Climate change threatens all ecosystems and the organisms within them. Freshwater habitats, in particular, appear to be experiencing rapid environmental change (Almond et al., 2020; Knouft & Ficklin, 2017). Over the last decade, there has been a growing interest in the effects of warming on freshwater populations, which includes changes in abundance, body size, reproductive success and survival of a range of organisms (e.g. Hovel et al., 2017; Ledger et al., 2013; Velthuis et al., 2017; Yvon‐Durocher et al., 2011). For instance, research in a natural warming experiment in Iceland has found that elevated temperatures increase the abundance of diatoms, with implications for secondary production (Junker et al. 2020; O'Gorman et al. 2017). All of this research has contributed to our knowledge that the effects of warming can be both direct and indirect. Even if there are no direct consequences of warming on a consumer population, they may suffer cascading consequences due to a change in resource availability. These ‘trophic cascades’ have been documented in the ecological literature for decades, from Stephen Carpenter's seminal work on fish–zooplankton–algal cascades in lakes (Carpenter et al., 1985), to more recent work on how they are altered under global change (Jackson et al., 2017; Murphy et al., 2020). For instance, warming can enhance the cascading effects of fish on zooplankton (and, subsequently, phytoplankton) by elevating feeding rates (Kratina et al., 2012). Alternatively, warming can also result in phenological mismatch, where fish fry emerges earlier in spring before zooplankton prey peaks, causing a bottom‐up trophic cascade where loss of prey reduces consumer survival (Jonsson & Setzer, 2015).

Importantly, research investigating the effects of warming on trophic cascades usually quantifies changes in abundance or biomass, rarely considering more subtle effects. For instance, warming can alter individual respiration rates (Cloyed et al., 2019), feeding selectivity (Gordon et al., 2018), behaviour (Kua et al., 2020) and food quality—all with consequences for other trophic levels. For instance, algae often produce less polyunsaturated fatty acids (PUFAs) in warmer conditions (Fuschino et al., 2011; Von Elert & Fink, 2018), which can have implications for consumers, such as the herbivorous grazer Daphnia. This filter‐feeding crustacean is incapable of de novo synthesis of long‐chained PUFAs, like most other arthropods (Harrison, 1990; but see Kabeya et al., 2018), and therefore such PUFAs must be derived directly from diet. PUFAs have two main functions: they help maintain membrane fluidity, especially at low temperatures (homeoviscous adaptation; Hazel, 1995) while the long‐chain PUFAs eicosapentaenoic acid (EPA, 20:5ω3) and arachidonic acid (ARA, 20:4ω6) serve as precursors for eicosanoids, a family of hormone‐like substances which play an important role in animal reproduction (Heckmann et al., 2008; Stanley‐Samuelson, 1994). Therefore, at low PUFA availability, Daphnia can experience performance limitations in terms of decreased growth and reproduction (Müller‐Navarra, 1995; Ravet et al., 2003; Wacker & Von Elert, 2001). In other words, resource quality can be just as, or more, important than resource quantity for Daphnia and other arthropods.

In this current issue of Functional Ecology, Tseng et al., investigated if changes in resource quality due to warming had consequences for higher trophic levels through trophic interactions. The authors discovered that, although algae produced more total PUFAs under warming, due to declines in cell size there was no net difference with the ambient treatment. However, the cold‐reared algae produced more neutral lipids (i.e. fat content) than warm‐reared algae. Somewhat surprisingly, these changes in lipid content had little cascading consequences for higher trophic levels. While Daphnia at ambient temperatures and fed cold‐reared algae had higher abundance than those fed warm‐reared algae, this positive effect was lost when Daphnia were kept at higher temperatures, and there were no cascading effects on the next trophic level (Chaoborus). One of the explanations Tseng et al. give for this finding is that Daphnia require less PUFAs at warmer temperatures because their role in the maintenance of membrane fluidity declines with warming (Hazel, 1995). Similar results have been observed in several other studies (Masclaux et al., 2009; Schlechtriem et al., 2006). For instance, Martin‐Creuzburg et al. (2012) showed that the effect of food quality (i.e. PUFA availability) on population growth of Daphnia magna was strongest at low temperatures. Addition of single PUFAs (ARA, EPA) had a consistent positive effect on the number of viable offspring, which showed a hump‐shaped temperature dependency. However, juvenile somatic growth was less affected by food quality, indicating an important role of life stage.

