Enrichment and characterization of dissolved organic matter in the surface microlayer and subsurface water of the South Yellow Sea

Abstract

Concentrations of dissolved organic carbon (DOC), total dissolved carbohydrates (TDCHO, including monosaccharides, MCHO and polysaccharides, PCHO), total hydrolysable amino acids (THAA, including dissolved free, DFAA and combined fraction, DCAA) and chlorophyll α were measured in the surface microlayer (SML) and subsurface water (SSW) of the South Yellow Sea (SYS) in May 2010. The concentrations of DOC, TDCHO, THAA and chlorophyll α in the SML were respectively correlated with their SSW concentrations, implying that transport from the SSW could be a major source of substances in the SML. One-way analysis of variance (ANOVA) showed that concentrations of DOC, MCHO and DCAA were significantly higher in the SML than in the SSW. Major constituents of DCAA and DFAA present in the study area were glycine, serine, alanine, glutamic acid, and aspartic acid. The enrichment factor (EF) of a compound is defined as the ratio of the concentration in the SML to that in the corresponding SSW. Carbohydrate species and amino acids were generally enriched in the SML, with mean EF values significantly greater than unity (t-test, P<0.05). The TDCHO concentrations were positively correlated to the chlorophyll α concentrations in the SML, suggesting that in situ production from phytoplankton might be another important source for the microlayer TDCHO. Our data showed that no significant correlations were found between the EFs of dissolved organic matter (DOM) and environmental parameters (e.g., wind speed, water depth, salinity, water temperature and distance from shore). TDCHO comprised on average 15.21% of DOC in the SML and 11.31% of DOC in the SSW, while THAA accounted for 9.03% of DOC in the SML and 7.76% of DOC in the SSW. The yields of TDCHO and THAA were consistently higher in the SML than in the SSW, whereas degradation index (DI) values in the SML were similar to those in the SSW. High yields of TDCHO and THAA in the SML probably suggested that samples in the SML were fresher than those in the SSW. The present study may have implications for the source, enrichment and bioavailability of DOM in the SML.