Rivers as biogeochemical highways: Decoding phosphorus transfer from catchments to coastal zones with insights from...

Phosphorus (P) exhibits a dual character as both a nutrient and a pollutant, sustaining aquatic ecosystem productivity while posing eutrophication risks under excessive inputs. The "nutrient-pollutant" duality of P is intricately linked to its biogeochemical processes and sediment transport dynamics. Over the past half-century, intensive dam construction and watershed development have driven structural transformations in "sediment-phosphorus" coupling fluxes across global river systems. This study employs the Yellow River-a global exemplar of dramatic sediment flux variations-as a natural laboratory. Through multi-scale observations and process-based modeling, we identify the mechanisms of human-driven phosphorus cycle restructuring: P in the Yellow River predominantly exists in particulate form, with potential bioavailable P (BAP) accounting for 17.02%-54.02% of particulate P (PP); human interventions have reduced sediment discharge to the lower reaches by 92.5%, resulting in an 87.58% and 87.72% decline in total P (TP) and PP loads, respectively; while dissolved P (DP) has increased. Over the past two decades, P has emerged as a limiting nutrient in the lower reaches and estuary of the Yellow River and a substantial reduction in P load could exacerbate N/P imbalances and diminish primary productivity. This study establishes a novel framework for understanding abrupt changes in riverine biogeochemical cycles in the Anthropocene, with implications for nutrient management. The proposed "sediment-nutrient" coupled management framework provides a scientific basis for balancing hydropower development with estuarine ecological security.