In response to warming, food quality is not only expected to change due to altered PUFA content per algal cell but also due to changes in community composition (Ahlgren et al., 1997), as the content of essential PUFAs is taxon‐specific (Ahlgren et al., 1990; Lang et al., 2011). At high temperatures and nutrient availability, bloom‐forming cyanobacteria are favoured (Lürling et al., 2017; Pearl & Hiusman, 2008), which are a low‐quality food as they lack essential PUFAs and sterols (Martin‐Creuzburg et al., 2005; Von Elert et al., 2003). Furthermore, intraspecific and interspecific variation in consumer physiological and life‐history traits might further shape their overall response to warming (Geerts et al., 2015; Vanvelk et al., 2020). For example, naturally coexisting Daphnia longispina genotypes were shown to differ in their susceptibility to PUFA limitations (Ilić et al., 2021) while Werner et al. (2019) found pronounced differences in heat tolerance within a natural population of Daphnia magna. Therefore, future studies should investigate the effects of temperature on natural communities, and monitor population dynamics and compositional changes (and thus potential food quality alterations in the phytoplankton) over time periods long enough to allow for potential evolutionary changes.

Other factors not considered by Tseng et al., such as encounter rate, body size and development time of the prey, play an important role in prey–predator interactions (Pastorok, 1981). These factors are expected to be affected by warming, either directly via changes in metabolic rates, or indirectly via changes in food quality (Giebelhausen & Lampert, 2001; Lampert, 2006). Tseng et al., found that the population size of Daphnia decreased with temperature, indicating that food quantity and encounter rate for Chaoborus will also decline while neckteeth induction is expected to increase, potentially increasing handling time (Tollrian et al., 2015). Additionally, Chaoborus is a gape‐limited predator (Tollrian, 1995), and therefore lower growth rates in Daphnia will result in longer time spent in the vulnerable size spectrum (Pastorok, 1981).

Tseng et al.'s work highlights the need to consider quality, as well as quantity, in trophic interactions in a warming world. Moving forward, network approaches should be used to understand how stressor effects on resource quality ripple through the food web. In our rapid changing world, it is also important to consider how warming will interact with other stressors (Jackson et al., 2016), which may mitigate or amplify one another's effects on food quality with implications for cascading interactions.

气候变化威胁着所有生态系统和其中的生物。特别是淡水生境似乎正在经历快速的环境变化（Almond等，  2020； Knouft＆Ficklin，  2017）。在过去十年中，人们对变暖对淡水种群的影响越来越感兴趣，其中包括丰度，体重，繁殖成功和各种生物的存活率的变化（例如Hovel等人，  2017 ; Ledger等人，  2013 ; Velthuis等，  2017 ; Yvon-Durocher等，  2011）。例如，在冰岛的一项自然变暖实验中的研究发现，升高的温度会增加硅藻的含量，这对二次生产具有影响（Junker等人 2020年; O'Gorman等人 2017年）。所有这些研究都有助于我们认识到变暖的影响既可以是直接的也可以是间接的。即使变暖对消费者没有直接影响，但由于资源可利用性的改变，他们可能会遭受连锁反应。这些“营养级联”在生态学文献中已有数十年的记载，源于史蒂芬·卡彭特（Stephen Carpenter）关于湖泊中鱼-浮游生物-藻类级联的开创性研究（Carpenter et al。，  1985）。），以及有关在全球变化下如何改变它们的最新工作（Jackson等，  2017； Murphy等，  2020）。例如，升温可以通过提高摄食率来增强鱼类对浮游动物（以及随后的浮游植物）的级联效应（Kratina等，  2012）。另外，变暖还可能导致物候失配，即鱼苗在浮游动物的捕食高峰之前的春季早些时候出现，从而导致自下而上的营养级联，猎物的损失降低了消费者的生存率（Jonsson＆Setzer，  2015）。

重要的是，研究变暖对营养级联的影响的研究通常可以量化丰度或生物量的变化，很少考虑更细微的影响。例如，变暖会改变个体的呼吸速率（Cloyed等，  2019），进食选择性（Gordon等，  2018），行为（Kua等，  2020）和食物质量，所有这些都会影响其他营养水平。例如，藻类在温暖的条件下通常会产生较少的多不饱和脂肪酸（PUFA）（Fuschino et al。，  2011 ; Von Elert＆Fink，  2018），这可能对消费者产生影响，例如草食性食草动物Daphnia。像大多数其他节肢动物一样，这种以过滤器为食的甲壳动物无法从头合成长链PUFA（Harrison，  1990；但请参见Kabeya等人，  2018），因此，此类PUFA必须直接来自饮食。PUFA具有两个主要功能：它们有助于维持膜的流动性，尤其是在低温下（顺应性适应； Hazel，  1995年），而长链PUFA二十碳五烯酸（EPA，20：5ω3）和花生四烯酸（ARA，20：4ω6）可以发挥作用。作为类二十烷酸的前体，类激素物质在动物繁殖中起着重要作用（Heckmann等，  2008； Stanley-Samuelson，  1994）。因此，在低PUFA可用性下，水蚤由于生长和繁殖的减少，它们可能会受到性能的限制（Müller-Navarra，  1995； Ravet等，  2003； Wacker＆Von Elert，  2001）。换句话说，资源质量对于水蚤和其他节肢动物而言，可能与资源数量同等或更重要。

