Abstract:
Dissolved organic matter (DOM) was isolated from surface waters of Delaware Bay along a transect from freshwater to the coastal ocean and fractionated by tangential flow ultrafiltration into high (1-30 kDa; HDOM) and very high (30 kDa-0.2 μm; VHDOM) nominal molecular mass fractions. Carbon content, stable carbon isotopes, and lipid composition were measured for each DOM fraction, and particles collected in parallel. Lipids, excluding hydrocarbons, comprised up to 0.33% of HDOM organic carbon, 1.6% of VHDOM carbon, and 10% of POC, the majority of which were fatty acids. Although lipids comprised a small fraction of HDOM, fatty acids and sterols provided valuable information on the origins of DOM. Molecular composition of particulate and dissolved lipids and bulk stable carbon isotopes demonstrated differences in organic sources along the estuarine gradient with distinct terrestrial signals in the river and turbid middle estuary and an algal signal in the lower estuary and coastal ocean. Both particulate organic matter and VHDOM samples were enriched in lipids on a carbon basis compared to the HDOM fraction, which suggests that the HDOM fraction was less labile than particulate organic matter or VHDOM. Selective degradation of labile lipids by the microbial community can account for the depletions of unsaturated fatty acids, sterols, and phytol within HDOM relative to particles.