Flow regime alters body size but not the use of aquatic subsidies in a riparian predatory arthropod

Alterations to river flow conditions have wide impacts on riparian organisms in terms of behavior and biomass. However, little is known about natural flood impacts on prey use and individual growth of riparian predators. Using stable carbon isotope analysis, we investigated flood impacts on aquatic-prey use and the size structure of an orb-web spider, Nephila clavata, during 3 years under different flood conditions in a black locust forest in the middle reaches of the Chikuma River. Large floods depressed aquatic-prey abundance, but did not affect terrestrial-prey abundance in the riparian forest. Consequently, spider growth was stunted after large floods. Spider body size was positively correlated with the body sizes of both aquatic and terrestrial insects in spider webs, where terrestrial insects were significantly larger than aquatic insects. The δ¹³C of aquatic insects was about 8‰ higher than that of terrestrial insects, and the δ¹³C of both insect groups did not vary significantly between months or among years. A negative relationship was found between body size and δ¹³C in spiders under different subsidies levels. Our results showed that flow regime altered spider growth through changes in aquatic subsidies level, but not aquatic-prey use by the spiders due to relative body sizes of predators and prey. Changes in relative body sizes of predator and prey may be an important factor in understanding nutrients, materials, and energy flows in aquatic and terrestrial linkages in the context of flow regime.