Flow discharge influences on input and transport of particulate and sedimentary organic carbon along a small temperate river

Abstract

The relative importance of hydrological conditions and upstream processes on the input and transport of organic carbon from land to sea was investigated in the Mullica River, a small system located in the southeastern New Jersey (USA), using molecular and δ13C compositions. Sediment and particulate organic carbon (POC) samples were collected along the river, from upstream (draining predominantly pine-oak forests) in the Pinelands to the Great Bay Estuary (bordered by salt marshes) under normal and high flow conditions. During base river flow, terrestrial biomarkers (e.g., diterpenoids – conifer biomarkers) were detected predominantly upstream and sharply decreased downstream. From mid to down river sites, a mixture of higher plants (primarily from the salt marsh vegetation) and microbial biomarkers (e.g., mycose, cholesterol) predominated. δ13C abundances of individual fatty acids presented a consistent pattern, with depleted values (e.g., δ13Csed=−30.9‰ on average) upstream, becoming heavier toward the Great Bay (e.g., δ13Csed=−23.1‰; δ13CPOC=−22.8‰). During the high discharge event, significant amounts of upland biomarkers (e.g., pimaric acid, ferruginol, ursolic acid) and depleted δ13C values were observed from the Pinelands (δ13Csed=−34.6‰) to mid river (δ13Csed=−32.0‰; δ13CPOC=−30.8‰), clearly demonstrating the transport of terrestrial material farther down river. However, downstream of this point, the occurrence of the terrestrial material strongly decreased, demonstrating that terrestrial plant biomarkers were not transported in significant amounts to the estuarine region, even at high river discharge. Since the mid river site is situated in the upper limit of salt water intrusions, it is likely that flocculation and sedimentation provide a depositional sink for terrestrial OC in the mid Mullica River, thereby depleting the terrigenous signature in downstream OC.