Seasonal forest resources support fish biomass in floodplain lakes of an Amazonian tributary

The Flood Pulse Concept is a foundational ecological theory that emphasizes the critical role of lateral connectivity between a river channel and its floodplain. Many tropical rivers inundate the surrounding floodplain in the flood stage, thereby receiving large amounts of terrestrial organic matter that can be decomposed by microbes and directly consumed by animals. This dynamic could simultaneously drive down oxygen concentrations while also supporting fish production. We used two lines of evidence to investigate the fate of terrestrial organic matter during the low‐ and high‐water seasons in the Juruá River, Amazonas, Brazil: spot measurements of dissolved oxygen and isotopic measurements (δ13C, δ15N) of fishes and food source pathways originating from C3 and C4 plants, phytoplankton, and periphyton. Dissolved oxygen concentrations were low (mean ≤ 3.0 mg/L) throughout the floodplain during high water, while higher values (mean = 6.5 mg/L) were evident during low water, suggesting variable rates of ecosystem respiration, production and atmospheric exchange across seasons. Most fish species, including the commercially and culturally important pirarucu (Arapaima sp.), had a strong dependence on terrestrial C3 plants during the falling‐water season (median source proportions 34%–77%), while fishes shifted to rely on the phytoplankton pathway (median proportions 11%–82%) during low water. Our results demonstrate that terrestrial C3 plant resources are channeled into the food web through detritivorous fishes, such as bodó (Liposarcus pardalis), and frugivorous fishes, such as pacu ( Mylossoma aureum ). During high water, a dispersed food web takes shape as fish move into the flooded forest, driven by terrestrial resources and accompanied by low oxygen conditions. During low water, a concentrated food web emerges in the remaining oxbow lakes, consistent with fast‐growing algal resources.