Amino Acids as Indicators of Organic Matter Sources and Degradation in Suspended Matter off the Pearl River: Indications for Resuspension in the Northern South China Sea

Suspended matter (SPM) was sampled in a grid off the Pearl River mouth in the northern South China Sea (SCS). SPM concentrations and the content of total organic carbon (TOC), total nitrogen (TN), amino acids (AA) and hexosamines (HA) and derived biogeochemical indicators were used to study organic matter sources on the shelf and slope. A surface SPM maximum curtailed the water mass of mixed riverine and marine origin off the Pearl River mouth with salinities <30. SPM in this river plume was rich in organic matter of fresh planktic origin. In areas outside the river plume chlorophyll (Chl-a) maxima were found at the subsurface nutricline. The AA composition shows that the degradation state of organic matter is very similar in all samples except in bottom water samples. Rather than degradation indicators, an indicator of SPM residence time in the ocean shows differences between samples from the upper 200 m and the deeper SPM samples. On the shelf and the shelf break a distinct SPM maximum was found above the sea floor. AA and HA spectra revealed that its organic matter was more degraded than the other SPM samples and that part of the organic matter in the bottom water turbidity maximum originated from the fine fraction of sediments. The state of organic matter implies that degradation of this resuspended material possibly adds to bottom water hypoxia; furthermore, contaminants originally deposited in shelf sediments can be redistributed into distal areas of the South China Sea.