The Hai River Basin (HRB) is considered to be one of the most polluted areas in China due to the high regional population density and rapid economic development. The estuaries of the HRB, which receive pollutants from terrestrial rivers, may subsequently suffer potential pollution and result in ecological risk of heavy metals. Six heavy metals (Cd, Cr, Cu, Ni, Pb, and Zn) were measured in estuarine surface sediments from 10 estuaries of the HRB to investigate their variation characteristics and ecological risks. The spatial difference of Cr, Ni, Pb, and Zn in sediments was higher than that of the rest two elements. The Yongdingxin Estuary (YDX) and Ziyaxin Estuary (ZYX) in the Northern Hai River System (NHRS) were the most severe in terms of heavy metal contamination. According to the Risk Assessment Code (RAC) classification, Cd associated with the exchangeable and carbonate fraction (the average of 21.3 %) indicated medium risk to high risk. More than 50 % of Cr, Cu, Ni, and Zn on average were associated with the residual fraction. Based on the sum of the first three fractions (exchangeable and carbonate + reducible + oxidizable), the mobility order of these heavy metals was Cd >Pb > Zn ≈ Cu > Ni > Cr. Compared to the background values of cinnamon soil, the potential ecological risk index (RI) values ranged from 25.6 to 168, with an average of 91.2, indicating a low ecological risk in estuarine sites of the HRB. Cd and Pb were the dominant contributors to the toxic-response factor (45.8 and 25.5 %, respectively). The results give insight into the different control measures pertaining to heavy metal pollution and risk for both relatively clean estuaries and urban seriously polluted areas, respectively, for the formation of protect strategies of aquatic environment in the HRB.

Changes of cropping systems are un-neglected driving forces to increase nitrogen imbalance of agricultural development in the watershed scale.

Riverine dissolved organic carbon (DOC) is an important carbon pool in the global biogeochemical cycle. DOC transport in river networks involves three processes: DOC input (including DOC release from terrestrial ecosystems and in situ production in river networks), in-river removal, and export through watershed outlets or estuaries. DOC transport in large river networks is gaining attention due to its important role in carbon and nutrient supply and CO 2 emission, for example. However, quantifying DOC input to rivers and in-river removal is still not properly understood. This study developed a synthesis model to quantify DOC transport in the Changjiang River Network (CRN) by coupling spiraling theory with Strahler river order. Our study suggested that the wetlands proportion and soil organic matter are valid parameters in a DOC model and our simulations showed that approximately 2.65-4.86 Tg C/year entered rivers and 1.16-2.12 Tg C/year was exported to the estuary as DOC, with a removal proportion of over 50% throughout the CRN in 1980-2015. The subbasins of the Dongting Lake, the Yichang-Jiujiang section of the mainstream, and the Min-Tuo River were primary contributors of DOC load, accounting for approximately 45% of the bulk DOC load of the CRN. The subbasins of Jinsha River and Yalong River in the upper reaches of the CRN and the subbasin of Wu River contributed less than 10% of the DOC load. DOC export by the CRN accounted for 0.3-1.2% of the global DOC flux from land to sea. Plain Language SummaryThe transport of dissolved organic carbon (DOC) in river networks is a key section of the global carbon biogeochemical cycle. DOC transport in river networks involves three processes: DOC input to rivers, in-river removal, and export through estuaries, and these processes can be altered by human activity and climate change. We have developed a synthesis model to individually simulate these three processes of riverine DOC transport by coupling hydrological and biogeochemical processes and have applied our model to the Changjiang River Network. Our study is helpful in comprehensively understanding riverine DOC transport from land to estuaries in large river networks. Key Points:• A synthesis model was developed to quantify DOC input to rivers, in-river removal, and export to the estuary in the CRN • The DOC removal proportion slightly decreases with increasing river order and is 33.6-60.1% at the subbasin and entire CRN scales • About 1.47-2.71 t C·km −2 ·year −1 entered the rivers, and 0.64-1.18 t C·km −2 ·year −1 was exported by the CRN to the estuary as DOC Supporting Information:• Supporting Information S1• Figure S1 • Figure S2 • Figure S3 • Table S1 . A synthetic model to quantify dissolved organic carbon transport in the Changjiang River system: Model structure and spatiotemporal patterns.