Spatial Variation in Food Web Structures, Energy Flows, and System Attributes Along the Pearl River and Their Indications of Protection and Restoration

River ecosystems are facing significant degradation from human activities, which impact both biotic (e.g., fish and invertebrates) and abiotic components (e.g., water and habitat). A comprehensive comparison of energy flow patterns and system attributes among river food webs under different levels of human interference is highly important for developing management strategies to protect river ecosystems. Along the subtropical Pearl River, six spatial zones, including agricultural, industrial, island, urban, factory, and estuarine areas, were chosen to construct the Ecopath models. The output results revealed that the highest trophic level of the Pearl River was 3.8-4.2, which was occupied by softshell turtles and piscivorous/carnivorous fish. The most diverse functional groups were found in island and estuarine zones due to their heterogeneous habitats (e.g., high submerged vegetation coverage and the transition area between fresh and brackish water). In contrast, the food web structure in the industrial zone was destroyed due to water pollution (e.g., sewage discharge) and habitat degradation. The increase in exotic species and the decrease in native top predators were two factors that result in the low efficiency of energy transmission. A series of trophic (e.g., Lindeman transfer efficiency and mixed predator-prey impacts), structural (e.g., keystoneness, omnivory, and Finn's path length), and theoretical (e.g., connectance and ascendency) indices revealed that the health and maturity of the Pearl River sections can be ranked as island > estuarine > agricultural > urban > factory > industrial zones. The food chains led by softshell turtles (<i>Pelodiscus sinensis</i>), piscivores (e.g., <i>Elopichthys bambusa</i>), molluscivores (e.g., <i>Mylopharyngodon piceus</i>), and herbivores (e.g., <i>Ctenopharyngodon idelus</i>) could be used to indicate the health and functioning of river ecosystems. Our results suggest that the ecological management of river ecosystems should focus more efforts on protecting original habitats (e.g., the island zone with fish feeding/spawning grounds), monitoring bioindicators with keystone trophic impacts in the food web, and evaluating the food chains that play important roles in upward energy transmission.