Recent Declines in Nutrient Concentrations and Fluxes in the Lower Changjiang River

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 NO₃^-, PO₄^3-, and NH₄⁺ and 2014 for SiO₃^2-. After these turning point years, NO₃^-, SiO₃^2-, and PO₄^3- concentrations decreased at annual rates of 2.953, 3.746, and 0.108 μM/year, respectively. Regarding NO₃^- and PO₄^3-, 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 NO₃^- and PO₄^3- 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.