Riverine phosphorus transport and its statistical coupling with discharge and suspended sediment

Phosphorus is an essential nutrient in aquatic ecosystems. Its concentration in surface waters regulates primary productivity, whereas excessive loading promotes eutrophication and associated water-quality degradation. Rivers are major conduits for phosphorus transport from land to downstream lakes, reservoirs, estuaries and coastal waters, with both water and sediment acting as primary carriers of phosphorus in river systems. Human activities and climate change have substantially altered water and sediment regimes in rivers worldwide. However, the resulting changes in the patterns and statistical characteristics of riverine phosphorus transport remain insufficiently quantified, despite their importance for managing aquatic ecosystem health.

Using the Mississippi River basin as a case study, we compiled long-term observations of river discharge, suspended sediment concentration and total phosphorus concentration from 1970 to 2020, and statistically analysed the patterns of riverine phosphorus transport and its relationships with water and sediment. To account for changes in water-quality and environmental policies within the basin, we further divided the record into two sub-periods (1971–1990 and 2001–2020) and considered the full period 1970–2020 for comparison. We developed a multiple linear regression framework to quantify interactions between phosphorus export, discharge and suspended sediment concentration, and to assess how watershed characteristics influence phosphorus transport under different flow conditions. This framework was used to characterise the temporal and spatial variability of phosphorus transport across the Mississippi River basin and to disentangle the effects of human activities and climate variability.

We find that phosphorus transport is source limited and negatively correlated with basin area under low-flow conditions. Human activities are strongly associated with phosphorus transport, with population density influencing total phosphorus concentrations both directly and indirectly through the TP–discharge and TP–suspended sediment concentration relationships. The interception effect of reservoirs on total phosphorus export increases with their regulation capacity, while trends in total phosphorus concentration are positively related to changes in precipitation and predominantly negatively related to vegetation cover. Our study provides a basin-scale perspective on source-to-sink fluvial phosphorus transport, offering critical insights for sustainable phosphorus management and for the integrated management of riverine and coastal ecosystems.