1 INTRODUCTION

Modification of natural flow dynamics in rivers can transform riverine food webs, changing the basal resources and trophic pathways driving consumer production (Kopf et al., 2019; Rees et al., 2020). There is a need to better understand how flow regimes transform food-web structure, but hindering that understanding is the challenge of resolving consumer–resource linkages. Characterising the diet of many aquatic consumers is problematic owing to the difficulty of identifying diverse, partially-digested taxa, and differential digestion rates among prey can lead to biased inferences about the relative importance of different prey to a consumer (e.g., Amundsen & Sánchez-Hernández, 2019). Modification of rivers also can transform the nutritional composition of resources at the base of freshwater food webs by altering the composition of organic material, associated microorganisms, and primary consumers (e.g., Atkinson et al., 2009; Dwyer et al., 2018; Growns et al., 2020). However, the consequences of nutritional transformations to consumer performance remain poorly understood (e.g., Dwyer et al., 2020; Ruess & Müller-Navarra, 2019). Fatty acids (FAs) can be applied to aquatic food-web studies (a) to better understand the effects of food-web transformation on consumer performance, and (b) as biomarkers, to help trace trophic pathways.

Analysis of FAs may improve our ability to predict how food-web transformation affects consumer performance (Guo et al., 2021). Growth and survival of animals can be limited by the availability of some FAs which can serve as sources of cellular energy (Jardine et al., 2020; Twining et al., 2016) and the proportions of FAs vary among basal resources and different habitats (McInerney et al., 2020). Long-chain polyunsaturated FAs (LC-PUFAs, a subset of PUFAs with ≥20 C in their acyl chains; e.g., Brett & Müller-Navarra, 1997; Guo et al., 2015; Hill et al., 2011) are especially important for maintaining a healthy physiological status and for supporting somatic growth (e.g., Kainz et al., 2010). Omega-3 (ω3) and omega-6 (ω6) PUFAs, such as eicosapentaenoic (EPA, 20:5ω3), docosahexaenoic (DHA, 22:6ω3) and arachidonic acid (ARA, 20:4ω6) are particularly important for a wide range of physiological functions and reproductive purposes (Wacker & von Elert, 2001). Since ω3 and ω6 PUFAS cannot be synthesised by most animals, they must be obtained from their diet, either as EPA, DHA and ARA, or as their precursor molecules α-linolenic acid (ALA, 18:3ω3, precursor of EPA and DHA) and linoleic acid (LIN, 18:2ω6, precursor of ARA) (Ahlgren et al., 2009). Monounsaturated FAs (MUFAs) oleic acid (OA, 18:1ω9) and palmitoleic acid (POA, 16:1ω7) have been positively associated with aquatic fungi (Funck et al., 2015) and diatoms (Taipale et al., 2013) respectively, yet are abundant in animals and are important sources of energy for fish (Tocher, 2003).

In many rivers and lakes, algae are the dominant source of energy at the base of food webs and generally are rich in LC-PUFAs (Ebm et al., 2021; Guo et al., 2016). In lowland river–floodplain ecosystems, terrestrial organic matter (litter) can be transferred to aquatic habitats in large quantities via flooding. The resulting detritus appears to provide an abundant and productive food resource for consumers (McInerney et al., 2017). Such observations are at odds with our understanding of the nutritional value of detritus, which is characterised by what is thought to be a low-quality FA profile; lack of LC-PUFAS and dominance of saturated FAs indicative of heterotrophic decomposers (e.g., 10:0, 15:0, 17:0 and their branched iso and anteiso-homologues, McInerney et al., 2020).

Use of FAs as biomarkers of trophic pathways is becoming common in aquatic food-web studies. When applying FAs as biomarkers there is an assumption that FAs are largely incorporated into consumer tissues with minimal modification (Taipale et al., 2013), such that the FA profile of the consumer matches that of its resources. However, this assumption will not hold if there is selective retention or modification of dietary FAs by consumers, resulting in FA profiles that differ significantly from their food items (Galloway & Budge, 2020). Few studies have resolved the degree to which FA profiles of prey are modified by consumer digestion and biosynthesis.

In the present study we used laboratory feeding experiments and a field study to improve our understanding of the trophic transfer of FAs in freshwater food webs, and the consequences of changes in the FA profiles of resources to consumer performance (following the recommendation of Galloway & Budge, 2020). Our study species was the crayfish, Cherax destructor (Parastacidae, referred to hereafter as crayfish), a mesoconsumer that is widespread and abundant in Australia, and a critical link between basal resources and apex predators in lowland freshwater food webs (Johnston & Robson, 2009; Lawrence et al., 2002). The influences of changed hydrological regimes on the diets of crayfish and resultant implications for their populations and for taxa that rely on them for food are not well-studied and remain a significant knowledge gap.

