Sources of estuarine dissolved and particulate organic matter: A multi-tracer approach

Assessing the fate of organic matter (OM) in estuaries is challenging due to its numerous sources, diagenetic processing, and physical and biogeochemical sinks. In order to evaluate differences in the sources and potential transformations of OM, particulate organic matter (POM) and high molecular weight dissolved organic matter (HMW DOM ( 3 kDa)) were characterized along the York River estuary (Virginia, USA) using lipid biomarker, elemental ratios, and stable isotope (δ13C and δ15N) analyses. Isotopic signatures of HMW DOM from the freshwater site were distinct from both its POM counterpart and HMW DOM from higher salinity regions of the estuary. Lipid compound classes were depleted in HMW DOM relative to POM. Saturated fatty acids (FA) dominated the dissolved organic pool while monounsaturated and polyunsaturated FA were the primary FA classes in POM. Concentrations of sterols diagnostic of allochthonous (i.e., plant/freshwater algae) sources were enriched in HMW DOM relative to the POM at higher salinities. Ternary plots based on FA biomarkers show that POM is characterized by contributions from polyunsaturated FA, labile compounds representing contributions from “fresh” phytoplankton/zooplankton sources. In contrast, FA in HMW DOM reflect bacterial and vascular plant signatures. Disparities in the composition of HMW DOM and POM likely reflect differences in the dominant biological sources, different susceptibilities to transformation and/or differential influences from abiotic processes (sorption–desorption, flocculation, etc.). Thus, the physical associations of OM (particulate vs. dissolved) may be a fundamental control on both the distribution and biogeochemical processing of OM whereby terrigenous DOM may be selectively exported while terrigenous POM is retained within the estuary. These observations have implications for the selective processing of different sources and forms of OM in rivers and estuaries prior to export to the coastal ocean.