A stable isotope study of linkages between stream and terrestrial food webs through spider predation

1. Transfer of carbon from freshwater to terrestrial ecosystems can occur through predation on adult aquatic insects, but the significance of this trophic pathway to the energetics of riparian communities is poorly understood. We used stable isotopes of carbon and nitrogen to explore linkages between aquatic insect production and the nutrition of web-building and free-living spiders alongside two streams in the North Island of New Zealand. 2. delta(13) C values for riparian tree leaves (means for each site = -32.2 and -30.3parts per thousand) were distinct from those of lichens collected from stream channel rocks and instream algae, both of which were similar (-23.4 to -22.4parts per thousand). delta(15) N values for leaves were similar at both sites (-3.4 and -2.7parts per thousand), but algae were considerably more depleted in delta(15) N atonesite suggesting significant differences in instream nitrogen sources between the twostreams. 3. Isotope values for potential aquatic prey of spiders indicated that aquatic algal production was their primary carbon source at both sites. Terrestrial invertebrates collected and assumed to be potential prey reflected a range of carbon sources and represented several trophic levels. 4. At one site, delta(13) C values indicated a primarily algae-aquatic insect pathway of carbon transfer to both web-building and free-living spider guilds. The other site appeared to have a primarily terrestrial carbon pathway for the free-living spider guild, and a mixed aquatic-terrestrial pathway for the web-building guild. 5. Overall, web-building spiders were estimated to obtain around 61% of their body carbon from aquatic production compared with 55% for free-living spiders. Our findings suggest that consumption of prey derived from aquatic sources can provide significant nutrition for spiders living along some stream channels. This pathway may represent an important feedback mechanism contributing to the energetics of riparian communities at sites where aquatic insect production is high.