1 INTRODUCTION

In the era of rapid climate change, heatwaves, referred to as short-term extreme hot weather lasting from hours to days, could disproportionally shape species performance and distribution (Jentsch et al., 2007; Sandblom et al., 2016). Globally, freshwater ecosystems are among the most thermally threatened (Closs et al., 2016; Yousefi et al., 2020). Regionally, the intensity of heatwaves in freshwater ecosystems could be further elevated, such as by thermal pollution from power plants (Raptis et al., 2016) and high surface energy flux from urbanisation (Nichol et al., 2020). Besides these, agriculture is another major factor that intensifies heatwaves in freshwater ecosystems, due to: (1) increased solar radiation after the reduction of riparian cover (Caissie, 2006); (2) reduced thermal refuges following channelisation (Dugdale et al., 2013); and (3) decreased cool groundwater recharge after agricultural water use (Loheide & Gorelick, 2006). Additionally, the impacts of heatwaves on aquatic organisms in agricultural landscapes could further be exacerbated by low dissolved oxygen from fertiliser use and toxicants from pesticide application (Op de Beeck et al., 2017; Verberk et al., 2016; Wang et al., 2003).

To understand how fish respond to thermal stress, physiological methods have been widely used. Among all physiological aspects, energy metabolism, measured as the rate of oxygen consumption, is arguably one of the most important for fish, as it influences locomotion, growth, and reproduction (Brown et al., 2004). Thermal sensitivity of metabolism has been explained by the oxygen- and capacity-limited thermal tolerance hypothesis (Pörtner, 2010), despite its limitations (Clark et al., 2013; Jutfelt et al., 2018). Oxygen- and capacity-limited thermal tolerance conjectures that during warming, tissue oxygen demand increases exponentially until it reaches a critical temperature, where oxygen demand for maintenance exceeds cardiorespiratory capacity, causing loss of performance (Blasco et al., 2020; Schulte, 2015). To evaluate the impacts of such critical temperatures on the thermal tolerance and metabolism of aquatic organisms, many physiological studies targeting long-term temperature increases have been conducted. However, short-term heatwaves have received considerably less attention (Morash et al., 2018), partially due to a lack of real-time, high-resolution field temperature information. This limits the extrapolation of lab-derived results to real-world conditions, causing us to overlook real-time metabolic costs and adjustments of fish in nature. Examples include increased standard metabolism of Atlantic salmon Salmo salar under a more variable environment (Oligny-Hébert et al., 2015), and elevated excess post-exercise oxygen consumption by Nile perch Lates niloticus after acute thermal challenges (Nyboer & Chapman, 2017). Thus, bioenergetic models that only include long-term temperature increases, while failing to consider heatwaves, are limited in predictive power.

By monitoring how fish react to extreme heatwaves, we cannot only have a better mechanistic understanding of the real-time adjustments of thermal tolerance, but also examine how prior experience to heatwaves could shape later reversible changes in thermal tolerance (i.e. heat hardening) (Bowler, 2005; Gunderson & Stillman, 2015; Schaefer & Ryan, 2006). Such carryover effects (O'Connor et al., 2014) could be important for fish overcoming the increasing frequency and intensity of heatwaves under global warming. However, heat hardening normally comes with costs, including reduced aerobic scope due to elevated metabolism, thus impairing fitness-related performance like prey consumption and predator avoidance (Farrell, 2009; Oligny-Hébert et al., 2015). Also, rapid increases in body temperature during heatwaves can lead to oxidative damage in ectotherms (Guzzo et al., 2019; Heise et al., 2006; Kaur et al., 2005). By monitoring the physiological responses of fishes under different frequencies and intensities of heatwaves, we can better understand their protective mechanisms and costs, then predict potential thermal thresholds for ecosystem stability.

