COS 25-4 - Small stream predators rely heavily on terrestrial matter energy input, regardless of riparian buffer size

Abstract: Stream ecosystems are dependent on terrestrial subsidies providing a strong bottom-up trophic effect throughout an aquatic food web. The effects of terrestrial land use on riparian systems and the adjacent streams can have direct and indirect effects on the predators that rely on those systems. A common forestry technique to separate streams from upland land use is the establishment of forested buffers. However, the exploration of the effects of forested buffers on food webs are few, with most focusing on streams surrounded by agricultural production rather than forestry. In this study, we explored the carbon and nitrogen-stable isotopic composition of two major predator groups, brown trout (Salmo trutta) and riparian spiders (Tetragnathidae and Lycosidae), to determine the predominant prey pathway in unbuffered, buffered, and forested riparian systems surrounding small forest streams. Using δ¹⁵N and δ¹³C isotopic signatures and Bayesian mixing models (MixSIAR), we estimated the proportion of aquatic versus terrestrial reliance in the diets of brown trout and each spider family at twelve stream sites categorized as unbuffered, buffered (1-120 m of forest buffer) or forested ( >120 m). Our aquatic and terrestrial prey did have significantly different isotopic signatures, and predators at all sites had ranges of both δ¹⁵N and δ¹³C isotopic signatures that fell within the signatures of the collected potential prey suggesting a reliance on multiple trophic levels of prey from both aquatic and terrestrial environments. However, our results present a picture of aquatic and riparian predators with a considerable dependence on terrestrial subsidies and prey items regardless of forested buffer size. Predators in unbuffered sites did not appear to consume a higher portion of aquatic prey over terrestrial prey compared to buffered or fully forested sites. Although we found no evidence of riparian buffers affecting the diet of fish or spiders, previous studies have highlighted the impact of forestry and upland land use on aquatic and riparian systems. It may be that the condition and use of the forested upland system may have a greater impact on small streams than the immediately adjacent buffered area.