Abstract: 

Excess nutrient discharge from human activities drives the expansion of coastal eutrophication and hypoxia, generating a harmful marine environment. Yet the interlinks of nutrient discharge, riverine transport and coastal physical-biogeochemical responses are commonly quantified separately, obscuring the links between coastal degradation and its inland and socioeconomic origins. Here, we develop a process-based economy-land-river-ocean framework that connects economic input-output networks with watershed routing and three-dimensional coastal physical-biogeochemical processes. The framework establishes two complementary attribution chains: nutrient chain traces coastal impacts back through nutrient discharge to its physical production sources, while supply chain traces the same discharge to the final consumption driving it. We apply this framework to the Pearl River Basin-coastal ocean system by integrating trade flows among 313 cities, a monthly five-species nutrient discharge inventory, the Soil and Water Assessment Tool (SWAT) and the China Sea Multiple-scale Ocean Modelling System (CMOMS). Nutrient chain-based attribution shows that anthropogenic nutrients released in the river basin are routed through the river network to the coastal ocean, where they drive nearly all of the eutrophication and hypoxia based on our removal experiments. We also reveal that the physical sources of those coastal impacts and the consumption that drives them are geographically separated. Nutrient chain-based attribution identifies agriculture as the dominant physical source, yet supply chain-based attribution shows that consumption in the Greater Bay Area (GBA), mainly of industrial and service products, drives a substantial part of the coastal impacts. We further identify two persistent coastal hotspots of eutrophication and hypoxia and show that they originate from distinct river-outlet pathways, yet both are driven by the same urban consumption, notably in the GBA. These findings highlight the need to trace coastal degradation from nutrient sources through the river network to the coastal ocean, across both natural processes and economic supply chains, providing a transferable basis for coordinated land-sea nutrient management.