Short-term resilience of aquatic-riparian linkages to artificial floods in a gravel-bed river

Background/Question/Methods

Riparian zone is an interface between aquatic and terrestrial ecosystems and provide a model system to test various hypotheses regarding the importance of subsidy. Tightly coupled adjacent ecosystems with efficient energy transfer across their boundary requires biological mechanisms to maximally utilize resources. Flood disturbance determines river communities and generates high spatio-temporal heterogeneity in subsidy (adult aquatic insects) to riparian areas. Nevertheless, how the recipient riparian consumers maximize the use of subsidy are scarcely known despite the ubiquity of floods. We hypothesized that aquatic-riparian food-web exhibits high resilience to floods through rapid recolonization of consumers. Our objectives were to examine riparian food-web and immediate responses of arthropods (Carabidae & Araneae) to artificial floods in a gravel-bed river, Japan. Consumers and their potential food resources for CN stable isotope analyses were collected, and emergence of aquatic insects (also larvae stranded on bars) was quantified in Satsunai river and else in 2013 and 2014. Abundance of consumers was quantified in 8 gravel bars in Satsunai river before and after an artificial flood (June 2014; a return period of <1 yr).    

Results/Conclusions

Numerically dominant species of carabids was Hemichlaenius noguchii. Riparian consumers derived their carbon more (>60%) from aquatic origins in particular Plecoptera insects (>80% of emergence). Araneae showed a similar average dependence on aquatic resources with a greater variability compared to carabids. Aquatic emergence was relatively abundant  in June second to the July occasion. All bars submerged in the water during the flood and most of the consumers left for riparian forests rather than being washed away. Abundance averaged across all bars remained the same level over the flood, and consumer distribution was skewed along the edges of gravel bars both before and after the flood. Aquatic emergence abundance was significantly reduced over the floods whereas abundance of larvae stranded on bars increased. Carabid abundance was relatively high in the area where stranded insects were abundant. Overall, riparian consumers (at least carabids) were highly resilient to the flood. Riparian-aquatic food-web linkage could remain stable during short-term floods because consumers with flexible feeding strategy and high mobility benefit from stranded larvae instead of emergence of adult aquatic insects.