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P>1. Low flows in rivers are predicted to increase in extent and severity in many areas in the future, yet the consequent impacts of river drying on terrestrial communities via (i) changes to riparian microclimatic conditions and (ii) the identity and abundance of emerging aquatic insects available to riparian predators have not been quantified.

2. We investigated the influence of low river flow on a riparian fishing spider, Dolomedes aquaticus, in five New Zealand rivers containing permanently flowing and drying reaches and, in one river, along a longitudinal drying gradient.

3. The biomass of aquatic insects, potential prey for D. aquaticus, declined with low river flows while the abundance of potential terrestrial prey remained similar at all sites. In the replicate rivers, and along the longitudinal drying gradient, spider biomass was lower, and size classes were skewed towards more small and fewer large spiders, in drying sites. A desiccation experiment in the laboratory indicated high sensitivity of the spiders, with prey presence increasing spider survival.

4. Differences in the spatial distribution, biomass and population size structure of spiders were observed along the longitudinal drying gradient and disappeared within 16 days of the water returning to all sites.

5. In total, low river flow affected the biomass of D. aquaticus, as well as their size class structure and spatial distribution. This indicates that low river flows have the potential to affect adjacent terrestrial ecosystems.