Anthropogenic impact on long-term riverine COD_Mn, BOD, and nutrient flux variation in the Pearl River Delta

In the Pearl River Delta (PRD), population growth and economic development have steadily increased the anthropogenic nutrient discharge into coastal waters. In this study, we employed the observed concentration and model reproduced runoff to quantify the interannual variation and the long-term (1985-2021) trends in riverine chemical oxygen demand (COD_Mn), biochemical oxygen demand (BOD), and nutrient fluxes. The annual COD_Mn and BOD fluxes increased slightly between 1999 and 2021. In comparison, the mean annual dissolved inorganic nitrogen (DIN) fluxes of the four eastern outlets increased significantly from 2.05 × 10⁵ t/a in 1985-1995 to 3.11 × 10⁵ t/a in 1999-2011 and then to 3.91 × 10⁵ t/a in 2014-2021. The outlets with the largest contributions to the COD_Mn, BOD, and DIN fluxes were Humen and Modaomen, which are both located near large cities. By calculating the COD_Mn fluxes upstream of the PRD, we found that the COD_Mn fluxes from downstream in the PRD increased faster than the fluxes from upstream. It follows that the increase in COD_Mn at outlets was mostly driven by the contributions of downstream major cities. In addition, the proportion of ammonia nitrogen flux in the DIN flux decreased from over 50 % to under 10 % at most outlets. This indicates that the toxicity of DIN fluxes has been mitigated. The DIN fluxes also showed a positive correlation with surface chlorophyll a and a negative correlation with bottom dissolved oxygen outside the Pearl River Estuary (PRE). This implies that the changes in phytoplankton growth and oxygen levels outside the PRE are closely linked to the variation in river-delivered nutrients, and the increasing riverine nutrient input may result in the expansion of intensified low-oxygen conditions outside the PRE.