Carbon and nitrogen isotopes analysis and sources of organic matter in surface sediments from the Sanggou Bay and its adjacent areas, China

Naturally existing stable carbon and nitrogen isotopes are important in the study of sedimentary organic matter sources. To identify the sources of sedimentary organic matter in Sanggou Bay and its adjacent areas, which is characterized by high-density shellfish and seaweed aquaculture, the grain size, organic carbon (OC), total nitrogen (TN), carbon and nitrogen isotopic composition (delta C-13 and delta N-15) of organic matter in the surface sediment were determined. The results showed that, in August, sedimentary OC and TN ranged from 0.17% to 0.76% and 0.04% to 0.14%, respectively. In November, OC and TN ranged from 0.23% to 0.87% and 0.05% to 0.14%, respectively. There was a significant positive correlation between OC and TN (R=0.98, P < 0.0001), indicating that OC and TN were homologous. In August, the delta C-13 and delta N-15 of organic matter varied from -23.06aEuro degrees to -21.59aEuro degrees and 5.10aEuro degrees to 6.31aEuro degrees, respectively. In November, delta C-13 and delta N-15 ranged from -22.87aEuro degrees to -21.34aEuro degrees and 5.13aEuro degrees to 7.31aEuro degrees, respectively. This study found that the major sources of sedimentary organic matter were marine shellfish biodeposition, seaweed farming, and soil organic matter. Using a three-end-member mixed model, we estimated that the dominant source of sedimentary organic matter was shellfish biodeposition, with an average contribution rate of 65.53% in August and 43.00% in November. Thus, shellfish farming had a significant influence on the coastal carbon cycle.