Can primary production contribute non-labile organic matter in the sea: Amino acid enantiomers along the coast south of the Changjiang Estuary in May

Amino acid enantiomers (AAEs) in suspended particulate organic matter (OM) were measured along the coast south of the Changjiang Estuary in May to determine the origin and nature of estuarine particulate OM. The degradation index (DI) for amino acid yields revealed that fresh OM corresponded to depletion of dissolved inorganic nitrogen, whereas degraded OM corresponded to excess nutrients. This indicates coupling between OM degradation and nutrient regeneration in surface waters. Two clearly different trends were found for the amino acid content of total particulate nitrogen. Offshore samples were characterized by amino acid enrichment (91%), whereas inshore samples were depleted in amino acids (14%), probably because of terrestrial impacts during the previous winter. Compared to samples previously taken from the lower Changjiang reaches (Shao et al., 2011), samples of surface coastal waters can be clearly identified by the D/L ratio of selected AAE, indicating the potential of AAEs for identifying OM sources. Peptidoglycan estimated on the basis of D-AAEs, ranged from 3 to 275 nM. For fresh particulate OM (DI > 0.5), the peptidoglycan concentration was positively correlated with OM freshness and particulate-nitrogen-normalized peptidoglycan was comparable to or even higher than that in terrestrial OM. This suggests that estuarine and coastal zones make a significant contribution to non-labile OM production. Further analysis suggests that heterotrophic bacteria and Synechococcus are notable contributors.