The chemical composition and cycling of particulate and macromolecular dissolved organic matter in temperate estuaries as revealed by molecular organic tracers

We have examined high molecular weight dissolved organic matter collected seasonally throughout the 200 km Delaware Estuary and nearby coastal ocean and compared its organic composition to particles collected in parallel. Dissolved organic matter (DOM) was collected and fractionated by ultrafiltration into 1–30 kDa (HDOM) and 30 kDa — 0.2 μm (VHDOM) molecular weight fractions. For one time period (June 1996) a suite of analytical methods was used to characterize dissolved and particulate fractions in detail and to provide complementary information on sources, impact of biological processing and the transport of organic material through the estuarine interface. Substantial, yet variable, portions of organic carbon were identified as carbohydrates, amino acids, lipids and lignin phenols, accounting for 30–90% of POC, 42–67% of VHDOM-C and 14–39% of HDOM-C. The average composition of organic material in Delaware Bay included 32% POC, 1.3% VHDOM, 23% HDOM and 42% LDOM (<1 kDa). Among these fractions, HDOM appears to mix most conservatively, with a strong negative relationship with salinity and a distinct chemical character. Comparisons of the Delaware with other systems indicate that carbohydrates are the main component of total DOM and its high molecular weight fractions which can be characterized, followed by total hydrolyzable amino acids. Lipids are universally low in those systems where they have been examined, but show an increase in concentration with molecular weight as well as increased presence of labile components.