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Biogeochemistry of dissolved organic matter (DOM) in estuarine and coastal areas is an important topic of carbon cycling studies on both global and local scales (Cai et al., 2012;Barrera et al., 2017;Seidel et al., 2014). The bulk DOM in aquatic environments has been shown to be heterogeneous in size, composition, and biological/chemical reactivity (Benner & Amon, 2015;Xu & Guo, 2017). Construction of dams and reservoirs along the main river channel and tributaries may also change the fluxes and composition of terrestrial materials transported by the large rivers Yu et al., 2011;. Thus, further knowledge is needed to better understand processes and mechanisms that control the composition, seasonal variations, and cycling pathways of DOM in large river dominated ocean margins on various spatial and temporal scales (Cao et al., 2016).In large river dominated estuarine areas, the concentrations and inventories of biogeochemical constituents, such as DOM and nutrients, are largely controlled by the export fluxes from rivers (Gao et al., 2015;Song et al., 2017). The biogeochemical cycling of many chemical species in coastal and marine environments is also closely connected with the physical and chemical properties of those terrestrial compounds (Cai

To elucidate nutrient variation patterns and trends over various timescales under combined effects of human activities and climate change, nutrient concentrations were monitored monthly in Lower Changjiang (Yangtze) River from November 2016 to August 2020. They were also monitored daily during an extreme flood in July 2020. Over daily and seasonal timescales, the Changjiang River discharges had a dominant influence on nutrient concentrations. By combining existing data over recent decades with those from the current study, we found that turning points for concentration trends for most nutrients emerged in the recent decade (2010–2020), i.e., 2012 for NO 3 − , PO 4 3− , and NH 4 + and 2014 for SiO 3 2− . After these turning point years, NO 3 − , SiO 3 2− , and PO 4 3− concentrations decreased at annual rates of 2.953, 3.746, and 0.108 μM/year, respectively. Regarding NO 3 − and PO 4 3− , their concentrations and fluxes increased from 1960s to 2012, similar to the increasing trends of anthropogenic N and P fertilizer inputs from the drainage basin. After 2012, concentrations and fluxes of NO 3 − and PO 4 3− showed significant decreasing trends, largely due to the control of N and P fertilizer usage. A comparison among eight rivers in East and South China (including the Changjiang River) indicated that basin latitudes were essential to determining areal nutrient yields, implying that latitude-related factors, such as temperature, precipitation, and areal population density, significantly impacted nutrient fluxes. This study emphasized that the deteriorating Changjiang River aquatic environment (which lasted from 1960s to 2010) has been successfully terminated over the last 10 years in 2010s.

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