Our first objective was to determine how the FA composition of diet affects crayfish growth and survival. We reared crayfish on three diets with significantly different FA profiles (see Appendix S1): (a) detritus, (b) invertebrates (Chironomidae) and (c) commercial crayfish pellets. These diets were selected to broadly encompass two of the major food resources that crayfish encounter in the wild (detritus and invertebrates), as well as a diet developed to be close to the optimal composition for crayfish growth and survival (commercial crayfish pellets), serving as a useful point of reference for the detritus and invertebrate diets. The percentage of LIN and OA FAs, in particular, varied strongly among diets and followed the inequality ‘pellets > invertebrates > detritus’. We expected that elevated proportions of LIN and OA in diets would be responsible for high growth and low mortality of crayfish (e.g., Thompson et al., 2010; Tocher, 2003), and that the pattern of growth across these treatments would be consistent with the inequality ‘pellets > invertebrates > detritus’ and that mortality across treatments would be described by the opposite inequality.

Our second objective was, broadly, to determine how the FA profile of crayfish is shaped by that of their diet. We sought answers to the following questions, in an experimental setting, when fed a constant diet: (a) To what extent is the body FA profile of crayfish influenced by that of their diet? and (b) Which FAs become over- and under-represented in crayfish bodies, relative to their dietary FA profiles as a result of digestion and biosynthesis? To answer these questions, we used the same experimental setup employed for evaluating the effects of FAs on performance. Answers to these questions are required to advance the application of FAs as food-web biomarkers.

Our third objective was to determine how the FA profiles of crayfish and their diet varies among natural habitats of a river–floodplain ecosystem. We sampled crayfish from habitats in the river channel and from floodplain wetlands, and analysed the FA profiles of their body tissue and their gut contents. Modification of riverine flow regimes, and riverscapes more generally, has resulted in loss of lateral hydrological connectivity among channel and floodplain habitats, yet our understanding of how that has affected food webs is poor, partly because we have a rudimentary understanding of how consumer diets vary among river–floodplain habitats. We anticipated differences in the FA profiles of crayfish and their diets between channel and wetland habitats. Consistent with the Flood Pulse Concept (Junk et al., 1989) and other observations of the high productivity of consumers on floodplains (McInerney et al., 2017), we expected diets and crayfish from floodplain wetlands to contain FAs indicative of higher quality food resources than crayfish in the river channel. Results pertaining to this field study are interpreted in light of those from our experimental investigation.

中文翻译：

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膳食脂肪酸谱塑造了小龙虾的生物合成和性能：对河流食物网的影响

1 简介

河流中自然流动动力学的改变可以改变河流食物网，改变基础资源和驱动消费者生产的营养途径（Kopf 等人，2019 年；Rees 等人，2020 年）。有必要更好地了解流动状态如何改变食物网结构，但阻碍这种理解是解决消费者与资源联系的挑战。由于难以识别不同的、部分消化的分类群，因此对许多水产消费者的饮食进行表征是有问题的，而且猎物之间的不同消化率可能导致关于不同猎物对消费者的相对重要性的有偏见的推论（例如，Amundsen & Sánchez-埃尔南德斯，  2019）。河流的改造还可以通过改变有机物质、相关微生物和主要消费者的组成来改变淡水食物网基础资源的营养组成（例如，Atkinson 等人，  2009 年；Dwyer 等人，  2018 年；Growns等人，  2020 年）。然而，人们对营养转变对消费者表现的影响仍然知之甚少（例如，Dwyer 等人，  2020 年；Ruess 和 Müller-Navarra，  2019 年）。脂肪酸 (FA) 可应用于水生食物网研究 (a) 以更好地了解食物网转变对消费者表现的影响，以及 (b) 作为生物标志物，帮助追踪营养途径。

对 FA 的分析可能会提高我们预测食物网转变如何影响消费者表现的能力（Guo 等人，  2021 年）。动物的生长和生存可能会受到一些可用作细胞能量来源的 FA 的限制（Jardine 等人，  2020 年；Twining 等人，  2016 年），并且 FAs 的比例因基础资源和不同栖息地而异（ McInerney 等人，  2020 年）。长链多不饱和脂肪酸（LC-PUFA，一种 PUFA 的子集，其酰基链中的温度≥20 C；例如，Brett & Müller-Navarra，  1997；Guo 等人，  2015 年；Hill 等人，  2011 年）) 对于维持健康的生理状态和支持体细胞生长尤其重要（例如，Kainz 等人，  2010 年）。Omega-3 (ω3) 和 omega-6 (ω6) PUFA，例如二十碳五烯酸 (EPA, 20:5ω3)、二十二碳六烯酸 (DHA, 22:6ω3) 和花生四烯酸 (ARA, 20:4ω6) 对于广泛的生理功能和生殖目的的范围（Wacker & von Elert,  2001）。由于大多数动物无法合成 ω3 和 ω6 PUFAS，它们必须从它们的饮食中获取，或者作为 EPA、DHA 和 ARA，或者作为它们的前体分子α-亚麻酸（ALA，18:3ω3，EPA 和 DHA 的前体）和亚油酸（LIN，18:2ω6，ARA 的前体）（Ahlgren 等人，  2009）。单不饱和脂肪酸 (MUFA) 油酸 (OA, 18:1ω9) 和棕榈油酸 (POA, 16:1ω7) 分别与水生真菌 (Funck et al.,  2015 ) 和硅藻 (Taipale et al.,  2013 )呈正相关，但在动物中含量丰富，是鱼类的重要能量来源（Tocher，  2003 年）。

