Influence of Algal Production and Decomposition on the Carbon Isotope Signature of Labile Particulate Organic Matter on a Productive Continental Shelf Under the Stress of Coastal Hypoxia

Abstract

To better understand the sources and recycling of labile organic matter in coastal water, we studied the carbon isotope signature of particulate organic carbon (POC) and particulate amino acids, a major group of labile organic compounds, on the shelf of the northern South China Sea. In addition, we were able to compare effects of hypoxia on labile organic matter at one station on the shelf. We found that carbon-weighted average d13C values of particulate amino acids (wAAd13C) were generally higher at more productive offshore stations than nearshore stations. This amino acid d13C distribution suggests that the labile fraction of organic matter in coastal water originated primarily from local primary production. In contrast, bulk POC d13C values showed little spatial variation on the NSCS shelf. The different patterns of wAAd13C and PO13C distributions indicate a combined effect of POM lability and physical processes on the isotopic distribution of organic materials on this productive shelf. Moreover, the amino acid d13C in coastal hypoxic water suggests local primary production as the most likely source of labile organic matter that drives hypoxia on the NSCS shelf. No clear difference was observed in particulate amino acid d13C distribution between hypoxic and oxic waters,

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