Asymmetric assimilation of an anthropogenic resource subsidy in a freshwater food web

Abstract

Anthropogenic subsidies occur readily, especially with growing human development neighboring natural systems. Determining how these subsidies permeate into native food webs is important, but currently not well understood. Here, we used a combination of stable isotopes and fatty acids as bio-tracers to explore the pathway(s) of assimilation of an anthropogenic resource (aquaculture feed) provided by freshwater cage aquaculture. Our results point to an asymmetric assimilation of aquaculture feed by native organisms into and through the food web. Specifically, two pelagic fish species (an intermediate consumer and a top predator) show elevated trophic positions (based on δ15N signatures driven by aquaculture feed) and higher proportions of essential fatty acids (18:2n-6 and 18:3n-3) in waters surrounding the aquaculture facility compared to control sites lacking aquaculture cages, suggesting feed consumption. Fish that appeared to assimilate feed also had higher n-3 long chain poly unsaturated fatty acids (LC PUFA) proportions, indicating a possible role of the feed subsidy for fish health. Alternatively, there was no evidence of feed assimilation through the littoral energy channel. Our findings provide new insight into an emerging research area that asks how anthropogenic resource subsidies affect species health and properties of food webs. Since many habitats experience both natural and anthropogenic subsidy inputs, the multiple tracer approach employed here shows promise for studying how subsidies may affect species health and behavior, as well as food web structure and ultimately stability.