Spatial and temporal variations in nitrogen and phosphorous nutrients in the Yangtze River Estuary

Based on spatial interpolation data from 2003 to 2010, combined with almost 30 years of nitrogen and phosphorus pollutant data, this research analyzed the variations in total nitrogen (TN), nitrate nitrogen (NO3-N), total phosphorus (TP) and phosphate-P (PO4-P) in the Yangtze River Estuary (YRE). On the annual and seasonal timescales, nitrogen and phosphorus concentrations exhibited increasing trends overall, and the fluctuations in the concentrations of TN, TP and PO4-P significantly increased during the last three decades, especially after 2003, because of the more prevalent human activities and nonpoint sources in the area. Specifically, a high-concentration area of TN was found downstream of the North Branch of the YRE. Considering the spatial distribution of the nutrients, combined with a tidal flood current and a time span of 7 years, the TN maximum increased from approximately 3.07 mg/L to 4.48 mg/L. The TP maximum also rose from approximately 0.25 mg/L to 0.34 mg/L because of a high-concentration area of TP in the South Branch of the YRE due to the confluence with the Huangpu River. Additionally, there was an expansion of high-concentration areas of TN (⩾3.0 mg/L) and of TP (⩾0.20 mg/L).