Tseng等人在本期《功能生态学》中研究了变暖引起的资源质量变化是否通过营养相互作用而对较高的营养水平产生了影响。作者发现，尽管藻类在变暖下会产生更多的总PUFA，但由于细胞大小的减少，与环境处理没有净差异。但是，冷养藻类比温育藻类产生更多的中性脂质（即脂肪含量）。令人惊讶的是，脂质含量的这些变化对于较高的营养水平几乎没有连锁反应。虽然在环境温度和喂食冷养藻类的水蚤比喂食温养藻类的水牛有更高的丰度，但是当水蚤吞噬时，这种积极作用就消失了保持较高的温度，并且在下一个营养级（Chaophorus）上没有级联效应。Tseng等人的解释之一。得出的结论是，水蚤在较热的温度下需要较少的PUFA，因为它们在维持膜流动性中的作用随着温度的升高而降低（Hazel，  1995年）。在其他几项研究中也观察到了相似的结果（Masclaux等，  2009； Schlechtriem等，  2006）。例如，马丁·克鲁兹堡（Martin‐Creuzburg）等。（2012）表明，食物质量（即PUFA可用性）对水蚤的种群增长的影响在低温下最强。添加单个PUFA（ARA，EPA）对存活后代的数量具有一致的积极影响，这显示出驼峰状的温度依赖性。但是，少年的体细胞生长受食品质量的影响较小，表明生命阶段具有重要作用。

响应变暖，不仅由于每个藻细胞中PUFA含量的变化，而且由于社区组成的变化，食品质量也将发生变化（Ahlgren等，  1997），因为基本PUFA的含量是特定于分类群的（Ahlgren等人，  1990； Lang等人，  2011）。在高温和养分可利用的情况下，青bloom形成的蓝细菌受到青睐（Lürling等人，  2017； Pearl＆Hiusman，  2008），这是一种劣质食品，因为它们缺乏必需的PUFA和固醇（Martin-Creuzburg等人，  2005年；冯·埃勒特（Von Elert）等人，  2003年）。此外，消费者生理和生活史特征的种内和种间变异可能会进一步影响其对变暖的总体反应（Geerts等，  2015； Vanvelk等，  2020）。例如，天然共存的水蚤（Daphnia longispina）基因型对PUFA限制的敏感性不同（Ilićet al。，2021），而Werner et al。（  2021）。（2019）发现大型蚤（Daphnia magna）的自然种群在耐热性上有明显差异。因此，未来的研究应调查温度对自然群落的影响，并在足够长的时间内允许潜在的进化变化，监测种群动态和组成变化（以及浮游植物潜在的食品质量变化）。

Tseng等人没有考虑的其他因素，例如encounter的发生率，体型和猎物的发育时间，在猎物与食肉动物的相互作用中起着重要的作用（Pastorok，  1981）。预计这些因素会直接通过新陈代谢率的变化而受到变暖的影响，或者通过食物质量的变化而间接受到变暖的影响（Giebelhausen＆Lampert，  2001； Lampert，  2006）。Tseng等人发现，水蚤的种群规模随温度降低而减少，这表明潮虫的食物数量和遭遇率也将下降，而颈齿感应率预计会增加，这可能会增加处理时间（Tollrian等人，  2015年）。此外，Chaoforus是捕食有限的食肉动物（Tollrian，  1995），因此，水蚤的低增长率将导致在脆弱的种群规模上花费更长的时间（Pastorok，  1981）。

Tseng等人的工作强调了在变暖的世界中，营养相互作用中必须同时考虑质量和数量。展望未来，应该使用网络方法来了解压力源如何通过食物网对资源质量产生连锁反应。在我们这个瞬息万变的世界中，考虑变暖将如何与其他压力源相互作用也很重要（Jackson等人，  2016年），这可能会减轻或放大彼此对食品质量的影响，并影响级联相互作用。