Human-driven long-term disconnect of nutrient inputs to the Yellow River basin and river export to the Bohai Sea

Abstract

Combining long-term measurements of nitrogen (N) and phosphorus (P) concentrations in the Yellow River (YR) (1981–2018) and river discharge (1981–2018) to the Bohai Sea (BS) with model-based estimates of nutrient delivery shows a disconnect between nutrient delivery and export fluxes to the BS. Basin-wide nutrient delivery to surface water, primarily from agriculture and sewage, exhibited an increasing trend over the whole period. However the nutrient export fluxes to BS declined from 1981 to 2002, followed by a period of large inter-annual variability. Nutrient export is strongly controlled by water discharge, the Water and Sediment Regulation Scheme (WSRS), and the large volume of water containing nutrients that is extracted for primarily irrigation. Dissolved organic N (average 15% in the period 2001–2018) and dissolved organic P (68%) accounted for an important share of total dissolved N and P, which is due to agricultural inputs and preferential retention of inorganic forms in the YR basin. River nutrient export fluxes alone cannot explain the changes in the frequency and area of red tides in the BS due to impacts of the WSRS, the long water residence time in the BS and climate change. This calls for the application of mechanistic models for describing the impact of combined long-term changes in the BS ecosystem.