Suchergebnisse

Abstract
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.

Abstract
Background
Fungicides are frequently used in agriculture and can enter freshwater ecosystems through multiple pathways. The negative impacts of fungicides on microorganisms, fungi in particular, and their functions such as leaf decomposition have been repeatedly shown. In our previous microcosm experiment with three consecutive cycles of fungicide exposure and colonisation of leaf substrate, we found clear functional changes, but no differences in fungal community structure could be detected using morphological identification by analysing the spores of aquatic hyphomycetes. In this study, we examined the effects on fungal and bacterial community composition in detail using ITS and 16S metabarcoding and comparing the results to morphologically assessed community composition.

Results
While we found fewer species with metabarcoding than with morphological identification, metabarcoding also enabled the identification of several fungal species that were otherwise unidentifiable morphologically. Moreover, by distinguishing individual amplicon sequence variants (ASVs) metabarcoding provided greater taxonomic resolution. In line with the morphological results, metabarcoding neither revealed effects of fungicides on the aquatic hyphomycetes nor on the total fungal or bacterial community composition. However, several ASVs responded significantly to fungicides, demonstrating variable tolerances within species.

Conclusions
Overall, the absence of detectable effects of fungicides on the community structure despite clear functional effects, suggests a complex relationship between community structure and the ecosystem function of leaf decomposition.