Streams and rivers in the midwestern U.S.A. are known for being both productive and speciose (Smith et al., 2010). With such diversity of prey and predators, a key question is: which species should be prioritised for consideration under climate change? A common prey fish in North America, fathead minnow Pimephales promelas, became our focus here for the following reasons. Firstly, the performance of prey fish under heatwaves is critical for ecosystem stability, as they act as important nutrient and energy pathways (Hebert et al., 2008; Johnson et al., 2005). Dominant prey species, such as fathead minnow, are critical from a food web perspective, as they are both naturally widespread across North America and widely stocked to support fisheries (Colvin et al., 2008; Page & Burr, 2011). Secondly, despite the upper thermal tolerance of smaller species being less impaired during warming relative to larger ones, the acclimation potential of smaller species is lower (Leiva et al., 2019; Rohr et al., 2018). Fathead minnow, as a eurythermal prey species, has around a 2°C lower critical thermal maximum (CT_max) compared to its eurythermal predators largemouth bass Micropterus salmoides and channel catfish Ictalurus punctatus when acclimated at 25°C (CT_max: 36.1°C vs. 37.8–38.7°C) (Beitinger et al., 2000; Carveth et al., 2006). Thus, heatwaves could have different consequences for small prey relative to large predators, influencing the balance in nutrient and energy pathways. The potential impacts of heatwaves on prey fish still lack enough attention, deserving a more comprehensive understanding.

To further understand the physiological mechanisms by which stream fishes respond to short-term heatwaves, we exposed fathead minnow to different frequencies and intensities of heatwaves and then quantified: (1) short-term CT_max; (2) enzyme responses related to antioxidant defence, as well as aerobic and anaerobic capacity; and (3) whole-organism oxygen consumption rate (ṀO₂). The frequency and magnitude of heatwaves were based on in situ monitoring of agricultural streams in Illinois, U.S.A. We predicted that fathead minnow undergoing higher frequency and magnitude of heatwaves would show higher CT_max, but with more costs in terms of prolonged increases in whole-organism metabolism, reduced metabolism-related enzyme performance, and reduced antioxidant. Quantifying the physiological responses of fathead minnow to heatwaves facilitates a better mechanistic understanding of thermal impacts on fish. It also motivates future bioenergetic modelling to consider both long-term and short-term thermal impacts.

中文翻译：

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鱼类对农业溪流中热浪的耐热性：什么不会杀死你让你更强壮？

1 简介

在气候快速变化的时代，热浪，被称为持续数小时至数天的短期极端炎热天气，可能会不成比例地影响物种的表现和分布（Jentsch 等人，  2007 年；Sandblom 等人，  2016 年）。在全球范围内，淡水生态系统是最受热威胁的生态系统之一（Closs 等人，  2016 年；Yousefi 等人，  2020 年）。从区域来看，淡水生态系统中的热浪强度可能会进一步升高，例如发电厂的热污染（Raptis 等人，  2016 年）和城市化带来的高地表能量通量（Nichol 等人，  2020 年））。除此之外，农业是加剧淡水生态系统热浪的另一个主要因素，原因是：（1）河岸覆盖减少后太阳辐射增加（Caissie，  2006 年）；(2) 通道化后减少的热避难所（Dugdale 等人，  2013 年）；(3) 农业用水后冷却地下水补给减少（Loheide & Gorelick,  2006）。此外，热浪对农业景观中水生生物的影响可能会因化肥使用中的低溶解氧和农药使用中的毒物而进一步加剧（Op de Beeck 等人，  2017 年；Verberk 等人，  2016 年；Wang 等人，  2003 年）。

为了了解鱼对热应激的反应，生理学方法已被广泛使用。在所有生理方面，以耗氧率衡量的能量代谢可以说是鱼类最重要的代谢之一，因为它影响运动、生长和繁殖（Brown 等人，  2004 年）。尽管存在局限性（Clark 等人，  2013 ； Jutfelt 等人 ，2018）。氧气和容量受限的热耐受性推测，在变暖期间，组织需氧量呈指数增长，直到达到临界温度，此时维持所需的氧量超过心肺能力，导致性能下降（Blasco 等人，  2020 年；Schulte，  2015 年） . 为了评估这种临界温度对水生生物的耐热性和新陈代谢的影响，已经进行了许多针对长期温度升高的生理研究。然而，短期热浪受到的关注要少得多（Morash 等人，  2018)，部分原因是缺乏实时、高分辨率的现场温度信息。这限制了将实验室得出的结果外推到现实世界的条件，导致我们忽略了自然界鱼类的实时代谢成本和调整。例子包括在更加多变的环境下大西洋鲑鱼Salmo salar的标准代谢增加（Oligny-Hébert 等人，  2015 年），以及尼罗河鲈鱼在急性热挑战 后增加的运动后过量耗氧量（ Nyboer和查普曼， 2017 年） . 因此，仅包括长期温度升高而未考虑热浪的生物能模型的预测能力有限。

