Stable isotope ratios of carbon and nitrogen in suspended organic matter: Seasonal and spatial dynamics along the Changjiang (Yangtze River) transport pathway

Seven cruises were conducted in the Changjiang (Yangtze River) Estuary and the adjacent western East China Sea (ECS) from 2010 to 2012 to study the seasonal variations of δ¹³C and δ¹⁵N in suspended organic matter. In addition, two cruises in the northeastern ECS in July 2011 and in Tsushima Strait in July 2012 were conducted to evaluate the distribution patterns of these isotopes over the entire Changjiang transport pathway. In summer, the surface δ¹³C was lowest in the Changjiang Channel, increasing from land to sea, reaching highest values in the central ECS, and then decreasing and remaining relatively constant. In winter, the surface δ¹³C in the western ECS showed lower values with less variation in general. At most stations, δ¹³C increased from the sea surface to the seabed, reflecting the degradation of sinking organic matter; however, these trends could be changed in the summer by surface phytoplankton accumulation. Combining data from all the Changjiang Estuary and western ECS cruises revealed that when the suspended particulate matter (SPM) was > 135mg/L, the δ¹³C values were fairly constant (-24.5‰ to -20.5‰); when the SPM was < 135mg/L, the δ¹³C values showed much greater variability (-28.4‰ to -16.6‰). The surface δ¹⁵N also showed generally higher values in the central ECS in summer and lower values in winter. The seasonal variations of δ¹³C and δ¹⁵N were largely attributed to the SPM composition change, i.e., more phytoplankton cells in the summer where as more resuspended sediment particles were present in winter.