Sources and fate of particulate organic matter along the river-estuary-coastal ocean continuum: Constraints from amino acid and amino sugar carbon isotopes

Abstract

Estuaries represent hotspots for organic matter cycling. Understanding the sources and fate of organic matter in estuaries is crucial for quantifying the transport of terrestrial organic carbon to the coastal ocean and air-sea carbon dioxide fluxes. Here we report the abundance and carbon isotopic signatures of bulk particulate organic matter (POM) as well as particulate amino sugars and amino acids in surface suspended particles along the salinity gradient in the Changjiang Estuary and adjacent coastal ocean. Our data show that bulk δ¹³C values are directly related to the δ¹³C values of essential amino acids, suggesting a control by primary production on bulk δ¹³C values. A large degree of fractionation (−26‰) between phytoplankton δ¹³C and dissolved inorganic carbon δ¹³C values was observed in regions with salinities greater than 28, leading to a decline in bulk δ¹³C values. Examining the δ¹³C patterns of individual amino sugars and amino acids reveals that terrestrial amino sugars are produced by mixed sources of bacteria, fungi, and algae, while terrestrial amino acids originate from vascular plants and bacteria. Along the salinity gradient, the source of amino sugars shifted to bacteria, whereas amino acids transitioned to algae. Moreover, the low carbon- and nitrogen-normalized yields of amino acids (∼10% and ∼22%, respectively) observed in the Changjiang River suggest an advanced diagenetic state of terrestrial POM. In contrast, elevated POM reactivity in moderate to high salinity zones indicates contributions from phytoplankton production. Using the bacterial biomarker muramic acid, we found that a substantial portion (∼19%) of terrestrial POM is of bacterial origin. Combining the distinct excursions in bulk, amino sugar, and amino acid δ¹³C values in low salinity (<20) regions of the estuary indicates that terrestrial organic matter is extensively removed in the estuarine regions. Together, these findings underscore significant alterations in the sources and properties of organic matter along the river-estuary-coastal ocean continuum, with bacterial reworking playing an important role.