通过监测鱼对极端热浪的反应，我们不仅可以更好地了解热耐受性的实时调整，还可以检查先前的热浪经验如何影响以后的耐热性可逆变化（即热硬化）（鲍勒,  2005 ; Gunderson & Stillman,  2015 ; Schaefer & Ryan,  2006 )。这种遗留效应（O'Connor et al.,  2014) 可能对鱼类在全球变暖下克服日益增加的热浪频率和强度很重要。然而，热硬化通常会带来成本，包括由于新陈代谢升高而导致有氧范围减少，从而削弱与健康相关的表现，如猎物消耗和捕食者回避（Farrell，  2009 年；Oligny-Hébert 等人，  2015 年）。此外，热浪期间体温的快速升高会导致等温动物的氧化损伤（Guzzo 等人，  2019 年；Heise 等人，  2006 年；Kaur 等人，  2005 年））。通过监测鱼类在不同频率和热浪强度下的生理反应，我们可以更好地了解它们的保护机制和成本，进而预测生态系统稳定性的潜在热阈值。

美国中西部的溪流和河流以高产和独特而著称（Smith 等人，  2010 年）。由于猎物和捕食者的多样性，一个关键问题是：在气候变化下应该优先考虑哪些物种？北美常见的猎物黑头鲦鱼Pimephales promelas成为我们关注的焦点，原因如下。首先，捕食鱼在热浪下的表现对于生态​​系统稳定性至关重要，因为它们是重要的营养和能量途径（Hebert 等人，  2008 年；Johnson 等人，  2005 年））。从食物网的角度来看，主要的猎物物种，如黑头鲦鱼，是至关重要的，因为它们在北美自然分布广泛，并且被广泛放养以支持渔业（Colvin et al.,  2008 ; Page & Burr,  2011）。其次，尽管较小物种的较高耐热性在变暖期间相对于较大物种的损害较小，但较小物种的驯化潜力较低（Leiva et al.,  2019 ; Rohr et al.,  2018）。黑头鲦鱼作为一种极热猎物，与极热捕食者大嘴鲈鱼和鲶鱼Ictalurus punctatus相比，其临界热最大值 (CT _{max ) 低约 2°C}当在 25°C 适应时（CT_最大值：36.1°C 对 37.8–38.7°C）（Beitinger 等人，  2000 年；Carveth 等人，  2006 年）。因此，相对于大型捕食者，热浪可能对小型猎物产生不同的影响，从而影响营养和能量途径的平衡。热浪对捕食鱼类的潜在影响仍缺乏足够的重视，值得更全面的认识。

为了进一步了解河流鱼类对短期热浪的生理机制，我们将黑头鲦鱼暴露在不同频率和强度的热浪中，然后量化：（1）短期 CT_最大值；(2) 与抗氧化防御相关的酶反应，以及有氧和无氧能力；(3)全身耗氧率( Ṁ O ₂ )。热浪的频率和幅度基于对美国伊利诺伊州农业溪流的现场监测 我们预测，经历更高频率和热浪幅度的黑头鲦鱼将显示更高的 CT _max，但在整个机体代谢的长期增加、与代谢相关的酶性能降低和抗氧化作用降低方面会付出更多的代价。量化黑头鲦鱼对热浪的生理反应有助于更好地理解热对鱼类的影响。它还促使未来的生物能建模考虑长期和短期的热影响。