Aquatic and terrestrial resources are not nutritionally reciprocal for consumers

Aquatic and terrestrial ecosystems are connected through reciprocal fluxes of energy and nutrients that can subsidize consumers. Past research on reciprocal aquatic–terrestrial subsidies to consumers has generally focused on subsidy quantity while ignoring major differences in the nutritional composition of aquatic and terrestrial resources. Because aquatic resources contain substantially more highly unsaturated omega‐3 fatty acids (HUFAs) than terrestrial resources, aquatic subsidies may play a unique role by supplying these critical compounds to both aquatic and terrestrial consumers. Here, we first characterized nutritional quality in terms of HUFA content in aquatic and terrestrial insect prey. We then used bulk stable isotope analyses to estimate subsidy use by stream and riparian consumers coupled with compound‐specific stable isotope analyses, which allowed us to document consumer HUFA sources. Finally, in order to understand the nutritional importance of aquatic‐derived HUFAs for riparian consumers, we conducted manipulative diet experiments on Eastern Phoebe (Sayornis phoebe) chicks in the laboratory. Aquatic insects were significantly enriched in HUFAs, mainly in terms of eicosapentaenoic acid (EPA), compared with terrestrial insects. Stream fishes relied mainly upon aquatic resources, while insectivorous birds varied in their use of aquatic subsidies across sites. However, like stream fishes, Eastern Phoebe chicks received HUFAs from aquatic insects, even when they were heavily reliant upon terrestrial insects for their overall diet. In the laboratory, dietary HUFAs, such as those supplied by aquatic insects, increased the growth rate and condition of Eastern Phoebe chicks. This study demonstrates that aquatic and terrestrial subsidies are not nutritionally reciprocal from the perspective of consumers because aquatic resources are the main source of critical fatty acids for both stream and riparian consumers. It also confirms previous findings on the nutritional importance of HUFAs for riparian birds, demonstrating that an insectivorous riparian lifestyle influences avian nutritional needs. Finally, our findings raise the possibility that birds and other riparian insectivores may experience nutritional mismatches with terrestrial prey if they do not have access to high‐quality aquatic subsidies as a consequence of aquatic habitat degradation or shifts in consumer and resource phenology. A free Plain Language Summary can be found within the Supporting Information of this article.

水生和陆地生态系统通过能量和营养物质的相互流动而相互联系，这些能量和营养物质能够为消费者提供补给。以往关于对消费者的水生 - 陆地相互补给的研究通常侧重于补给量，而忽略了水生和陆地资源营养成分的主要差异。由于水生资源比陆地资源含有多得多的高度不饱和ω - 3脂肪酸（HUFAs），水生补给可能通过向水生和陆地消费者提供这些关键化合物而发挥独特作用。在此，我们首先根据水生和陆地昆虫猎物中的HUFAs含量来描述营养质量。然后我们利用整体稳定同位素分析来估计溪流和河岸消费者对补给的利用情况，并结合特定化合物稳定同位素分析，这使我们能够记录消费者的HUFAs来源。最后，为了了解水生来源的HUFAs对河岸消费者的营养重要性，我们在实验室对东菲比霸鹟（Sayornis phoebe）雏鸟进行了饮食操控实验。与陆地昆虫相比，水生昆虫的HUFAs含量显著更高，主要是二十碳五烯酸（EPA）。溪流鱼类主要依赖水生资源，而食虫鸟类在不同地点对水生补给的利用情况各不相同。然而，与溪流鱼类一样，东菲比霸鹟雏鸟从水生昆虫中获取HUFAs，即使它们的整体饮食严重依赖陆地昆虫。在实验室中，膳食中的HUFAs，比如由水生昆虫提供的那些，提高了东菲比霸鹟雏鸟的生长速度和身体状况。这项研究表明，从消费者的角度来看，水生和陆地补给在营养方面不是相互的，因为水生资源是溪流和河岸消费者关键脂肪酸的主要来源。它也证实了先前关于HUFAs对河岸鸟类营养重要性的研究结果，表明食虫的河岸生活方式影响鸟类的营养需求。最后，我们的研究结果提出了一种可能性，即如果鸟类和其他河岸食虫动物由于水生栖息地退化或消费者与资源物候的变化而无法获得高质量的水生补给，它们可能会与陆地猎物出现营养不匹配的情况。在本文的支持信息中可以找到一份通俗易懂的语言概要。