Abstract：
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 (CODMn), biochemical oxygen demand (BOD), and nutrient fluxes. The annual CODMn 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 CODMn, BOD, and DIN fluxes were Humen and Modaomen, which are both located near large cities. By calculating the CODMn fluxes upstream of the PRD, we found that the CODMn fluxes from downstream in the PRD increased faster than the fluxes from upstream. It follows that the increase in CODMn 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.
摘要：
在珠江三角洲（PRD），人口增长和经济发展稳步增加了向沿海水域的人为养分排放。在这项研究中,我们利用观测到的浓度和模型重现径流来量化河流化学需氧量（CODMn）的年际变化和长期（1985-2021）趋势，生化需氧量（BOD），和营养通量。在1999年至2021年期间，年度CODMn和BOD通量略有增加。相比之下,东部四个出口的年平均溶解无机氮（DIN）通量显着增加，从1985-1995年的2.05×105t/a增加到1999-2011年的3.11×105t/a，然后增加到2014-2021年的3.91×105t/a。对CODMn贡献最大的网点，BOD,DIN通量是虎门和摩道门，两者都位于大城市附近。通过计算PRD上游的CODMn通量，我们发现，来自PRD下游的CODMn通量比来自上游的通量增加得更快。因此，网点CODMn的增加主要是由下游主要城市的贡献推动的。此外,在大多数出口，氨氮通量在DIN通量中的比例从50％以上下降到10％以下。这表明已经减轻了DIN通量的毒性。DIN通量也与珠江口（PRE）外的表面叶绿素a呈正相关，与底部溶解氧呈负相关。这意味着浮游植物生长和PRE外部氧气水平的变化与河流输送的营养物质的变化密切相关，增加的河流营养输入可能会导致PRE以外的低氧条件加剧。
