Riparian hunting spiders do not rely on aquatic subsidies from intermittent alpine streams

Drying in alpine streams might decrease aquatic-terrestrial trophic linkages by reducing terrestrial predation on aquatic prey. We tested this hypothesis by investigating whether a common riparian predator (hunting spiders) in alpine environments assimilated a lower proportion of aquatic prey with increasing stream intermittency. We used high temporal-resolution data from electrical resistance sensors to map patterns of naturally-occurring flow intermittency across 30 headwater streams of Val Roseg, a glacierized catchment in the Swiss Alps. We collected riparian hunting spiders, as well as potential terrestrial and aquatic macroinvertebrate prey, from streams and their associated riparian zones across two seasons (Alpine spring and summer). We estimated aquatic contributions to spider diets (pA) using (i) a gradient approach with aquatic invertebrate and spider carbon stable isotope ratio values (δ13C), and (ii) Bayesian carbon and nitrogen (δ15N) isotope mixing models. Spider pA from the gradient method were not statistically different from zero in spring (0.08 ± 0.10) and low in summer (0.16 ± 0.04). Mixing models also estimated low dependence on aquatic prey in both seasons, although with potentially higher contributions in summer. Spider diet did not vary with increasing flow intermittency in either season. Our results suggested that alpine hunting spiders obtain most of their carbon from terrestrial prey. The slight increase in spider pA during summer may correlate with peak emergence periods for aquatic insects, indicating opportunistic feeding by this riparian predator.

Methods
Our paper presents an observational study of riparian hunting spiders along intermittent headwater streams in Val Roseg, a glacierized Alpine catchment, in June and August 2018. Flow intermittency was measured from July 2017 using electrical resistance sensors to log presence/absence of water. We collected spiders from the riparian zones of the streams in both June (Alpine spring) and August (Alpine summer), then estimated the relative contributions of emergent aquatic insects to spider diets using analysis of C and N stable isotopes. Information on data processing is summarised in the associated ReadMe file.