Nutrients and particulate organic matter discharged by the Changjiang (Yangtze River): Seasonal variations and temporal trends

From September 2009 to August 2010, intensive monthly sampling of nutrients was conducted at two stations at the mouth of the Changjiang (Yangtze River). Particulate organic carbon (POC), particulate nitrogen (PN), and their stable isotope values (δ¹³C and δ¹⁵N) were also measured in selected samples of all months. Most nutrients (nitrate, phosphate, ammonia, and nitrite) as well as POC, PN, and δ¹³C displayed peak values when the highest or lowest Changjiang monthly discharges occurred, suggesting the Changjiang discharges strongly influence the seasonal variations of these chemicals. The sharply increases in concentrations of ammonia and nitrite in winter probably suggest nitrification was greatly depressed during this cold period. Using five interpolation methods, the annual discharge fluxes of nutrients, POC, and PN from the Changjiang to the East China Sea shelf were calculated. Combining this nutrient data with data from previous studies, the seasonal Mann-Kendall test, in which the influence of seasonal variation was considered, suggests concentrations of nitrate and phosphate in the Changjiang have significantly increased during recent decades at rates of 2.2 M yr ^-1 and 0.03 M yr^-1, respectively; no significant trend for silicate was noted. Decreased POC annual fluxes along with sharply decreased suspended particulate matter yields were also seen in recent years (1993-2010). However, no distinct changes of δ¹³C, δ¹⁵N, and the POC/PN ratio, which describe the particulate organic matter properties, were observed during this period.