Small scale hydrogeomorphic features influence macroinvertebrate food webs in two Great Plains rivers

Abstract

Food resources that support river food webs and food web structure have been shown to be influenced by hydrogeomorphology, and its influence on food webs has been gaining support with ecologists over the years. I analyzed the influence of local hydrogeomorphic variables on the structure and function of food webs in two U.S. Great Plains rivers, the Little Missouri and Niobrara. I used stable isotope analysis to reveal hydrogeomorphic relationships with δ13C in the food web, consumer resource use, trophic community metrics and size corrected standard ellipse area (SEAc) a measure of consumer niche breadth. I found river sinuosity and percent fine grain sediment to have a large influence on the food web. Increasing sinuosity was associated with a decrease in the stable isotope composition (δ13C) of the entire food web and trophic diversity (p=0.038, R2=31.1%), but an increase in trophic niche specialization (p=0.013, R2=41.1%). Increasing percent fine sediment was also associated with a decrease in the δ13C of the food web, as well as higher consumption of autochthonous resources and terrestrial coarse particulate organic matter (CPOM); and a decrease in trophic redundancy (fewer species occupying the same trophic niche). These results suggest that the decrease in stable substrate probably caused an overall decrease in primary productivity and limited autochthonous growth to pools and slackwater areas. This decrease in primary productivity also caused the decrease in overall food web δ13C. The consumption of autochthonous resources and CPOM increased with slow water due to the lack of stable substrate in the majority of the system. Results also indicated that species became more specialized in their trophic niche likely due to decreased diversity. My study gives support to the importance of local hydrogeomorphic variables such as sediment size and sinuosity, on food web structure and trophic interactions in Great Plains rivers. Future studies could expand the number of study rivers in the Great Plains to increase the number and diversity of hydrogeomorphic variables and organisms analyzed.