decreased by elevated CO2 (14–15%). The differential effects of elevated temperature and elevated CO2 on maximum metabolic rate resulted in aerobic scope being reduced only in the elevated CO2 treatment. Critical swimming speed also increased with elevated temperature and decreased with elevated CO2, matching the results for maximum metabolic rate. Periods of elevated CO2 already occur in the coastal habitats occupied by juvenile snapper, and these events will be exacerbated by ongoing ocean acidification. Our results show that elevated CO2 has a greater effect on metabolic rates and swimming performance than heatwave conditions for juvenile snapper, and could reduce their overall performance and potentially have negative consequences for population recruitment.

Locomotor activity in field captured crepuscular four-striped field mice, Rhabdomys dilectus and nocturnal Namaqua rock mice, Micaelamys namaquensis during a simulated heat wave.

Activity of animals is influenced by ambient temperature and increasing temperatures brought about by climate change may impose a heat stress risk. Previous studies investigating the effect of heat waves on activity usually measure animals at different, but constant temperatures, however, rarely are they studied under a natural temperature cycle. General activity, behavioural flexibility and frequency of water drinking counts during a normal day, hot day and a simulated heat wave temperature cycle were studied in the crepuscular four-striped field mouse, Rhabdomys dilectus, and the nocturnal Namaqua rock mouse, Micaelamys namaquensis. Both R. dilectus and M. namaquensis showed typical daily locomotor activity under control conditions. During the heat wave, peak activity times changed for R. dilectus, but both species exhibited higher bouts of activity for the heat wave during the day compared to the control, which was accompanied by an increased amount of time spent drinking water. The increased activity during the heat wave is likely due to enhanced water requirements and potentially a form of behavioural thermoregulation as animals may be uncomfortable and try to move to cooler areas. Thus, in the absence of a typical microclimate, access to water may allow rodents to overcome heat stress from extreme temperatures without having to shift their temporal active times.