Background

Many aquatic insects emerge as adults from water bodies to complete parts of their life cycle in terrestrial ecosystems and are potential prey for riparian predators. The benefits of riparian predators from aquatic insects include higher contents of long-chain polyunsaturated fatty acids (PUFA) compared to terrestrial insects. Aquatic insects are therefore considered a high-quality food. Food containing high levels of PUFA can enhance growth and immune response of spiders. However, agricultural stressors like nutrient increase, pesticides and habitat degradation can affect the biomass of aquatic insects and in turn the diet of spiders. Studies quantifying the influence of land use on fatty acid (FA) profiles of emergent aquatic insects and riparian predators are lacking. We quantified differences in exports of FA, saturated FA, monounsaturated FA, and PUFA, FA profiles of aquatic insects and spiders between forested and agricultural sites over the primary emergence period within one year. The FA export to the riparian food web is crucial to understand energy fluxes between ecosystems. Furthermore, we monitored environmental variables to identify associations between agricultural stressors and FA profiles of aquatic insects and spiders.

Results

We found differences in FA export and profiles of aquatic insects between land-use types. The quantity of total FA export via aquatic insects was lower in agricultural sites (95% CI 1147–1313 µg m⁻²) in comparison to forested sites (95% CI 1555–1845 µg m⁻²), while the biomass export was higher in agricultural sites. Additionally, in spring the PUFA export was significantly lower (up to 0.06 µg d⁻¹ m⁻²) in agricultural than forested sites. Agricultural stressors explained only little variation in the FA profiles of aquatic insects, e.g., 4% for caddisflies and 12% for non-biting midges. Percentage of shading and pool habitats were identified as most important variables explaining the variation in FA profiles.

Conclusion

The quality of aquatic insects as food source for riparian spiders was smaller in agricultural than forested sites, which can decrease the fitness of riparian predators. To improve our capacity to predict potential adverse effects in the riparian food web, future studies should identify the mechanisms underlying a lower PUFA content.

中文翻译：

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土地利用改变了水生昆虫高价值脂肪酸的跨生态系统转移

背景

许多水生昆虫作为成虫从水体中出现，在陆地生态系统中完成其生命周期的一部分，并且是河岸捕食者的潜在猎物。与陆生昆虫相比，水生昆虫对河岸捕食者的好处包括长链多不饱和脂肪酸（PUFA）含量更高。因此，水生昆虫被认为是优质食品。含有高含量PUFA的食物可以增强蜘蛛的生长和免疫反应。然而，养分增加、农药和栖息地退化等农业压力因素会影响水生昆虫的生物量，进而影响蜘蛛的饮食。目前还缺乏量化土地利用对新兴水生昆虫和河岸捕食者脂肪酸 (FA) 分布影响的研究。我们量化了森林和农业地区一年内初出苗期 FA、饱和 FA、单不饱和 FA 和 PUFA、水生昆虫和蜘蛛 FA 分布的差异。FA 向河岸食物网的输出对于了解生态系统之间的能量通量至关重要。此外，我们还监测环境变量，以确定农业压力源与水生昆虫和蜘蛛的 FA 谱之间的关联。

结果

我们发现不同土地利用类型之间的 FA 出口和水生昆虫概况存在差异。^{与森林地区 (95% CI 1555–1845 µg m -2} ) 相比，农业地区通过水生昆虫的 FA 出口总量较低 ( 95% CI 1147–1313 µg m ^-2 )，而生物量出口较高在农业用地。此外，春季农业地区的多不饱和脂肪酸出口量显着低于森林地区（高达 0.06 µg d ^-1  m ^-2）。农业压力源仅解释了水生昆虫 FA 谱的微小变化，例如，石蛾为 4%，非叮蠓为 12%。遮荫和水池栖息地的百分比被认为是解释 FA 分布变化的最重要变量。

结论

农业地区作为河岸蜘蛛食物来源的水生昆虫的质量比森林地区的要小，这会降低河岸捕食者的适应性。为了提高我们预测河岸食物网潜在不利影响的能力，未来的研究应该确定低多不饱和脂肪酸含量背后的机制。