Composition, Sources, and Age of Dissolved and Particulate Organic Matter in the Delaware River and Estuary

Estuaries are important sites of organic matter (OM) transformation, exchange, and burial but remain one of the least understood regions in the global carbon cycle. The carbon cycle within these regions is complex due to strong gradients in biological and physical processes, and increasing anthropogenic impacts. This is further complicated by the many sources of particulate and dissolved organic matter (OM) in estuaries, including materials derived from terrestrial and anthropogenic sources as well as aquatic and marine primary production. This study combined lipid biomarker analyses with stable and radiocarbon signatures of lipids and source-specific biomarkers to better understand the sources and aging of OM in Delaware River and Bay, a model estuarine system. The lipid composition of particulate organic matter (POM, \u3e 0.7 μm) and ultrafiltered dissolved organic matter (UDOM, 1kDa – 0.1 μm) was investigated along the salinity gradient in the Delaware River and Bay during five separate cruises. Sources of OM associated with POM and UDOM were examined using chlorophyll a, C:N ratios, stable carbon and nitrogen isotopes (δ13C and δ15N), total lipid extracts, and fatty acid (FA) biomarker compounds. Multiple hierarchical models explored which environmental characteristics were the primary drivers of POM and UDOM composition. These models revealed that chlorophyll a, POC, and TSS influenced POM sources and composition along the estuary, while a variety of drivers influence UDOM composition. Stable carbon (δ13C) and radiocarbon (Δ14C) measurements of dissolved inorganic carbon, bulk particulate organic carbon (OC), and neutral and polar lipids from particulate organic matter (TLEPOM) and ultrafiltered dissolved organic matter (TLEUDOM) were measured in order to gain insights about the source and age distribution of lipids along the Delaware River and Bay. Overall, Δ14C values for neutral TLE were more depleted (i.e., had “older” radiocarbon ages) than polar TLE. Radiocarbon ages for neutral TLEPOM were younger than neutral TLEUDOM by approximately 10,000 YBP, while polar TLEPOM and polar TLEUDOM were similar in age. Using a 14C isotope mass balance, changes in contributions of modern and fossil OC were quantified along the estuary for TLEPOM and in TLEUDOM. Complementary to determining the radiocarbon ages of different lipid classes, this study was the first to apply compound specific radiocarbon analyses to fatty acids (FA) associated with estuarine POM. Δ14C values indicate that the ages of terrestrial and algal FA change along the estuary. Terrestrial FA increased in age along the estuary due to downstream sources, while algal FA became “younger” along the estuary due to contributions from autochthonous sources. FA biomarker and radiocarbon analyses revealed changing composition of OM along the Delaware River and Bay: (1) older, terrestrial sources of OM characterized riverine OM, (2) the ETM was a location of shifting sources and introduction of “older” POC, and (3) the bay was dominated by younger, marine sources of OM. Lipid age was not based on within estuary processes but on the delivery of “aged” OM from the watershed and along-estuary mixing of different sources. Overall, this study provided new insights about the sources and ages of OM along the estuarine salinity gradient and the complex processes by which they are controlled