Shift in diet composition of a riparian predator along a stream pollution gradient.

Terrestrial insectivores in riparian areas, such as spiders, can depend on emergent aquatic insects as high-quality prey. However, chemical pollution entering streams from agricultural and urban sources can alter the dynamics and composition of aquatic insect emergence, which may also affect the riparian food web. Few studies have examined the effects of stressor-induced alterations in aquatic insect emergence on spiders, especially in terms of chemical pollution and diet composition. We used DNA metabarcoding of gut content to describe the diet of Tetragnatha montana spiders collected from 10 forested streams with differing levels of pesticide and wastewater pollution. We found that spiders consumed more Chironomidae and fewer other aquatic Diptera, including Tipulidae, Ptychopteridae and Culicidae, at more polluted streams. Pollution-related effects were mainly observed in the spider diet, and were not significant for the number nor composition of flying insects trapped at each site. Our results indicate that the composition of riparian spider diets is sensitive to stream pollution, even in the absence of a change in the overall proportion of aquatic prey consumed. A high reliance on aquatic prey at polluted streams may give spiders an increased risk of dietary exposure to chemical pollutants retained by emergent insects.