Abstract
Due to the essential roles of dissolved organic matter (DOM) in both microbiol food loop and marine carbon cycling, changes in marine DOM composition have an important impact on the marine ecosystem and carbon cycling. In October 2014 and June 2015, two field investigations for the DOM in the upper 200 m were conducted in the slope region of the northern South China Sea to characterize the DOM composition via amino acid enantiomers. In June, our sampling locations were under upwelling impact induced by an eddy-pair event, whereas in October there were no eddies. High-frequency sampling (a few hours interval) over 24 h reveals that the variability of the amino acid carbon yield (min. 0.2%) and the D/L alanine ratio (min. 0.02) is larger than its corresponding analytical and propagated errors, suggesting solid short-term changes for these two molecular-based indicators. Section samples from June showed a lower D/L alanine ratio (0.43 vs. 0.53) and a GABA mol% (1.0% vs. 1.6%) relative to the section samples from October, suggesting that DOM in June is more fresh (less degraded) compared to that in October. A higher serine mol% (19.5% vs. 13.2%) and lower D/L serine ratio (0.06 vs. 0.24) from the diel observation in June relative to October further indicates that phytoplankton, rather than bacteria, plays an more important role in DOM composition alternation. This is consistent with the higher phytoplankton biomass found in June, promoted by the eddy-pair.
Highlights
A comparison of the short-term changes we suggest a careful consideration of GABA% (Table S3)
The absolute value of the prop(expressed as standard deviations) and the corresponding seasonal propagated errors agated error for GABA% was of the order of 0.3–0.4%, which was similar to, or even larger indicated that short-term changes were significant for Amino acids (AAs) carbon yield, Gly%, and D / L Ala than, the GABA% value itself, which was 0.3–2.0% in this study
A comparison of the short-term changes and the corresponding seasonal propagated errors indi-11 of 16 cated that short-term changes were significant for AA carbon yield, Gly%, and D/L Ala ratio, whereas the other two molecular indicators were not (e.g., GABA%; Figure 5)
Summary
Amino acids (AAs) account for a small fraction of the bulk DOM, but they offer a powerful tool for determining the composition of organic matter and are widely used in contemporary organic matter studies, e.g., [1,5,6,7,8]. AAs are derived mainly from living organisms, and variations in both their total amounts and relative abundances are tightly coupled to the metabolism of organisms. Both the concentrations and relative abundances of AAs are dependent on the status or progress of their metabolism (in the short term, such as within a day) and upon their source (in the longer term, such as seasonally)
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