在许多河流和湖泊中，藻类是食物网底部的主要能量来源，通常富含 LC-PUFA（Ebm 等，  2021；Guo 等，  2016）。在低地河流-泛滥平原生态系统中，陆地有机物（垃圾）可以通过洪水大量转移到水生栖息地。由此产生的碎屑似乎为消费者提供了丰富且高产的食物资源（McInerney 等，  2017）。这些观察与我们对碎屑营养价值的理解不一致，碎屑的特征是被认为是低质量的 FA 谱；缺乏 LC-PUFAS 和饱和 FAs 的优势，表明异养分解者（例如，10:0、15:0、17:0 及其支链异和反异同系物，McInerney 等人，  2020）。

使用 FA 作为营养途径的生物标志物在水生食物网研究中变得越来越普遍。当应用 FA 作为生物标志物时，假设 FA 在很大程度上被整合到消费者组织中，且修改最少（Taipale 等人，  2013 年），因此消费者的 FA 概况与其资源相匹配。然而，如果消费者选择性地保留或修改膳食 FA，则该假设将不成立，从而导致 FA 配置文件与其食品有显着差异（Galloway & Budge，  2020 年）。很少有研究能够解决消费者消化和生物合成对猎物 FA 谱的影响程度。

在本研究中，我们使用实验室喂养实验和实地研究来提高我们对 FA 在淡水食物网中的营养转移以及资源 FA 分布变化对消费者表现的影响的理解（遵循 Galloway & Budge 的建议） ,  2020 年）。我们的研究物种是小龙虾，Cherax destructor（Parastacidae，以下简称小龙虾），一种在澳大利亚广泛而丰富的中消费类动物，也是低地淡水食物网中基础资源和顶级捕食者之间的重要联系（Johnston & Robson,  2009；劳伦斯等人，  2002）。改变的水文状况对小龙虾饮食的影响及其对其种群和依赖它们作为食物的分类群的影响尚未得到充分研究，并且仍然存在重大的知识差距。

我们的第一个目标是确定饮食中的 FA 成分如何影响小龙虾的生长和生存。我们以三种具有显着不同 FA 特征的饮食饲养小龙虾（见附录 S1）：（a）碎屑，（b）无脊椎动物（摇蚊科）和（c）商业小龙虾颗粒。选择这些饮食以广泛涵盖小龙虾在野外遇到的两种主要食物资源（碎屑和无脊椎动物），以及为接近小龙虾生长和生存的最佳成分而开发的饮食（商业小龙虾颗粒），作为碎屑和无脊椎动物饮食的有用参考点。特别是 LIN 和 OA FAs 的百分比在不同饮食中差异很​​大，并遵循“颗粒 > 无脊椎动物 > 碎屑”的不平等。 2010 年；Tocher，  2003 年），并且这些处理的增长模式将与“颗粒 > 无脊椎动物 > 碎屑”的不平等一致，并且不同处理的死亡率将由相反的不平等来描述。

我们的第二个目标是，广泛地说，确定小龙虾的 FA 谱是如何由它们的饮食决定的。我们在实验环境中寻求以下问题的答案，当喂食恒定饮食时：（a）小龙虾的身体 FA 分布在多大程度上受其饮食的影响？(b) 相对于消化和生物合成的结果，相对于它们的膳食 FA 谱，哪些 FA 在小龙虾体内的比例过高或过低？为了回答这些问题，我们使用了相同的实验设置来评估 FA 对性能的影响。需要回答这些问题才能推进 FA 作为食物网生物标志物的应用。

我们的第三个目标是确定小龙虾的 FA 谱及其饮食如何在河流-泛滥平原生态系统的自然栖息地中变化。我们从河道和洪泛区湿地的栖息地采集了小龙虾，并分析了它们身体组织和肠道内容物的 FA 谱。河流流态和更普遍的河流景观的改变导致河道和洪泛区栖息地之间的横向水文连通性丧失，但我们对这如何影响食物网的理解很差，部分原因是我们对消费者饮食如何变化有初步的了解在河流泛滥平原栖息地之间。我们预计河道和湿地栖息地之间小龙虾的 FA 特征及其饮食存在差异。与洪水脉冲概念一致（Junk et al.,  1989) 和其他对洪泛区消费者高生产力的观察（McInerney 等人，  2017 年），我们预计洪泛区湿地的饮食和小龙虾含有 FA，表明其食物资源质量高于河道中的小龙虾。与本实地研究有关的结果是根据我们的实验研究得出的结果进行解释。