Carbon and nitrogen isotopic compositions of particulate organic matter in the upwelling zone off the east coast of Hainan Island, China

Highlights: • The coastal upwelling played an important role in determining the source of POM. • Terrestrial organic matter of nearshore was transported to the offshore through tidal movement. • The contribution of marine organic matter in the upwelling area was higher than other area. The isotopic compositions (δ¹³C and δ¹⁵N) and C/N ratios of suspended particulate organic matter (POM) were investigated off the east coast of Hainan Island in the South China Sea during summer. Coastal upwelling influenced the nearshore stations of transects S2 and S3, and higher δ¹³C and δ¹⁵N values suggested that coastal upwelling played a significant role in determining the POM sources. The POM at the nearshore area of transect S1 was controlled by the coastal current and freshwater discharge. Additionally, organic matter may be transported to the offshore area via tidal movements in transects S1 and S3. Based on the stable isotope analysis in an R model, the marine organic matter contribution in the upwelling area (19%) was higher than that in the other areas (transect S1 and the offshore area) (7%). The δ¹³C and δ¹⁵N values and C/N ratios reflect the carbon and nitrogen sources and their cycling in the upwelling zone off the east coast of Hainan Island.