Trophic linkages are formed when consumers eat resources from a different food web. These links can produce positive feedbacks that enhance productivity and resilience in other ways unlinked ecosystems. Amazonian black-water rivers have very limited primary productivity, yet they sustain highly diverse fish assemblages and important fisheries. This apparent paradox led us to investigate trophic linkages between terrestrial and aquatic food webs. We analyzed a large dietary dataset from stomach contents (N = 807 samples) and stable isotopes of C and N (N = 180) of 12 omnivorous fish species to test the hypothesis that food resources derived from seasonally flooded forests support fish production in nutrient-poor Amazonian rivers. We focused on medium-sized species because they are highly abundant and a major component of subsistence and commercial floodplain fisheries in Amazonian black-water rivers.
Results/Conclusions
Fish of all species consumed allochthonous foods from floodplain forests in higher proportions relative to autochthonous foods. Diets of all species were dominated by forest vegetation and canopy arthropods. Although forest vegetation (particularly fruits and leaves) contributed a larger proportional volume to diets, mixing models (MixSIAR) for C and N isotopes predicted a greater contribution of canopy arthropods to fish biomass. Most canopy arthropods found in fish diets are herbivorous and lack an aquatic life cycle stage and therefore function as trophic links between floodplain forests and fishes. Because the access of fishes to canopy arthropods is modulated by flood pulse dynamics, our work highlights the importance of considering temporal and spatial dimensions in food web research. Given the link between forests and fisheries, the log-term persistence of floodplain fisheries in oligotrophic systems depends on our ability to protect seasonally flooded forests which are increasingly threatened by large-scale deforestation, the proliferation of hydroelectric dams and now climate change.